β-phenyl quenching of triplet excited ketones: How critical is the geometry for deactivation?

Article abstract of DOI:10.1021/jo060200e

The phenomenon of β-phenyl quenching has been examined by laser-flash photolysis in a series of α- and/or β-substituted ketones 4-8 with similar excited-state characteristics. It is found that α-substitution markedly increases the triplet lifetimes in con

Full text of DOI:10.1021/jo060200e

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â-Phenyl Quenching of Triplet Excited Ketones: How Critical Is the
Geometry for Deactivation?
Subhas Samanta,† Brijesh Kumar Mishra,† Tamara C. S. Pace,‡
Narayanasami Sathyamurthy,† Cornelia Bohne,*,‡ and Jarugu Narasimha Moorthy*,†
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India, and Department of
Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6
moorthy@iitk.ac.in; bohne@uVic.ca
ReceiVed January 30, 2006
The phenomenon of â-phenyl quenching has been examined by laser-flash photolysis in a series of R-
and/or â-substituted ketones 4-8 with similar excited-state characteristics. It is found that R-substitution
markedly increases the triplet lifetimes in contrast to â-substitution. The force field calculations for the
various staggered conformers of ketones 4-6 and 8-syn show that the lowest-energy conformation in all
these ketones has the carbonyl group and the â-phenyl ring gauche to each other. Despite this geometrical
requirement, the longer lifetimes observed are interpreted as being due to the influence of the R-substituent
on the rotational freedom of the planar benzoyl moiety as a whole. The experimental results are suggestive
of the attainment of what appears to be a critical geometry for quenching. This scenario may be likened
to Norrish type II reactions, where the R-substituent has long been known to suppress the elimination
pathway and promote Yang cyclization. In addition, we have shown that the diastereomers of
R,â-disubstituted ketones exhibit distinct lifetimes.
Introduction
electron-deficient triplet excited carbonyl oxygen is responsible
for the deactivation.2,3 In support of this mechanism, the triplet
lifetimes of various ketones have been found to decrease with
a variation of increasingly electron-rich â-aryl rings.3 Indeed,
the involvement of charge transfer in intermolecular quenching
of triplet excited-state ketones by arenes is well established.4
In the case of intramolecular â-phenyl quenching in solution,
the conformation in which the â-aryl ring is gauche to the
carbonyl group has been presumed to be crucial for charge-
The rapid deactivation of the lowest and thermally populated
n,π* triplet excited states in aryl alkyl ketones containing an
aryl ring at the â-position, e.g., â-phenylpropiophenone, has
been known for well over three decades.1,2 Although the exact
mechanism of deactivation has not been fully understood, it is
firmly believed that charge transfer from the â-aryl ring to the
† Indian Institute of Technology.
‡ University of Victoria.
(1) (a) Whitten, D. G.; Punch, W. E. Mol. Photochem. 1970, 2, 77. (b)
Wagner, P. J.; Kelso, P. A.; Kemppainen, A. E.; Haug, A.; Graber, D. R.
Mol. Photochem. 1970, 2, 81. (c) Stermitz, F. R.; Nicodem, D. E.;
Muralidharan, V. P.; O’Connell, C. D. Mol. Photochem. 1970, 2, 87.
(2) (a) Scaiano, J. C.; Perkins, M. J.; Sheppard, J. W.; Platz, M. S.;
Barcus, R. L. J. Photochem. 1983, 21, 137. (b) Wismontski-Knittel, T.;
Kilp, T. J. Phys. Chem. 1984, 88, 110. (c) Netto-Ferreira, J. C.; Leigh, W.
J.; Scaiano, J. C. J. Am. Chem. Soc. 1985, 107, 2617.
(3) Leigh, W. J.; Banisch, J. H.; Workentin, M. S. J. Chem. Soc., Chem.
Commun. 1993, 988.
(4) (a) Guttenplan, J. B.; Cohen, S. G. J. Am. Chem. Soc. 1972, 94, 4040.
(b) Schuster, D. I.; Weil, T. M.; Halpern, A. M. J. Am. Chem. Soc. 1972,
94, 8248. (c) Wolf, M. W.; Brown, R. E.; Singer, L. A. J. Am. Chem. Soc.
1977, 99, 526. (d) Wagner, P. J.; Lam, M. H. J. Am. Chem. Soc. 1980,
102, 4166. (e) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J.
Am. Chem. Soc. 1986, 108, 7727.
10.1021/jo060200e CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/10/2006
J. Org. Chem. 2006, 71, 4453-4459
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Samanta et al.
