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ABSTRACT
A STUDY OF CHARGE TRANSFER CHARACTER IN THE
PHOTOREDUCTION OF VARIOUS Q-TRIFLUOROACETOPHENONES
BY
Henry Man Hung Lam
The photoreduction of a—trifluoroacetophenone by tolu—
ene in acetonitrile has been examined. Since the ability
of alkylbenzenes to quench triplet a-trifluoroacetophenone
is linearly dependent on their ionization potentials, the
presence of charge transfer process in the triplet quenching
is affirmed. The slope from a plot of log quenching rate
constant versus ionization potential indicates only ~'21%
electron transfer in the triplet charge transfer complex.
The photoreduction of several substituted g-trifluoro-
acetOphenones having either nv* and WW* lowest triplet has
been studied. The WF* lowest triplet ketones have lower
reactivity than the nn* low-lying triplet ketones. The
plot of log quenching rate constant versus the sum of
triplet energy and reduction potential does not show a good
linear relationship and thus obscures the interpretation of
the data. Perhaps the different nature of the two lowest
excited states may explain the difference in reactivities.
However, the WW* lowest triplet ketones may be quenched by
Henry Man Hung Lam
an hydrogen abstraction process rather than a charge trans-
fer process as observed in the quenching of nv* lowest
triplet ketones.
A STUDY OF CHARGE TRANSFER CHARACTER IN THE
PHOTOREDUCTION OF VARIOUS a-TRIFLUOROACETOPHENONES
BY
Henry Man Hung Lam
A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Chemistry
1975
ACKNOWLEDGMENT
I wish to thank Professor Peter J. wagner, who has
provided me with the opportunity to complete this work.
His guidance, and inspiration are sincerely appreciated.
I am particularly thankful to my fellow graduate stu-
dents for their helpful assistance.
I would also like to acknowledge financial assistance
from the Department of Chemistry throughout the course of
my study.
ii
TABLE OF CONTENTS
INTRODUCTION . . . . . . . . . . . .
General Mechanistic Process . .
The Nature of the Excited States
The Correlations between the Reactivities of
Hydrogen Abstraction and the Nature of the
Excited State . . . . . . . .
Charge Transfer Mechanism . . .
Previous Study of the Photoreduction of
TrifluoroacetOphenone . . . .
Objectives . . . . . . . . . .
Kinetics Expressions . . . . .
RESULTS 0 O O O O O O O O O O O O 0
Quantum Yield Determinations .
Triplet Quenching Studies . . .
Intersystem Crossing . . . . .
Absorbance and Emission Spectra
DISCUSSION . . . . . . . . . . . . .
EXPERIMEM‘AL O O O O O O O O Q O O 0
PART I: PROCEDURE . . . . . .
A. Preparation and Purification
1. Ketones . . . . . .
2. Solvent . . . . . .
3. Hydrogen Donors . .
4. Quenchers . . . . .
5. Internal Standard .
iii
10
11
12
15
15
15
16
17
29
39
39
39
39
41
41
42
42
TABLE OF CONTENTS
1. Irradiation Procedure
-- Continued.
Photolysis
2. Analysis of Sample .
C.
D.
PART II:
A.
B.
C.
