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The first law of photochemistry, the Grotthuss-Draper law,
states that light must be absorbed by a compound
in order for a photochemical reaction to take place.
The second law of photochemistry, the Stark-Einstein law, states
that for each photon of light absorbed by a
chemical system, only one molecule is activated for subsequent
reaction. This "photoequivalence law" was
derived by Albert Einstein during his development of the quantum
(photon) theory of light.
Type of Glass Wavelength Cut-off
Photochemistry
Photochemistry
The study of chemical reactions, isomerizations and physical
behavior that may occur under the influence of visible and/or
ultraviolet light is called Photochemistry. Two fundamental
principles are the foundation for understanding photochemical
transformations:
The efficiency with which a given photochemical process occurs
is given by its Quantum Yield (). Since many
photochemical reactions are complex, and may compete with
unproductive energy loss, the quantum yield is usually
specified for a particular event. Thus, we may define quantum
yield as "the number of moles of a stated reactant
disappearing, or the number of moles of a stated product
produced, per einstein of monochromatic light absorbed.", where
an einstein is one mole of photons. For example, irradiation of
acetone with 313 nm light (3130 ) gives a complex mixture
of products, as shown in the following diagram. The quantum
yield of these products is less than 0.2, indicating there are
radiative (fluorescence & phosphorescence) and non-radiative
return pathways (green arrow). The primary photochemical
reaction is the homolytic cleavage of a carbon-carbon bond shown
in the top equation. Here the asterisk represents an
electronic excited state, the nature of which will be defined
later.
Several secondary radical reactions then follow (shown in the
gray box), making it difficult to assign a quantum yield to the
primary reaction. The biacetyl product, formed in the third
reaction, may itself be excited by light or accept excitation
energy
from an excited acetone molecule, further complicating this
process.
By comparison, the light induced chlorination of methane, or
other alkanes, has a large quantum yield, often near 106,
because of the secondary chain reactions that follow the primary
cleavage of the Cl-Cl bond. Equations illustrating such
halogenation reactions have been presented elsewhere in this
text.
Mechanistic Background
Absorption of visible and/or ultraviolet light by a molecule
introduces energy
sufficient to break or reorganize most covalent bonds. From the
relationship E =
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Pyrex < 275 nm
Corex < 260 nm
Vycor < 220 nm
Quartz < 170 nm
hc / , we see that longer wavelength visible light (400 to 800
nm) is less energetic(70 to 40 kcal/mole) than light in the
accessible shorter wavelength (200 to 400
nm) near ultraviolet region (150 to 70 kcal/mole). Consequently,
ultraviolet light is
most often used to effect photochemical change. Care must also
be taken to
construct lamps and reaction vessels from glass that is
transparent to the desired
wavelength range. The low wavelength cut-off for some common
glass types are
given in the table on the right.The light required for a
photochemical reaction may come from many sources. Giacomo
Ciamician, regarded as the
"father of organic photochemistry", used sunlight for much of
his research at the University of Bologna in the early 1900's.
Depending on the compounds being studied and the information
being sought, bright incandescent lamps (chiefly infrared
and visible light), low, medium and high pressure mercury lamps
(185 - 255 nm, 255 -1000 nm & 220 -1400 nm
respectively), high intensity flash sources and lasers have all
been used. In careful studies of specific chromophores,
sources of monochromatic light may be desired.In this section we
shall focus chiefly on the nature and behavior of the electronic
excited states formed when a photon is
absorbed by a chromophoric functional group. As a rule, such
excitation results in a change in molecular orbital
occupancy, an increase in energy, and changes in local bonding
and charge distribution. The principles set forth in the UV-
Visible Spectroscopy chapter will provide a helpful
foundation.
We begin by considering the electronic excitation of a simple
diatomic molecule such as Cl2 or Br2. Both absorb light,
chlorine in the 300 to 380 nm region. and bromine in the 360 to
510 nm region. The
diagram on the right illustrates the initial electronic
excitation. Both the ground (lowest
energy electronic state) and excited states are shown as energy
profiles populated by
vibrational energy states (green lines) as well as rotational
states (not shown). The
electron reorganization that occurs when the ground electronic
state is excited by
absorption of a photon takes place much more rapidly than any
movement of the atom
nuclei that eventually follow. In other words, electron shifts,
when viewed from the
perspective of the nuclear coordinates, occur as if the heavier
nuclei were fixed in
place. This consequence of the Born-Oppenheimer approximation
led James Franck
and R. Condon to formulate the Franck-Condon Principle:
Electronic transitions
occur much faster than nuclei can respond.
Overall bonding in an excited state is usually lower than in the
ground state. Thus, the
XX bond length is increased in the excited state. At normal
temperatures essentially
all molecules will exist in the ground vibrational state (zero
level). The Franck-Condon
principle requires that excitation occur by a vertical
transition, shown by the red line, resulting in the population of
higher
vibrational levels in the excited state. Several events may then
take place.
1. The vibrational energy may be lost as heat, relaxing the
excited state to its zero vibrational level.
2. The excited state may return to the ground state by emitting
a photon (light blue line). If this happens from the zero
vibrational level the frequency or energy of the emitted light
will be lower than that of the initially absorbed light. This
radiative decay is called fluorescence if it takes place rapidly
from the initial excited state. It is termed
phosphorescence if it occurs slowly by way of other excited
states.
3. If a higher vibrational level of the excited state is
populated, either by the initial Franck-Condon transition or by
collisional activation, the molecule may cleave into two X
atoms. Note that vibrational level 5 of the excited state is
roughly coincident with bond breaking.