CHART 1
transfer-mediated deactivation.3,5,6 Proximity effect has also been
shown to be important for â-phenyl quenching deactivation in
the solid state.7 Accordingly, the remarkably shorter lifetime
observed for â-phenylpropiophenone containing a methyl sub-
stituent at the â-position, i.e., â-methyl-â-phenyl-p-methoxy-
propiophenone (vide infra), has been rationalized on the basis
of conformational restrictions,5 which promote the existence of
a major population of molecules in the gauche conformation
from which the charge-transfer deactivation may ensue readily.
It is precisely this requirement of the gauche relationship
between the carbonyl group and the â-aryl ring that we exploited
a few years ago in rationally designing and demonstrating the
diastereomeric discrimination in a photophysical property, viz.,
triplet lifetimes, for the first time.6
In our recent work, we have shown that the substituents at
the R and â positions and their relative stereochemistry
determine the photochemical outcome of R,â-disubstituted
â-arylpropiophenones;8 whereas the diastereomers with anti
stereochemistry were found to undergo Yang cyclization
predominantly, the syn isomers were found to undergo Norrish
type II elimination. Furthermore, for the diastereomers with a
preferred gauche geometry in their lowest-energy conformations,
intriguingly longer lifetimes were observed as compared to the
lifetime for the prototypical p-methoxy-â-phenylpropiophenone.
This prompted us to investigate if the charge-transfer deactiva-
tion is critically dependent on the achievement of a particular
geometry for effective deactivation. Although the importance
of achievement of a specific geometry for deactivation has been
previously suggested,2c no systematic study on this mechanistic
aspect has been reported. In the present study, the lifetimes of
ketones 1-3, documented already in the literature,5 are com-
pared with the lifetimes determined by laser-flash photolysis
for ketones 4-8, which were rationally designed to investigate
the effect of R- and â-substituents on the intramolecular
quenching mechanism (Chart 1).
Our choice of the p-methoxy derivatives was guided by the
fact that in unsubstituted ketones such as â-phenylpropiophenone
with a pure n,π* lowest excited triplet state the deactivation is
so rapid (τ ca. 1 ns) that the decay kinetics cannot be followed
with nanosecond laser-flash photolysis.2 The methoxy substitu-
tion in the benzoyl ring leads to the so-called state switching,
which causes the lowest excited state to be π,π* in character.9
Thus, the charge-transfer quenching can occur only from the
thermally populated proximate n,π* excited state leading to
longer triplet lifetimes measurable by nanosecond transient
absorption spectroscopy (cf. Discussion).2c,3,5 A further conse-
quence of state switching is that the p-methoxy derivatives are
fairly unreactive in their triplet states. Herein, we present our
results of laser-flash photolysis (triplet lifetimes) of a set of
ketones with similar excited-state characteristics, which strongly
point to a dramatic influence of the substituent at the R-position
and achievement of a critical geometry for the long-known
â-phenyl quenching phenomenon in triplet excited states.
Results
Synthesis and Characterization of Ketones. The ketones
4 and 8 were synthesized by quenching the lithium enolate of
p-methoxypropiophenone with benzyl bromide and R-phenyl-
ethyl bromide. A similar procedure was employed for the
preparation of ketone 7, but by employing â-phenyl-p-meth-
oxypropiophenone in place of p-methoxypropiophenone. Ke-
tones 5 and 6 were synthesized via 1,4-addition of phenylmag-
nesium bromide (PhMgBr) to the precursor chalcones (Scheme
1). The stereochemistry of the diastereomers of ketone 8 was
assigned by comparing the diagnostic 1H and 13C NMR signals
of the R-methyl protons with those of the closely related systems
reported in the literature.8b Accordingly, the 1H NMR signal of
(5) Boch, R.; Bohne, C.; Netto-Ferreira, J. C.; Scaiano, J. C. Can. J.
Chem. 1991, 69, 2053.
(6) (a) Moorthy, J. N.; Patterson, W. S.; Bohne, C. J. Am. Chem. Soc.
1997, 119, 11904. (b) Moorthy, J. N.; Monahan, S. L.; Sunoj, R. B.;
Chandrasekhar, J.; Bohne, C. J. Am. Chem. Soc. 1999, 121, 3093.
(7) (a) Boch, R.; Bohne, C.; Scaiano, J. C. J. Org. Chem. 1996, 61, 1423.
(b) Ng, D.; Yang, Z.; Garcia-Garibay, M. A. Tetrahedron Lett. 2001, 42,
9113.