LIST OF REFERENCES
KINETIC DATA
Quenching Studies
Reduction Potential
iv
Absorption and Emission Spectra
Quantum Yield of Bibenzyl Formation
Determination of Intersystem Crossing
Quantum Yield
Page
43
43
43
46
46
47
47
49
56
57
LIST OF TABLES
Table Page
1. Least squares SIOpe and intercept values from
¢ 1 versus [BH] 1 plot for the photoreduction
0 indicated ketones with toluene . . . . . . 17
2. Triplet quenching parameters for the reaction
of indicated ketone with 1M toluene by using
the quencher, naphthalene in acetonitrile . . 18
3. Triplet quenching parameters for trifluoroaceto-
phenone . . . . . . . . . . . . . . . . . . . 18
4. Kinetics data for photoreduction of 0.05M
indicated ketones with toluene in acetonitrile 19
5. Ultra violet absorption bands of indicated
ketones . O I O O O O O O O O O O O O O O C O 19
6. Phosphorescence data of indicated ketones . . 20
7. Substituent effects on triplet ketones . . . 37
8. Quantum yield of bibenzyl formation for 0.05M
ketone in acetonitrile with toluene, indi-
cated Cn and conditions . . . . . . . . . . . 47
Part A: a-Trifluoroacetophenone . . . . . . 47
Part B: pfiMethoxy-a-trifluoroacetophenone . 47
Part C: pfiMethyl-a-trifluoroacetophenone . . 48
Part D: mfchloro-a-trifluoroacetophenone . . 48
Part E: m-Trifluoromethyl-a-trifluoroaceto-
phenone . . . . . . . . . . . . . . 48
Part F: EflTrifluoromethyl-a—trifluoroaceto-
phenone . . . . . . . . . . . . . . 48
9. System with 0.05M a-trifluoroacetophenone and
0.1M toluene in acetonitrile quenching with
specific quencher in indicated Cn and
conditions . . . . . . . . . . . . . . . . . 49
Part A: Anisole . . . . . . . . . . . . . . 49
Part B: Benzene . . . . . . . . . . . . . . 49
Part C: Benzotrifluoride . . . . . . . . . . 50
Part D: Benzonitrile . . . . . . . . . . . . 50
Part E: Chlorobenzene . . . . . . . . . . . 50
V
LIST OF TABLES -- Continued.
Table
10.
11.
12.
13.
14.
15.
15.
Part F: Fluorobenzene . . . . . . . . . . .
Part G: EfDimethoxybenzene . . . . . . . . .
Part H: lgfinichlorobenzene . . . . . . . . .
System with 0.05M a-trifluoroacetOphenone,
0.001M C20 and indicated amount of toluene in
acetonitrile quenching with naphthalene . . .
Part A: 1M Toluene . . . . . . . . . . . . .
Part B: 0.4M Toluene . . . . . . . . . . . .
System with 0.05M pfmethoxy-a-trifluoroaceto-
phenone, 0.001M C20 and indicated amount of
toluene in acetonitrile, quenching with
naphthalene . . . . . . . . . . . . . . . . .
Part A: 1M Toluene . . . . . . . . . . . . .
Part B: 2M Toluene . . . . . . . . . . . . .
0.05M merifluoromethyl-a-trifluoroaceto—
phenone-0.1M toluene system in acetonitrile
quenching with indicated concentration of
naphthalene . . . . . . . . . . . . . . . . .
0.05M archloro-a-trifluoroacetophenone-1M
toluene system in acetonitrile quenching with
indicated concentration of naphthalene . . .
0.05M pfiMethyl-a-trifluoroacetophenone-lM
toluene system in acetonitrile quenching with
indicated concentration of naphthalene . . .
0.05M prrifluoromethyl-a-trifluoroaceto-
phenone—0.1M toluene system in acetonitrile
quenching with indicated concentration of
naphthalene . . . . . . . . . . . . . . . . .
Triplet sensitized cis-trans isomerization
of piperylene . . . . . . . . . . . . . . . .
vi
53
53
54
54
55
55
56
Figure
1.
2.
10.
11.
12.
Jablon'ski's
LIST OF FIGURES
diagram . . . . . . . . . . . .
Reciprocal quantum yield plot for indicated
ketones with toluene . . . . . . . . . . . .
Reciprocal quantum yield plot for indicated
ketones with
Stern-Volmer
toluene . . . . . . . . . . . .
plots for naphthalene quenching
of photoreduction of indicated ketones by
1M toluene .
Stern-Volmer plots for naphthalene quenching
of photoreduction of indicated ketones by
indicated concentration of toluene . . . . .
Stern-Volmer plots for naphthalene quenching
of photoreduction of indicated ketones by
0.1M toluene
Stern-Volmer
reduction of
plots for the quenching of photo-
a-trifluoroacetophenone with 0.1M
toluene by indicated quenchers . . . . . . .
Stern-Volmer
reduction of
0.1M toluene
Stern-Volmer
reduction of
0.1M toluene
plots for the quenching of photo-
a-trifluoroacetophenone with
by indicated quenchers . . . . .
plots for the quenching of photo-
a-trifluoroacetophenone with
by anisole . . . . . . . . . . .
Phosphorescence of pftrifluoromethyl-a-tri-
fluoroacetOphenone at 770K . . . . . . . . .
PhosPhorescence of mftrifluoromethyl—a-tri-
fluoroacetophenone at 770K . . . . . . . . .