Virtually all organic compounds have more than two atoms, so the
potential energy state diagram of X2 must be adjusted
for the increased number of bonding relationships. One way of
doing this is to retain the energy coordinate while
dispensing with the dimensional (r) coordinate. The Jablonski
diagram shown below is an example, in which the spatial
orientation of the various electronic states is not specified.
Nevertheless, Franck-Condon transitions are expected.
One important feature conveyed by the diagram is that more than
one electronic excited state is likely to exist for a given
molecule, six are drawn and labeled in the diagram. Each
electronic state will have a group of vibrational (and
rotational)
states, depicted by light blue lines above each state marker.
Transitions between electronic states often occur to higher
vibrational levels which then relax to lower levels by
collisional loss of heat (translational energy).
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Excited states may be classified as singlet or triplet based
upon their electron spin angular momentum. The electrons in
most non-metallic organic compounds are paired (opposite spins)
in bonding and non-bonding orbitals, resulting in a net
zero spin diamagnetic molecule for the ground state. Such states
have a single energy state in an applied magnetic field,
and are called singlets. Electronic states in which two
electrons with identical spin occupy different orbitals (the
Pauli
exclusion principle) have a net spin of 1 (2 1/2) and are
paramagnetic. In a magnetic field such states have three energy
levels (+1, 0, -1) and are called triplets. Molecular oxygen is
a rare example of a triplet ground electronic state.
The distinction between singlet and triplet states is important
because photon induced excitation always leads to a state of
the same multiplicity, i.e. singlet to singlet or triplet to
triplet. Since most ground states are singlets, this means that
the
excited states initially formed by absorption of light must also
be singlets. Internal conversion of excited states to lower
energy states of the same multiplicity takes place rapidly with
loss of heat energy (relaxation). Alternatively, an excited
state
may return to the ground state by emitting a photon (radiative
decay). In the study of acetone described above, nearly 80%
of the excited singlet states lose energy by internal conversion
and about 3% by fluorescence. Conversion of a singlet state
to a lower energy triplet state, or vice versa, is termed
intersystem crossing and is slower than internal conversion.
Radiative decay from a triplet state is called phosphorescence
and is generally quite slow. The approximate timescales for
these transitions are given in the following table.
Process Transition Timescale (sec)
Light Absorption (Excitation) S0 Sn ca. 10-15
(instantaneous)
Internal Conversion Sn S1 10-14 to 10-11
Vibrational Relaxation Sn* Sn 10
-12 to 10-10
Intersystem Crossing S1 T1 10-11 to 10-6
Fluorescence S1 S0 10-9 to 10-6
Phosphorescence T1 S0 10-3 to 100
Non-Radiative Decay S1 S0
T1 S0
10-7 to 10-5
10-3 to 100
The non-radiative decay noted in the last row may take place by
intermolecular energy transfer to a different molecule. This
collisional process is termed quenching if the focus is on the
initially excited species, or sensitization if the newly
created
excited state is of interest. Photochemical sensitization
commonly occurs by a T1 + S0 S0 + T1 reaction, where the
bold red-colored species is the sensitizer. The new triplet
excited state may then undergo characteristic reactions of its
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own.
Photochemical Reactions
Alkene Isomerization
A photochemical reaction occurs when internal conversion and
relaxation of an excited state leads to a ground state
isomer of the initial substrate molecule, or when an excited
state undergoes an intermolecular addition to another reactant
molecule in the ground state. The cis-trans photochemical
isomerization of stilbene is a reaction of the first kind, as
shown
in the following diagram. Both cis and trans-stilbene undergo *
electron excitation by absorption of uv light. Whereas
isolated double bonds require 180 nm light for such excitation,
conjugation with the phenyl substituents lowers the
transition energy to about 300 nm, a more easily achieved
source. The molar absorptivity of the cis-isomer is less than
that
of the trans-isomer because steric crowding of the ortho sites
causes the phenyl groups to twist slightly out of coplanarity.
The stability of the stereoisomers of stilbene is due to a 62
kcal/mole barrier to rotation about the double bond produced by
the -bond. This bonding is absent in the * excited state
(magenta curve in the diagram). Both the initial S1 states
formed from the cis and trans ground states are slightly twisted
(the cis by 25 & the trans by 13) with the C=C double
bond being lengthened by about 4.5%. These local S1 states
quickly relax to a common lower energy twisted configuration
( 90). Non-radiative internal conversion of this S1 twisted
state leads to the transition state region of S0, which decays
equally to the ground states of the cis and trans isomers. This
simple configurational isomerization about a single double
bond is referred to as a One-Bond-Flip (OBF) event.
A small proportion (6%) of the trans-S1 state fluoresces back to
the trans-isomer, but there is less than 0.1% fluorescence
from the cis-S1 state. Triplet states (not shown) may also be
formed, but there is no observable phosphorescence in
solution, and non-radiative decay from such states results in
similar cis-trans isomerization. The quantum yield of trans to
cis isomerization decreases in viscous solvents, accompanied by
an increase in fluorescent decay. Freezing the solutions
to a rigid glass halts the isomerization and results in maximum
fluorescence efficiency.
Inspection of the cis-S1 local minimum near = 0 shows that only
70% of the molecules in this state relax to the lower
energy twisted S1 state. The remaining 30% undergo an
electrocyclic rearrangement to an isomeric 4a,4b-
dihydrophenanthrene (DHP) S1 state, as illustrated above by
clicking on the diagram. Molecules occupying this new
excited state then relax to either DHP or cis-stilbene ground
states. Over time, DHP accumulates and may be converted to
phenanthrene by mild oxidation with air or iodine. Substituted
phenanthrenes thus become available from trans-stilbene
precursors prepared by Wittig or Grignard procedures.