(8) (a) Moorthy, J. N.; Mal, P. Tetrahedron Lett. 2003, 44, 2493. (b)
Singhal, N.; Koner, A. L.; Mal, P.; Venugopalan, P.; Nau, W. M.; Moorthy,
J. N. J. Am. Chem. Soc. 2005, 127, 14375.
(9) (a) Turro, N. J. Modern Molecular Photochemistry; Benjamin/
Cummings Publishing Co.: Menlo Park, NJ, 1978; p 628. (b) Gilbert, A.;
Baggott, J. Essentials of Molecular Photochemistry; Blackwell Scientific
Publications: Oxford, 1991; p 538.
4454 J. Org. Chem., Vol. 71, No. 12, 2006

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â-Phenyl Quenching of Triplet Excited Ketones
SCHEME 1. Synthesis of Ketones 4-8
FIGURE 1. Transient absorption spectrum for ketone 4 in acetonitrile
collected after a delay of 48 ns.
R-methyl protons of the anti diastereomer is found to appear at
ca. 0.2 ppm downfield relative to that of the syn diastereomer.
A similar trend has been noted in the 13C NMR signals as well,
where the signal due to an R-methyl carbon of the anti isomer
has been found to appear at ca. δ 17.0, and that of the syn occurs
at ca. δ 13.0.
Laser-Flash Photolysis and Triplet Lifetimes. Laser-flash
photolysis (Excimer Laser, 308 nm)10 of ketones 4-8 led to
the detection of strong transient absorptions with their maxima
centered around 390 nm. The transients responsible for the
absorptions were attributed to triplet states of ketones by
comparison of their spectral features to those already docu-
mented in the literature for ketones 1-3.2,5 The typical T-T
(triplet-triplet) absorption spectrum recorded for ketone 4 is
shown in Figure 1. Decay profiles observed for the syn and
anti diastereomers of ketone 8 are shown in Figure 2. All decays
followed a monoexponential function. The lifetimes determined
in two solvents, viz., CH3OH and CH3CN, are given in Table
1. The triplet lifetimes are uniformly found to be longer for all
ketones in methanol as the solvent than for those in acetonitrile.
This solvent effect has been previously observed for ketones
1-3.5 The enhancement of lifetimes for phenyl ketones in a
polar protic solvent has previously been attributed to solvation
of the carbonyl group via hydrogen bonding, which presumably
hinders the deactivation by intramolecular quenching2c and
stabilization of π,π* triplet excited states.11
The triplet lifetimes for ketones 1-3 have been reported
already in the literature.5 Although the parent triplet excited
â-phenyl-p-methoxypropiophenone has a lifetime of ca. 240 ns,
the substitution of a methyl group in the â-position and in the
p-position of the â-phenyl ring as in 2 and 3 has been previously
shown to lower the lifetime to ca. 30-90 ns (entries 2 and 3,
Table 1). Indeed, a similar lifetime is observed for â,â-diphenyl-
p-methoxypropiophenone 5 (entry 5). In marked contrast, methyl
substitution at the R-position as in 4 and 6 increases the triplet
lifetime to 1270 and 680 ns, respectively (entries 4 and 6). The
triplet in the case of R-benzyl-â-phenyl-p-methoxypropiophe-
none 7 is remarkably long-lived, despite the fact that two
â-phenyl groups are present, and from a statistical point of view,
FIGURE 2. Decay of the triplet excited state of 8-anti (1) and 8-syn
(2) in methanol monitored at 400 nm.
TABLE 1. Triplet Lifetimes (ns) of Ketones 1-8 at Room
Temperature
entry
ketone
methanol
acetonitrile
1
2
3
4
5
6
7
8
9
1a
239
95
31
1270 ( 60
90 ( 10
680 ( 30
1190 ( 90
510 ( 40
1060 ( 80
50
19
9
310 ( 10
<20
280 ( 20
340 ( 20
140 ( 20
230 ( 10
2a
3a
4
5
6
7
8-syn
8-anti
a From ref 5.
the quenching efficiency could be expected to double (entry
7). The diastereomers of ketone 8 exhibit a difference in their
triplet lifetimes; whereas the triplet lifetime for the anti
diastereomer is ca. 1060 ns, that for the syn diastereomer is
found to be significantly shorter, 510 ns in methanol.