Phosphorescence of m-chloro-a-trifluoro-
acetophenone at7701‘c'............
vii
Page
4
21
22
22
23
23
24
25
25
26
26
27
LIST OF FIGURES--Continued.
Figure
13.
14.
15.
16.
17.
18.
Phgsphorescence of a-trifluoroacetOphenone at
77 K O O O O O O O O O O O O O O O O O O O O
Phosphorescence of pfmethyl-a-trifluoroaceto-
phenone at 77°K . . . . . . . . . . . . . . .
Phosphorescence of pfmethoxy-a-trifluoro-
acetophenone at 770K . . . . . . . . . . . .
Correlation of rate constant for quenching of
triplet a-trifluoroacetophenone by aromatics
with their ionization potentials in acetOP
nitrile . . . . . . . . . . . . . . . . . . .
Log kr for quenching of acceptor triplets by
toluene gs triplet energy minus reduction
mtential O O O O O O O O O O O O O O O O O 0
Fluorescence quenching rate constants k as a
function of the free enthalpy AG . . . . . .
viii
Page
27
28
28
30
33
35
INTRODUCTION
Photoreduction of ketones has become a common topic
for investigation since the original discovery by Ciamician
and Sil‘ber1 that the action of sunlight on benzophenone in
2-propanol yields benzopinacol and acetone. Studies of the
mechanism of photochemical pinacolization were intensely
0 on ou I u
2C5H5CC5H5 + CH30HCH3 ——-> (c611,),c -C(C5H5)2 + casccn3 (1)0 I
0H 0H
conducted since 1900. Most of the work done in the 1950's
emphasized synthetic application because the chemical yields
in some of these photochemical reactions were good and the
products were more readily prepared than by non-photochemi-
cal methods. The advancement of technology during the past
few years enabled more thorough investigation on the mechan-
ism of photoreduction.
The photoreduction of ketones, either intra— or inter-
molecular, involving hydrogen abstraction could be divided
into radical—like and charge-transfer processes. Radical—
like intramolecular photoreduction,usua11y referred to as
the Norrish type II reactions, involves the transfer of y
hydrogen to the carbonyl oxygen in the excited state and the
subsequent reaction of the 1,4-biradica1.2
1
H R R
. ,_., + ( 2 )
I] ‘—+ OH’ R
Intermolecular photoreduction represents the abstrac-
tion of hydrogen from a substrate, which can be an alcohol,3
hydrogen bromide,4 toluene,5 alkanes,6 and amines,7 by the
electronically excited carbonyl compound.
:c=o* +I-IB -—-—> -c-0H +B° (3)
The resulting radicals may proceed to form different pro-
ducts depending on the reaction conditions.8
2 -(?.-OH > -c—c- (4)I I
0H 0H
(A)
—C:!-OH + B' > -C-B (5)I
OH
(B)
B- + B- -—--> B-B (6)
(c)
-¢-0H 4- BH >- -C-OH +B' (7)
13
(D)
The reaction of benzophenone with toluenéh9gives products
A, B, and C which are shown in equations 4, 5, and 6, but
the photoreduction of acetone by tributyl stannane10 gives
only product D. Although aliphatic ketones usually undergo
3
radical-like photoreduction with alcohols, many aromatic
ketones as fluorenone, aminobenzophenone, a and B-naphthyl
carbonyl compounds are not efficiently photoreduced by
alcohols. Recent studies reveal that these aromatic ketones
could be photoreduced efficiently in a charge-transfer
mechanism by amines which contain a-hydrogens.11
;c==o* + RCH2NR5
l
[-c;-0' RCHz'fiRh (8)
1
-¢—0H + RéHNR;
General Mechanistic Process
Before any further discussion on photoreduction of
ketones, a brief descriptionof the fate of the electronic
excitation is necessary to achieve a better understanding
of the mechanism.
Figure 1 is a modern elaboration of Jablonski's diagram,
representing the energies of the various low-lying states of
a typical molecule with respect to the ground state So. In
solution, when an organic molecule absorbs light, it is
elicited to an excited singlet state, and then decays rapidly
to the lowest singlet state («:10" sec). The lowest singlet
state either deactivates to ground state by fluorescence
and radiationless decay or populates triplet states by inter-
system crossing. Since the rate of intersystem crossing