Another minor product from the photolysis of stilbene has been
identified as 1,2,3,4-tetraphenylcyclobutane (distilbene),
shown in the following diagram. Unlike the previously described
unimolecular isomerizations, this compound is formed by a
bimolecular reaction between an electronically excited stilbene
and a ground state stilbene molecule. The dimer
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Photosensitized Stilbene Isomerization
Sensitizer Triplet Energy Cis:Trans Ratio
benzophenone 69 kcal/mole 60:40
accumulates slowly, irradiation of a 0.07 M solution of
trans-stilbene in benzene producing 27% (as two stereoisomers)
after two months of exposure. As expected, higher stilbene
concentrations increase the rate of dimer formation. Thus, a
0.56 M solution of trans-stilbene in ethyl acetate yielded 11%
dimer in four hours of irradiation. Mercury lamps were used as
the uv light source in all these cases.
Electron donating substituents on the benzene rings facilitate
the dimerization, as does restricting the ability of the double
bond to isomerize or achieve orthogonally twisted excited
states. Examples of these influences will be displayed above by
clicking on the diagram.
The stilbene reactions described above have been attributed to
singlet excited states. Triplet excited states exist, but their
formation by intersystem crossing is inefficient. In order to
study the behavior of triplet excited states it is often
necessary
to generate them by energy transfer from a higher triplet
excited state of a suitable sensitizer molecule. Spin
conservation
requires that spin exchange take place during the collisional
energy transfer. Thus, a sensitizer triplet (Zt), generated
from
sensitizer molecule Zs, reacts with a ground state stilbene
molecule (Ms) in the following manner:
Zt() + Ms() Zs() + Mt().
The following diagram illustrates important features of this
sensitization reaction. The ideal sensitizer (Z) absorbs light
preferentially with respect to the substrate (M), and undergoes
efficient intersystem crossing to a triplet excited state (T1).
This energetic state then serves to activate a substrate
molecule to a lower energy triplet state by collisional
exothermic
energy and spin exchange, returning the sensitizer to its ground
state. A variety of useful sensitizers have been identified,
and by clicking on the diagram a few of these will be drawn
below.
In the early 1960's Prof. George Hammond's group at California
Institute of Technology undertook an extensive study of the
photosensitized isomerization of stilbene and related alkenes
and dienes.
A few of their findings are presented in the table on the
right, the same photostationary cis:trans ratio being
observed from either isomer as the starting point. By
clicking on the diagram a second time a diagram
illustrating this system will be displayed above. The
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benzil 53 kcal/mole 85:15
pyrene 47 kcal/mole 90:10
eosin 43 kcal/mole 0:100
unexpected change in steady state isomer distribution withthe
triplet energy of the sensitizer could not be rationalized
as a single classical energy transfer. All sensitizers
having
a greater triplet energy than 60 kcal/mole act by rapid
collisional vertical energy transfer (heavy blue arrows) to
the first encountered stilbene isomer. The initial cis and trans
triplets undergo fast, and in the case of trans reversible,
conversion to a lower energy twisted triplet, which then decays
to cis and trans-stilbene at rates that favor cis by a factor
of
1.5. As the triplet energy of the sensitizer drops below 57
kcal/mole, two changes occur. First, the efficiency of energy
transfer to cis-stilbene diminishes and the trans isomer is
selectively activated, resulting in a greater [cis]/[trans]
photostationary state. Next, direct nonvertical excitation to
the trans and twisted triplets may take place (light blue wavy
arrows). These competing excitations and subsequent decay to cis
and trans ground states lead to remarkable variations
in isomer ratios. Finally, the very low energy triplet state of
eosin is unable to effect any electronic excitation of stilbene,
and
simply catalyzes thermal equilibration, which strongly favors
the trans-isomer (trans is 6 kcal/mole more stable than cis)
These sensitized isomerizations do not give any significant
amount of the electrocyclic rearrangement or dimerization
reactions observed from singlet excited states.
Carbonyl Compounds
Background
Conjugated ketones are often good sensitizers, thanks to the
efficiency of intersystem crossing from the n * singlet to
the triplet. However, before discussing this application it will
be helpful to consider the bonding structure of the carbonyl
group itself. In the case of the simple compound formaldehyde,
the Lewis formula consists of two CH sigma bonds, a C
O sigma bond, a C=O pi bond and two non-bonding electron pairs.
As
shown in the diagram on the right, two Lewis structures,
differing in the
hybridization of oxygen, may be drawn, The structure on the left
is a
common representation in which an sp2 oxygen replaces one of
the
CH2 groups of an ethylene molecule. However, based on the
principle
that stable molecules will adopt the strongest bonding possible,
the right
hand structure becomes an attractive alternative. An sp2sp
sigma
bond should be stronger and shorter than a sp2sp2 sigma bond,
and the shorter bond distance will enhance the pi-
bonding. Double bonds are of course shorter and stronger than
equivalent single bonds. The average values given in the
following table indicate that C=C is 13% shorter and 76%
stronger than a CC bond, whereas C=O is 15% shorter and
over 100% stronger than a CO bond, possibly reflecting the sp
hybridization of the oxygen. An important difference
between these models is found in the nature of the non-bonding
electron pairs on oxygen. In the sp2-oxygen model these
occupy very similar (degenerate) orbitals, but in the sp-oxygen
model one pair is in a relatively low energy sp-orbital and the
other in a higher energy p-orbital. Molecular orbital
calculations clearly show that the latter model is a better
representation
than the former.