Relative Energies of Staggered Conformations of Ketones
1, 2, 4-6, and 8. United force field calculations (UFF) were
carried out for ketones 1, 2, 4-6, and 8 with the Gaussian 03
suite of programs.12 To compute the energies of the different
staggered conformations, the following strategy was adopted.
Initially, for ketones 2 and 4, the dihedral angle about the CR-
Câ bond was kept constant at a certain angle and the rest of
the geometry was optimized. Subsequently, the dihedral angle
was varied incrementally by 10° and the energy was computed
for each such geometry. From the resultant dihedral angle energy
profiles for 2 and 4, the staggered conformations were identified.
In the case of ketones 1, 5, 6, and 8, the low-energy staggered
conformations were optimized fully. Figure 3 shows the
consolidated energies of all staggered conformations for ketones
(10) Liao, Y.; Bohne, C. J. Phys. Chem. 1996, 100, 734.
(11) Wagner, P. J.; Kemppainen, A. E.; Schott, H. N. J. Am. Chem. Soc.
1973, 95, 5604.
J. Org. Chem, Vol. 71, No. 12, 2006 4455

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Samanta et al.
which the methyl and benzoyl groups overlap corresponds to
the highest energy.
Discussion
It is well established that substitution of an electron-donating
group in the benzene ring of an aryl alkyl ketone at an ortho or
para position results in the so-called state switching.9 For
example, the character of the lowest-energy triplet excited state
of unsubstituted â-phenylpropiophenone changes from n,π* to
π,π* upon methoxy substitution as in â-phenyl-p-methoxypro-
piophenone. Thus, the lowest excited triplet state for all the
ketones 1-8 is π,π* (T1) with the n,π* state (T2) being
proximately placed; the latter is populated via thermal equilibra-
tion with the former state. Norrish type I cleavage and â-phenyl
quenching are reactions that occur from triplet states with n,π*
character, which in the case of ketones 1-8 is the T2 state.
Assuming, as in the previous studies, that the thermal equilibra-
tion between the T1 and T2 states2c,6,11,13 is faster compared to
the deactivation processes, the triplet decay rate constant (kT),
which is the inverse of the triplet lifetime, is given by:
kT ) kint + K(kâ + kcleavage
)
(1)
where kint is the intrinsic decay rate constant for the π,π* triplet,
K is the equilibrium constant between T1 and T2, kâ is the rate
constant for â-phenyl quenching, and kcleavage is the rate constant
for the Norrish type I reaction. The value of K may be assumed
to be the same for ketones 1-8 because the chromophore, i.e.,
p-methoxypropiophenone, is the same for all these compounds.
The model compound that may serve for kint is p-methoxypro-
piophenone,6b which has a lifetime of 5.6 µs in acetonitrile and
33 µs in methanol. These lifetimes are more than an order of
magnitude larger than the lifetimes for 1-8, and for this reason,
the intrinsic decay of 1-8 from the T1 state needs not be taken
into account when analyzing the differences in the lifetimes
observed. The low photoreaction quantum yield observed for 1
and â-phenylpropiophenone indicates that the Norrish type I
reactivity cannot compete with â-phenyl quenching. In 4-8,
the only transient observed by laser-flash photolysis is the triplet
state, which shows further that the Norrish type I cleavage does
not occur for these ketones. Therefore, the differences in the
lifetimes for ketones 1-8 must be related to differences in the
efficiencies for â-phenyl quenching.
The â-phenyl quenching involves charge transfer, and the
quenching efficiency can be modulated by changing the donor
ability of the â-phenyl ring3,5 (cf. lifetimes of 1 and 3 in Table
FIGURE 3. UFF-calculated relative energies (kcal/mol) for all
staggered conformers of 1, 2, 4-6, and 8.
1, 2, 4-6, and 8. The objective of these calculations was to
obtain relative energies for the different staggered conforma-
tions. The relative stability of different conformers for the
different ketones, in principle, can be computed ab initio.
However, this was not done in present examples because of
the structural complexity of the ketones and the number of
ketones involved. In addition, past experience shows that force
field calculations do give relative stabilities, albeit qualitatively.5,6b
The full-energy dihedral angle profiles for ketones 2 and 4 show
that the relative order of the energies of the eclipsed conforma-
tions follows that of the energies of the staggered conformations.
For example for compound 2, the conformer with a gauche
interaction between the methyl and benzoyl groups has a higher
energy than the conformer with a gauche interaction between
the phenyl and benzoyl groups. The same effect is observed
for the eclipsed conformations such that the conformation for
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Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
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Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
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G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
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4456 J. Org. Chem., Vol. 71, No. 12, 2006