• PHOTOCHEMICAL AND THERMAL E/Z ISOMERIZATIONS OF SOME a,S -UNSATURATED" IMINIUM SALTS By PANKRATZ, B.Sc. A Thesis Submitted to the School of G=aduate Studies in Pa=tial Fulfilment of the Requi=ements fo= the Deg=ee Docto= of Philosophy McMaste= Unive=sity
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•
PHOTOCHEMICAL AND THERMAL E/Z ISOMERIZATIONS
OF
SOME a,S -UNSATURATED" IMINIUM SALTS
By
WL~IANNE PANKRATZ, B.Sc.
A Thesis
Submitted to the School of G=aduate Studies
in Pa=tial Fulfilment of the Requi=ements
fo= the Deg=ee
Docto= of Philosophy
McMaste= Unive=sity
-.
•
•
E/Z ISOMERIZATIONS
OF
IMINIUM SALTS
..
DOCTOR OF PHILOSOPHY (1986)(Chemistry)
-
McMASTER" UNIVERSITYHamilton, Ontario
.-
TITLE: Photochemical and Thermal Elz Isomerizations of Some~,S-Unsaturated Iminium Salts
Some Spectroscopic Data for Rhodopsin,Bacteriorhodopsin, and the All-Trans~Retinylidene Iminium Salt
Absorption Maxima for Rhodopsin,Bacteriorhodopsin, and RetinvlideneIminium' Salts in Various Sol;ents'
Physical Data for Iminium Salts 55-63'
Absorption Data for Iminium Salts 55-63
lH NMR Chemical Shift Data forIminium Salts 55-63
13C NMR Chemical Shift Data
lH NMR Chemical Shift Data for ~Iminium Salts 64-72
lH NMR Chemical Shift Data forIminium Salts 73-90
Absorption Maxima as a Function ofSubstituent
Quantum Yields of Photoisomerization
Medium Effects on PhotoisomerizationQuantum Yields
Photostationary State Compositions
Absorption Maxima and Relative QuantumYields of Photoisomerization of thepara and n.eta methoxy iminium salts59 and 95
Reduction Potentials
V .~. ~ - ~·~·~h Sc R ~e ~o~a=~a~~on o~ ~o w~~ an .a~ --Iminium Salt 57
xii
Page
18
25
54
55
56
57
61
62
73
82
84
86
106
118
120
Table 4-3
Table 4-4
Table 5-1
Table 6-1
Table 6-2
Table. 6-3
Table 6-4
Table 6-5
Table 6-6
Table 6-7
,.. ~.
Quenching of tris(2,2'-bipyridine)ruthenium(II)dichloride, 97,Luminescence by Iminium Salts 55-60in Acetonitrile
Isomerization of Iminium Salt 60 onIrradiation of tris(2,2'-bipyridine)ruthenium(II)dichloride, 97
Isomerization Rate Constants at 100~C
in TFA and O.OlM H2S04/TFA
Physical and Spectroscopic Data forN-methyl, N-phenyl-3-(m-methoxyphenyl)2-propenylidene iminium perchlorate, 95
lE NMR Data for Iminium Salt 95
. Raw Quantum Yield Data for the IminiumSalts 55-59 in Trifluoroacetic Acid
Raw Quantum Yield Data for Irninium Salts60-63 in Trifluoroacetic Acid
Raw Quantum Yield Data for Irninium Salts59 and 60 in Various Solvents
Raw Rate Data for the Isomerization ofIminium Salt 58 to 67 in TFA at 100°C
Approximate Rate Constants for theIsomerizations of Iminium Salts 55-63in TFA-d-at 100°C
xiii
125
127
135
146
146
157
158
159
165
166
LIST OF FIGURES .
Page
Charge Environment of Rhodopsin
. Figure 1-1
•Figure 1-2
Figure 1".3
Figure 1-4
Figure 1-5
Charge Distribution in the GroundStates of Model Compounds ofRetinylidene Iminium Salts
Charge Environment of Bacteriorhodopsin
Absorp~ion Maxima of Dihydroretinylid~ne
Iminium Salts .'.
/'
Effect of Non-Conjugated Charged Groupson ·Absorption Maxima of Rhodopsin andBacteriorhodopsin Models
20
22
27
28
29
Figure 1-6 Bond Lengths of Some Iminium Salts 40
Figure 2-1 Vinyl Region of the 19 NMR Spectraof N-methyl, N-phenyl-3-(p-methoxyphenyl)-2-propenylidene iminium perchlorate, 59,in TFA (Al E,E isomer (Bl Z,E isomer in athermally produced mixture, and (Cl E,Zand z,Z isomers in an irradiated mixture 60
Figure 2-2 13 C NMR spectra of N-methyl, N-phenyl3-(p-chlorophenyl)-2-propenylidene iminiumperchlorate, 56 (Al TFA solution, and(Bl solid state • 65
Figure 3-1
Figure 3-2
Figure 3-3
Figure 4-1
Absorption Spectrum of N-methyl, N-phenyl3-phenyl-2-propenylidene iminiumperchlorate, 57, in TFA
Cyclic Voltammogram of N-methyl,N-phenyl3-(p-chlorophenyl)-2-propenylidene iminiumperchlorate, 56 (Al background(Bl iminium salt
xiv
69
102
103
115
138
~ ..'