Bond Length, CC, 1.54 C=C, 1.34 CO, 1.44 C=O, 1.22
Bond Energy, kcal/mole CC, 84 C=C, 146 CO, 86 C=O, 177
To examine the molecular orbitals of formaldehyde Click
Here.
A general diagram illustrating the two major electronic
excitations of a carbonyl group is shown below. This
terminology
has been defined in the UV-Visible Spectroscopy chapter. The
weak nature of the n * absorption is due to the poor
overlap of the p-orbital housing the higher energy electron pair
with the * orbital (they are essentially orthogonal). In
contrast, and * orbitals overlap almost completely, so the
probability of a * excitation being achieved by a photon
of proper energy is high. This spatial overlap requirement has
been termed a selection rule by spectroscopists. A second
important selection rule concerns the probability of electronic
transitions between states of different spin multiplicity, the
spin selection rule. This rule reflects the high probability of
transitions between different singlet states, or between
different triplet states, but the low probability of
singlet-triplet or triplet-singlet transitions. A table summarizing
these rules is
given below.
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Formaldehyde, acetaldehyde and acetone show strong * absorption
at 180 to 190nm ( = 103 to 104), and weak n
* absorption at 280 to 300 nm ( = 12 to 20). Conjugation with
the -electrons of a double bond or a benzene ring
shifts both absorptions to longer wavelength and increases the
strength of absorption.
Compound Cyclohexanone (CH3)2C=CHCOCH3 C6H5CHO C6H5COCH3
max * 200 nm ( 2000) 230 nm ( 12,300) 242 nm ( 14,000) 238 nm (
13,000)
max n * 285 nm ( 14) 325 nm ( 90) 328 nm ( 50) 320 nm ( 40)
Aryl ketones such as acetophenone (right hand compound in the
table above), undergo rapid intersystem crossing of the n
* singlet excited state to an energetically close * triplet
state. The latter then quickly decays to the lower energy
n * triplet, as shown for benzophenone in the diagram below.
This circuitous route for the low probability direct
conversion reflects modified selection rules formulated by Prof.
M. A. El Sayed of UCLA (last two entries in the following
table). This pathway is not available to most aliphatic ketones,
so their intersystem crossing rates from n * singlets to
triplets are slow.
As noted in the diagram, there is very little fluorescent decay
from the S1 state, and radiationless decay to S0 is less than
1% of the intersystem crossing to T2. In the absence of
quenching reactions, T1 returns to S0 by a mixture of
phosphorescent and radiationless decay.
Benzophenone Excited States
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Some Carbonyl Selection Rules
Transition Probability Factor
S0 S1(n , *)
S0 S1( , *)
forbidden
----------------
allowed
orbital overlap
in space
(also symmetry)
Sn S1 S0Tn T1
allowedno change in
spin multiplicity
S(n , *) T(n , *)
S( , *) T( , *)forbidden
change in
spin multiplicity
S(n , *) T( , *)
S( , *) T(n , *)allowed
change in
orbital configuration
Reactions
H Abstraction, Bond Cleavage & Cycloaddition
The n * excited states of carbonyl compounds display a rich
chemistry in their own right.
Since the oxygen has an unpaired electron, it behaves in much
the same way as an alkoxy
radical, as noted in the chapter on free radical chemistry.
Hydrogen abstraction and addition
to double bonds are typical reactions. Cleavage of neighboring
carbon-carbon bonds may
also occur, the two most common of these being designated Type I
and Type II. General equations for these primary
reactions, starting from aryl ketone excited states, are
described in the following diagram. The n * excited state shown
in brackets at the center of the diagram may also be described
as the resonance hybrid drawn on the right. Note that the
ground state polarity of the carbonyl group has been reversed in
this excited state.
The triplet excited state of aryl ketones has a lifetime of
about 100 ns, and most of the subsequent reactions are very
fast
(rates range from 104 to 109 M-1 sec-1). Consequently, the
effect of quenchers such as 1,3-pentadiene on product
distribution provides valuable information about the mechanisms.
These quenchers have T1 energies much lower and S1energies higher
than corresponding ketone states. The rate of quenching is nearly
diffusion controlled, and is therefore
proportional to the concentration of the quencher. Since the
ground state triplet of oxygen reacts rapidly with triplet
excited
states, air must be excluded when studying these reactions.
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Examples of hydrogen abstraction and alkene addition reactions
will be displayed above by clicking on the diagram.
Reduction of benzophenone to benzpinacol is common in most
hydrocarbon solvents, even in pure benzene. The
diphenylmethanol radical formed by hydrogen abstraction is
relatively stable and, once formed, dimerizes to benzopinacol
in nearly quantitative yield. In contrast, the more reactive
isopropanol radical is converted to acetone by transferring its
hydroxyl hydrogen to benzophenone, thus forming another
diphenylmethanol radical.
The photoaddition of carbonyl compounds to olefins had been
observed by the Italian chemist E. Paterno in 1909, but the
structure of the oxetane products remained unknown until
elucidated by Bchi and coworkers (M.I.T.) in 1954. This
reaction
is now known as the Paterno-Bchi reaction. The mechanism shown
here was established for the electron-rich alkene
dioxene, a compound that favors electron transfer to
benzophenone. Other alkenes are also believed to react by way of
a
1,4-biradical intermediate, and significant regioselectivity is
found with many unsymmetrical alkenes. Two additional
examples of this reaction are drawn in the gray-shaded box at
the bottom. The second illustrates the intramolecular variant.
A related example of carbonyl addition to alkenes is the use of
benzophenone as a light activated initiator of polymerization
for monomers such as styrene and methyl methacrylate.
Examples of type I fragmentations are displayed below. These
reactions are fast, and often proceed with a high quantum
yield. The first example, shown at the top, illustrates a
typical homolytic cleavage of a carbonyl substituent, followed by
a
fast hydrogen atom transfer between the caged radical pair.
Three intramolecular type I reactions, all proceeding by way of
biradical intermediates, demonstrate the variety of products
that may be formed by hydrogen transfer disproportionation
and/or ring closure. The cyclic transition states for such
intramolecular hydrogen transfer usually include five or six,
and
occasionally seven atoms.
By clicking on the diagram, examples of type II reactions will
be displayed. These reactions all begin by an intramolecular
hydrogen atom abstraction, similar to the intermolecular
abstraction in the reductive dimerization of benzophenone.The
general example at the top of the diagram shows how a key
1,4-biradical intermediate, formed by intramolecular hydrogen
abstraction, may disproportionate to several different products,
including type II cleavage to the enol tautomer of a ketone
(green shaded box) and ring closure to a cyclobutanol (the Yang
Reaction). In hydrocarbon solvents intramolecular
hydrogen transfer back to carbon is common, reducing the quantum
yield of these products by over 50%. This reverse
hydrogen transfer is inhibited by hydrogen bonding solvents,
which stabilize the newly formed OH group. As expected, the
rate at which the biradical is formed is sensitive to
substitution at the -carbon. If R=H (a methyl group) the rate is
100
times slower than if R=CH3 (abstraction of a 3- hydrogen). This
reactivity order reflects the bond dissociation energies of
the CH bonds in the same fashion as noted for free radical
halogenation of alkanes. Also, replacement of all -hydrogens
by deuterium reduces the rate of biradical formation
substantially.
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The ratio of type II cleavage and Yang cyclization products from
a given substrate depends on several factors. The steroid
ketone in example 2 gives both reactions, with cyclization
predominating. Example 3 is instructive, first because the rate
of
biradical formation is much higher than in many other cases, as
a result of conformational bias favoring the reactive
orientation (as drawn); and second, because cyclobutanol
formation induces severe strain. Only type II fragmentation is
observed. Finally, example 4 proceeds by a -hydrogen abstraction
(7-membered cycle), as the only possible
transformation of its kind. The resulting 1,5-biradical then
closes to the product shown. Remarkably, phosphorescence
studies indicate that this reaction occurs from a low-lying *
triplet state, rather than the n * triplet usually
associated with this class of reaction.
Type II reactions depend, of course, on close approach of the
carbonyl oxygen to a -hydrogen. The conformational
mobility of the examples drawn above permits this to occur
within the lifetime of the excited triplet. However, it is not
difficult
to find molecules for which this is not possible.
Benzoylcyclohexane, shown at the top of the following diagram, is
such a
compound. Because of its equatorial location on the six-membered
ring, the carbonyl oxygen cannot reach any of the -
hydrogens. If the benzoyl group is forced into an axial
orientation by a large cis-tert-butyl group on c-4, then the oxygen
can
easily reach the axial cis -hydrogens (colored red. The
resulting type II photochemistry depends on the other C-1
substituent. If R = H, type II cleavage is the only product;
however this changes to exclusive stereoselective cyclization if
R
= CH3. The conformational drawings in the gray-shaded box
suggest a reason for this remarkable change. Following the
initial hydrogen abstraction, the 1,4-biradical is well aligned
for ring closure, but the benzyl radical must be rotated 90
before it is properly aligned for bond cleavage (far right
structure). A C-1 methyl substituent hinders this rotation, leading
to
formation of the strained cyclobutanol product.
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To learn more about Type II reactions Click Here.
Photoenolization
When the reactive carbonyl function and a -hydrogen are
conjugated via an aromatic ring or double bond, the
1,4-diradical
created by hydrogen abstraction quickly relaxes to a conjugated
enol tautomer. This is illustrated in the following diagram
for a -hydroxyl substituent, examples on the left, and an
ortho-tolyl ketone on the right. If an aromatic ring has been
disrupted by the photoenolization, as in all the cases but
avobenzone (lower left), the enoltautomer is unstable and
rapidly
reverts to the initial aromatic carbonyl compound. This might
appear to be a useless transformation, but it finds practical
application as a sunscreen ingredient. The three compounds on
the left are examples of current sunscreen components.
Oxybenzone screens UVA, salicylate esters (R=C8 to C10) screen
UVB and avobenzone screens both.
Careful studies of the photoenolization of ortho-alkyl
benzophenones and acetophenones have enhanced our
understanding of this deceptively simple transformation. By
clicking on the diagram the behavior of ortho-
methylacetophenone will be elaborated. Excitation of the anti
and syn mixture of this ketone leads to two stereoisomeric
n * triplet states, TE and TZ respectively. The former has a
much longer lifetime () than the latter since it must undergo
a conformational change before -H abstraction from the
ortho-methyl substituent can take place. In both instances a
twisted triplet biradical is formed. This biradical may be
intercepted by electron transfer to PQ2+ followed by rapid
proton
loss. This event is easily monitored by the intense blue color
of the PQ+ radical cation, and serves to set the lifetime of
the
biradical at 580 nsec. Relaxation of the biradical leads to a
mixture of (E) and (Z)-xylylenols, the latter undergoing rapid
H-
transfer back to the starting ketone. (Z)-Xylylenol formation
directly from a syn n * singlet state is possible and would
not be detected.