--.;.-
Figure 4-2
Figure 5-1
Figure 5-2
•
---
Cyclic Voltammogram of N-methyl,N-(p-nitrophenyl)-3-phenyl-2-propenylideneiminium perchlorate, 60 119
Hammett correlations for the rate constantsfor isomerization of iminium salts withsubstituents on the C3 aryl ring, 55-59,in TFA ( ) and 55, 57, 59 inH2S04/TFA(---) ° .... 138
Hammett correlations for the rate constantsof isomerization of iminium salts with •substituents on the N-aryl ring, 57, 60-63,in TFA( __O_) and H2S04/TFA(---lo
, ,
J
.- .
xv
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INTRODUCTION
CHAPTER .. 1
Vision, the process whereby light refl~cted from
objects around us is translated into images, is currently being
studied from both chemical and biochemical perspectives.
Organic molecules called rhodopsins act as the visible light
absorbing pigments, and are located in cells in the retina of
the eye.~72,3,4 The human eye contains cone cells and rod
cells. The cone cells are responsible for vision in bright
light, and can distinguish colours by absorbing lignt of one
of three parts of the visible spectrum: blue, green or red
light. The rod cells are responsible for vision in .low light
levels, and cannot distinguisn-colours. All species of
animal .that can se~ contain v~sual pigments that are similar
to the human pigments, however most animals have only one
type of visual pigment and do not see in colour. Since
animal retinas are easier to obtain than human ones, our
understanding of the visual process comes from studies of
animal rhodopsins, especially cattle rhodopsin.
When rhodopsin absorbs light, a reaction sequence is
initiated that causes changes to the membrane in which the
pigment is located, and eventually leads to a nerve impulse
that is sent to the brain. There are approximately thirty
million rhodopsin molecules in one rod cell in the eye. These
cells are very sensitive to light, but also adaptable, as they
1
-2
can detect as low as a few photons ·to as high as 10 6 photons
of light per second. ·Each photon absorbed triggers a series
of enzyme reactions that ends with hydrolysis of cyclic
guanosine monophosphate (GMP) in the cell. Amplification of
~~the original
sensitivity,
absorption event is responsible for the high
in that one photon absorbed causes up to 10 4
-molecules of cyclic GMP to be hydrolysed.
In the absence of light, an electrical current passes
through the cell, generated by a sodium ion flow. Light
absorption causes this current to decrease, possibly through
the action of a transmitter that links the enzymic reactions~
with the changes in the membrane that stop sodium ions from
passing through the cell wall. The change in electrical
current causes a nerve impulse to be sent to the brain, where
interpretation and creation of an image takes place.
Pigme~ts that are similar to rhodopsin are found in at
least-two other organisms. These light-absorb~ngmolecules
are used not for vision, but for converting light energy into
energy the organism can use for cell functions. Bacterio-
rhodopsin is one of four such proteins found in the surface
membranes of a strain of bacteria called Halobacteria. Light
absorption by t~e protein leads to the synthesis of adenosine
triphosphate (ATP), an energy storage system of living cells. 4 ,5..A green algae species, Chlamydomonas, moves in response to
light, also because of a light absorbing pigment. 3
3
I. Rhodopsin
The chromophore of rhodopsin, 1, is one geometric
isomer of retinal, ll-cis retinal, and is common to all
animals. It forms an iminium bond with a lysjne amino acid of
the apoprotein, opsin.
1 R R - opel"'-
"
Rhodopsin in human rod cells is red-coloured. The
protein backbone does not absorb light in the visible region,
so the long wavelength absorption band of rhodopsin, the one
that is responsible for its colour, is caused by light absorp-
tion of the chromophore only. The opsin chain surrounds the
chromophore, and exerts an influence on its absorption spec-
trum. Although each species of animal has the same chr~mo-
phore, each has a different opsin, and the absorption maxima
of the rhodopsins are also different, ranging from 440 to 600
nm. 6
Light absorption by rhodopsin initiates a reaction
cycle, Scheme 1-1. The intermediates were initially identified
by their absorption maxima and have since been characterized
--by other spectroscopic techniques. The first intermediate,
bathorhodopsin, absorbs light at longer wavelengths than doesr
rhodopsin. Photocalorimetry studies have found that batho-