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Depending on the reaction conditions, (E)-xylylenols may undergo
a variety of reactions or slowly relax back to the initial
ketone. Clicking on the above diagram a second time will show
examples of other reactions (Part 3). Unlike benzophenone
itself, the ortho substituted compound in the upper left corner
does not undergo any pinacol reduction. Instead, -H
abstraction produces a mixture of interconverting twisted
biradicals which decay predominantly to (E,E) and (Z,E)-
xylylenols. Only the former is sufficiently long lived to yield
distinctive products. At temperatures below 0 C conrotatory
electrocyclization forms cis-1,2-diphenylbenzocyclobutenol in
high yield. At room temperature or above, conrotatory ring
opening regenerates the (E,E)-xylylenol which may be trapped by
cycloaddition to maleic anhydride.
Competition between photoenolization and type II cleavage has
been explored in the manner shown above, by clicking on
the diagram a third time (Part 4). The syn conformer of butyl
(and isoamyl) o-tolyl ketone gives exclusive photoenolization,
60% from the n * singlet state, the corresponding triplet state
undergoing xylylenol formation faster than type II
cleavage. The anti conformer reacts exclusively from the triplet
state, which either isomerizes to the syn triplet or
generates the biradical precursor to type II products. As noted
in the previous section, t-butanol favors type II cleavage. The
8-methyl-1-tetralone served as a reference molecule, constrained
with the carbonyl oxygen syn to the methyl substituent.
Conjugated -Orbital Functions
Dienes and Polyenes
Acyclic conjugated dienes, trienes and polyenes absorb strongly
in the 220 to 300 nm region of the near ultraviolet. The
resultant singlet excited states undergo a variety of reactions,
as shown in the following diagram for 1,3,5-hexatriene and
two 2,5-dialkyl derivatives. Two configurational isomers (E
& Z) are possible, together with three coplanar s-cis and
s-trans
conformations for each (the cEt structure is not drawn). The
E-configuration is normally more stable than its Z-isomer, and
for R=H or CH3 the elongated tEt conformation is preferred. The
various conformers of both the E & Z-isomers are rapidly
interconverted at room temperature, the equilibrium composition
depending on the size of substituents such as R. The light
induced isomerization about the central double bond of the
trienes (e.g. EZ) is an example of OBF.
Now, electronic * excitation of any triene increases the
-bonding between carbons 2 & 3 as well as 4 & 5, as may
be seen in the HOMO (3) and LUMO (4*) molecular orbitals of the
parent triene. Taken together with the very short
lifetimes of these excited states (10 nsec), this suggests that
photochemical products should reflect the rotamer
composition of the ground state. Put another way, excited state
rotamers are not expected to equilibrate prior to reaction,
the NEER principle (Non-Equilibration of Excited Rotamers). An
example of NEER is found in the absence of 1,3-
cyclohexadiene among the photoproducts from the parent triene
(R=H), compared with its increased quantum yield as R is
changed to methyl and then tert-butyl (increased cZc
concentration).
For E as well as Z-trienes, reactivity parallels an increase in
the population of non-planar s-cis rotamers. Furthermore, 1,2-
divinylcyclopentene (drawn in the gray-shaded box) is
photochemically unreactive and exhibits fluorescence in
solution,
demonstrating that twisting about the central (3,4-) double bond
of the triene is an essential factor in these reactions.
Irradiation of trans,cis,cis-1,3,5-cyclononatriene, shown at the
bottom of the diagram, by 254nm light establishes a 60:40
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photoequilibrium with the cis-fused bicyclononadiene on its
right. This equilibrium is shifted to the right by longer
wavelength 300nm light, which also induces further
electrocyclization to tricyclo[4.3.0.0]non-3-ene. As a rule,
cyclohexadienes absorb light at longer wavelength than similar
non-cyclic hexatrienes. Higher temperatures permit the
thermal electrocyclization of
trans,cis,cis-1,3,5-cyclononatriene to the trans-fused
bicyclononadiene on its left.
Finally, diradical mechanisms for the formation of
allylcyclopropene (ACP), vinylcyclobutene (VCB) and
bicyclohexene
(BHE) products will be shown above by clicking on the diagram.
ACP may be formed from either the tEt or cZt excited
states, but VCB and BHE require the latter rotamer. The MVCP
product on the left is unique to R = CH3.
As noted above, the reduced bond order of a formal double bond
in its first excited state leads to the OBF mechanism for
isomerization in fluid solution. The torsional movement in this
change is sensitive to the viscosity of the solution and to
other structural constraints that may exist. Thus, the facile
isomerization and weak fluorescence, typical for trans-stilbene
in fluid solutions, change to low isomerization and strong
fluorescence in rigid media at low temperatures. Here, the
rapid
radiationless deactivation of the excited state by OBF is
impeded and a normally non-fluorescent compound becomes
fluorescent.
A more complex, higher energy motion that achieves double bond
isomerization in restrictive environments has been
described and named the Hula-Twist. As shown for a cis-stilbene
derivative in the following diagram, the OBF process
involves a 180 flip of the aryl group on the right side of the
double bond. By contrast, the HT process proceeds by
simultaneous rotation of a small CH moiety (light blue ellipse)
about the double bond and an adjacent single bond (both
colored brown). This not only converts the cis-isomer to its
trans-configuration, but also effects a conformational change
(note the orientation of the red hydrogen relative to the aryl
substituent on its left).
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Show Hula-Twist Isomerization
The difference between OBF and HT isomerization of a Z to
E-triene will be shown above by clicking on the diagram. It has
been proposed that photoisomerization of biologically important
polyenes, such as retinal and calciferol may occur by the
HT mechanism because translocation of a single H-atom is less
volume demanding than the turning over of one-half the
molecule, as required by the OBF process. This would be
especially important when the chromophore is encapsulated in a
protein or bilayer membrane.
Instructive examples of the photochemical behavior of
biologically important polyenes may be examined by clicking
these
buttons.
Rhodopsin Bilirubin Vitamin D
Enone Reactions
[2+2]-Cycloaddition
In the earlier discussion of stilbene photoisomerization a
[2+2]-cyclodimerization was noted. The construction of
cyclobutane rings from alkene components is rare in ground state
chemistry, so a general photochemical synthesis of this
kind would provide a valuable addition to our assortment of
useful carbon-carbon bond forming reactions. Double bonds
activated by conjugation with a carbonyl group have proven
especially effective in this respect, both in forming [2+2]
dimers
and adding to isolated alkene functions. A particularly
important example of photodimerization involves the damage to
DNA
caused by uv light. Thymine (or uracil in the case of RNA) is
one of four heterocyclic base components of nucleic acids.
On exposure to ultraviolet light, thymine may undergo
[2+2]-dimerization with a suitably oriented nearby thymine, as
shown
in the diagram below. When this happens toxic products are
generated that are directly responsible for cell death via
mutagenic action, suppression of DNA transformation and/or
activation of carcinogenic pathways. A natural repair
mechanism employing a photolyase enzyme system reverses the
dimerization.
Four possible dimers of thymine (and uracil) may be formed. The
cis-syn isomer shown above is the chief dimer formed
by irradiation of DNA, or from frozen aqueous solutions of
thymine or uracil. Other isomers are formed in varying amounts
from room temperature solutions of these bases. By clicking on
the diagram a drawing showing the pertubation of DNA by
this dimerization will be displayed.
In reactions with unsymmetrically substituted alkenes, cyclic
enones yield fused cyclobutane rings with variable regio and
stereoselectivity. As shown in the following diagram, the
addition of 2-cyclopentenone to propene gives a mixture of
regioisomers, both having a cis-fused bicyclo [3,2,0] heptane
unit. In contrast, addition of 2-cyclohexenone to 1,1-
dimethoxyethene proceeds with good regioselectivity and modest
stereoselectivity favoring a trans-ring fusion. The last
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(i) Cyclization to a fused cyclobutane product.
(ii) Disproportionation by a 1,5-hydrogen transfer, yielding an
or alkenyl-substituted ketone.
(iii) Fragmentation back to the starting compounds.
example demonstrates that unsaturated carbonyl functions other
than ketones may lead to cycloaddition products, and that
functional substituents on the alkene moiety may influence the
regioselectivity.
By clicking on the diagram, three more examples will be
displayed, The first two confirm the modest [2+2]
regioselectivity
noted in the first display and show additional monocyclic
products. The third reaction discloses there is very low
retention
of configuration in the cycloaddition of alkene stereoisomers.
This stands in marked contrast to the high selectivity that
characterizes [4+2] cycloadditions known as the Diels-Alder
reaction. A similar lack of stereoselectivity is found in
intramolecular [2+2] cycloadditions, as may be seen by clicking
the diagram a second time. Three additional examples of
the intramolecular variant (I. II & III) are also presented
in this new display. The importance of this method for the
assembly
of complex molecules is shown by the short efficient synthesis
of the sesquiterpene isocomene accomplished by M.
Pirrung.
The poor selectivities and modest quantum yields observed for
these cycloaddition reactions have engendered several
mechanistic interpretations. Identification of the enone excited
state as a * triplet comes from selective quenching
studies as well as transient absorption spectroscopy.
Bauslaugh's proposal that this triplet adds to the alkene to form
a
1,4-biradical proposal is still the simplest and most satisfying
explanation of the facts. As shown in the following diagram,
the initial bonding may take place at either the or -carbon of
the enone. The resulting biradicals would also be triplets,
and on intersystem crossing to a singlet could yield ground
state products by one of three paths.
The latter event accounts for low quantum yields, and is
presumed to be fast because isomerization of recovered alkene
is
small when cis and trans-2-butene are used as the alkene
reactant. For the addition of cyclopentenone to isobutene, four
possible biradicals may be drawn, the estimated stability order
being II > I > III > IV. Finally, although the biradical
species
are very short lived (ca. 60 ns), it has been possible to trap
them by use of the foul-smelling, highly-toxic, hydrogen donor
hydrogen selenide. The results of such a trapping study will be
displayed below by clicking on the diagram.
Several important conclusions were reached in this case. First,
no initial addition to the more substituted carbon of the
alkene took place, since tert-butyl substituted cyclopentanones
would have been produced (via III & IV) and none were
found. Second, although initial bonding at the -carbon (yielding
II) was nearly twice as fast as -bonding, over 96% of II
fragmented to starting materials, compared with 79% of I.
Consequently most of the trapped products came from I. These
products were formed either by hydrogen atom delivery to both
radical sites (giving saturated compounds) or hydrogen
delivery followed by hydrogen abstraction by HSe. (giving
unsaturated compounds), as noted in the gray-shaded box
bottom-right..
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The formation of trans-fused bicyclo [4,2,0] octane products
from 2-cyclohexenone has been explained by considering the
conformations of the 1,4-biradical intermediates. This will be
illustrated above by clicking the diagram a second time. Here,
three rotamers (1, 2 & 3) for both the and chains are drawn.
When the radical sites are far apart (2 and 3),
fragmentation of the biradical is likely. The trans-ring fusion
will be formed from conformation 1, and cis from 3. The
latter bonding will be subject to some axial hindrance. For the
-manifold, 1 should yield a cis-fused product and 2 the
trans-isomer. However the latter bonding would again be
hindered.
The de Mayo Reaction
In 1962 Paul de Mayo (Univ. Western Ontario) found that the enol
of pentane-1,3-dione gave a 2-acetyl-1-
methylcyclobutanol adduct when irradiated in the presence of a
cycloalkene. As shown by the top reaction in the following
diagram, this cyclobutanol (R1 = R2 = CH3) may then undergo a
facile base-catalyzed retroaldol fragmentation to a 1,5-
diketone. This sequence thus achieves the attachment of a short
alkyl chain to each carbon atom of the double bond.
Further base induced reactions, such as epimerization of the
initial cis-configuration, and aldol cyclization of the
diketone
may follow. Following its initial report, this powerful
[2+2]-cycloaddition reaction sequence has been put to many uses.
One
of these is the synthesis of -tropolone shown by the bottom
equation.
Two additional applications of the de Mayo reaction in synthesis
will be displayed above by clicking on the diagram. The
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upper reaction is noteworthy for the unexpectedly high
regioselectivity of the [2+2] cycloaddition. Ring opening of
the
bicyclo[3.2.0]heptane moiety proceeds by an ethylagous
elimination or Grob fragmentation. Conversion of the resulting
bicyclic ketoacetal to -himachalene was straightforward. The
lower reaction demonstrates that poor stereoselectivity in
the cycloaddition does not necessarily present a problem. Here,
the four new stereocenters in the cycloadduct are lost
and/or epimerized by the retroaldol opening of the four-membered
ring and subsequent aldol cyclization.
Cyclohexadienone Reactions
Cross-Conjugated Derivatives
Derivatives of 2,5-cyclohexadienone are common in nature, and
their photochemical transformations posed a challenge to
early researchers. Some reactions of
4,4-diphenyl-2,5-cyclohexadienone are presented in the following
diagram. Although
2,3-diphenylphenol was a product from irradiation in aqueous
dioxane, it is actually formed from 6,6-
diphenylbicyclo[3.1.0]hex-3-ene-2-one, the initial rearrangement
product. It should be noted that a similar
bicyclo[3.1.0]hexane isomer was formed by irradiation of
4,4-diphenyl-2-cyclohexenones, as shown in the gray-shaded box
at the bottom right.
These photochemical rearrangements occur by way of triplet
excited states, which are conveniently depicted as diradicals.
Mechanisms for these reactions will be displayed above by
clicking on the diagram. Bond reorganization may take place in
the triplets, which eventually intersystem cross to singlet
species. Charge separation in these states may then lead to
rearrangement to a stable product.
The photo-isomerization shown above is one example of a general
family of reactions known as di--methane
rearrangements, other examples of which are illustrated in the
following diagram. These transformations are often photo-
sensitized, indicating they proceed by way of triplet excited
states. As the name suggests, substrates exhibiting this
rearrangement are comprised of two -functions separated by a
saturated (sp3-hybrid) carbon atom (designated by a red
dot in the illustration). In the event, a 1,2-shift of one
ene-group, followed by a bond formation between the terminal sites
of
the remaining -function leads to a vinylcyclopropane product.
One of the unsaturated functions may be a carbonyl or
imine group, as in examples 2. and 3.
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A photo-isomerization of this kind was used by H. Zimmerman
(Wisconsin) to prepare the novel self-replicating diene
semibullvalene from barrelene.
Conjugated Derivatives
Derivatives of 6,6-disubstituted 2,4-cyclohexadienones are also
photochemically reactive. Some examples are given in the
following diagram. Electrocyclic ring opening to an unsaturated
ketene is the favored transformation. Since nucleophilic
compounds such as water, alcohols and amines add rapidly to
ketenes, the resulting carboxylic acid or derivative is the
final isolated product. If no nucleophilic reactants are
present, the conjugated ketene diene will recyclize to the
starting
compound. Slower reactions leading to phenolic products may then
occur. In the last (bottom) example, additional methyl
substituents reduce the reactivity of the trans-ketene
intermediate, so that only strongly nucleophilic amines are able to
trap
it. The ketene is removed by light induced isomerization and
cyclization. As in the previous dienones, a triplet excited
state
undergoes decay to polar singlets that are thought to decompose
in the manner depicted in the gray-shaded area.
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Among the natural dienones displaying interesting photochemical
behavior, the sesquiterpene santonin is exceptional, and
has been the subject of exhaustive study. Click the button to
learn more about this subject.
Santonin
The names and association (at the time of their major work) of
many chemists whose research has led to the development
of modern photochemistry are presented in the following
table.
Name Location
D. H. R. Barton Imperial College, London
G. H. Bchi Massachusetts Institute of Technology
O. Chapman Iowa State University
P. de Mayo University of Western Ontario
G. S. Hammond California Institute of Technology
R. S. Liu University of Hawaii
D. C. Neckers Bowling Green State University
J. Saltiel Florida State University
J. C. Scaiano Ottawa Chemistry Institute
G. B. Schuster Georgia Institute of Technology
N. J. Turro Columbia University
P. J. Wagner Michigan State University
A. Weedon University of Western Ontario
N. C. Yang University of Chicago
H. E. Zimmerman University of Wisconsin
For more click here.
Return to Table of Contents
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