The excited state behavior of aminostilbenes. A new example of the meta effect

Full text of DOI:10.1021/ja964465u

3834
J. Am. Chem. Soc. 1997, 119, 3834-3835
The Excited State Behavior of Aminostilbenes.
A New Example of the Meta Effect
Frederick D. Lewis* and Jye-Shane Yang
Department of Chemistry, Northwestern UniVersity
EVanston, Illinois 60208
ReceiVed December 31, 1996
The enhanced photochemical reactivity of meta- vs para-
substituted benzenoid compounds was first described by Hav-
inga.1 The “meta effect” was rationalized by Zimmerman2
resulting from selective transmission of electron donation or
withdrawal on the basis of simple Hu¨ckel theory. We report
here a new example of the meta effect: namely, the marked
difference in excited state behavior of the trans isomers of m-
vs p-aminostilbenes 1-3 (Chart 1).3 In comparison to the para
derivatives, the singlet states of the meta derivatives have greater
charge transfer character, significantly longer lifetimes, and
higher fluorescence quantum yields. Moreover, the meta but
not the para isomers are subject to quenching by methanol, and
in one case, regioselective addition of methanol to a singlet
stilbene is observed.
Figure 1. The absorption spectra of aminostilbenes 1p (-‚‚-), 1m
(-x-), 2p (‚‚‚), 2m (- - -), 3p (-‚-), and 3m (s) in cyclohexane solution.
Chart 1
The spectroscopy and photochemistry of donor-acceptor-
substituted stilbenes such as 4-(dimethylamino)-4′-cyanostilbene
(DCS) have been extensively investigated.4-6 The pronounced
solvent dependence of the fluorescence behavior of such
molecules has been attributed to rapid relaxation of the initially
formed Franck-Condon excited state to a charge-transfer (CT)
state. Trans f cis isomerization also originates from the CT
state via twisting about the central double bond to form the
perpendicular excited species which decays to a mixture of cis
and trans isomers.6,7 The singlet state behavior of 4-amino-4′-
(methoxycarbonyl)stilbene (1p) is similar to that of DCS. The
absorption spectrum of 1p (Figure 1) is red-shifted with respect
to that of stilbene (λmax ) 295 nm). Increasing solvent polarity
results in a small red shift in the absorption maximum but a
substantial red shift in the fluorescence maximum (Table 1) and
a loss of the vibrational structure observed in nonpolar solvents.8
A dipole moment of 14.4 D can be estimated for the CT state
of 1p using the Lippert-Mataga equation9 and a solvent cavity
radius of 5 Å. As for DCS, the singlet lifetime of the CT state
of 1p is short in nonpolar solvents and increases modestly in
more polar solvents, as do the fluorescence and photoisomer-
ization quantum yields (Table 1).
Table 1. Fluorescence Emission Data and Quantum Yields for
Trans f Cis Photoisomerization (ΦE,Z) for Aminostilbenes 1-3 in
Nonpolar and Polar Solvents
sol-
vent 1m
1p
2m
2p
390
446
12.1
0.023 0.78
0.027 0.43
3m
3p
392
451
11.8
0.084
0.051
λmax,flu
(nm) MeCN 521 501
µe (D)b
14.9 14.4
cHex 407 412
394
467
393
459
13.2
12.4
c
ΦF
cHex 0.61
MeCN 0.12
cHex 6.15
0.024 0.76
0.062 0.29
τF (ns)
(preexp
%)
0.22
0.32
0.41
5.51 (31) 0.22
8.58 (69)
7.33 (17) 0.18
11.7 (83)
1.01 (99) 0.27
5.89 (1)
4.09 (46) 0.20
5.49 (54)
9.86 (21) 0.23
12.5 (79)
8.16 (57) 0.28
11.3 (43)
0.055
0.12
MeCN 6.20
MeOH a
d
The behavior of the meta isomer 1m differs from that of 1p
in several respects: (a) The lowest energy absorption maximum
of 1m is at shorter wavelength and has a lower oscillator strength
ΦE,Z
PhH 0.036 0.19
MeCN 0.036 0.23
0.078
0.19
0.31
0.38
0.17
a Fluorescence almost completely quenched. b Calculated according
to νflu ) -2µe2/hca3{(ꢀ - 1)/(2ꢀ + 1) - (n2 - 1)/(4n2 + 2)} + constant
with the assumption of solvent cavity of a ) 5 Å. c Phenanthrene (ΦF
) 0.14) as the actinometer for all compounds excited at 294 nm.21
d Light source is 313 nm.
(1) Havinga, E.; De Jong, R. O.; Dorst, W. Recl. TraV. Chim. Pays-Bas
1956, 75, 378.
(2) (a) Zimmerman, H. E.; Sandel, V. R. J. Am. Chem. Soc. 1963, 85,
915. (b) Zimmerman, H. E. J. Am. Chem. Soc. 1995, 117, 8988.
(3) (a) The amino-substituted stilbenes 1-3 were synthesized by reduc-
tion of the corresponding nitrostilbenes,3b which were made by Wittig
reactions using the procedures of Lee and Marvel.3c (b) Bellamy, F. D.;
Ou, K. Tetrahedron Lett. 1984, 839. (c) Lee, B. H.; Marvel, C. S. J. Polym.
Sci. Chem. Ed. 1982, 20, 393.
(Figure 1).10 (b) The fluorescence maxima of 1m are compa-
rable to those of 1p in nonpolar solvents and display larger
solvent-induced shifts. Thus, the calculated excited state dipole
moment of 1m is larger than that of 1p (Table 1). No
vibrational structure is observed for the fluorescence of 1m even
in cyclohexane solution or low-temperature glasses. (c) Both
the singlet lifetime and fluorescence quantum yield for 1m in
cyclohexane solution are 30-fold larger than the values for 1p.
The fluorescence lifetime of 1m is somewhat longer in benzene
or diethyl ether than in cyclohexane or acetonitrile solution. The
(4) (a) Gruen, H.; Go¨rner, H. Z. Naturforsch. 1983, 38a, 928. (b) Ephardt,
H.; Fromherz, P. J. Phys. Chem. 1989, 93, 7717. (c) Gruen, H.; Go¨rner, H.
J. Phys. Chem. 1989, 93, 7144. (d) Sun, L.; Go¨rner, H. J. Phys. Chem.
1993, 97, 11186.
(5) (a) Gilabert, E.; Lapouyade, R.; Rullie`re, C. Chem. Phys. Lett. 1988,
145, 262. (b) Lapouyade, R.; Czeschka, K.; Majenz, W.; Rettig, W.; Gilabert,
E.; Rullie`re, C. J. Phys. Chem. 1992, 96, 9643. (c) Le´tard, J.-F.; Lapouyade,
R.; Rettig, W. J. Am. Chem. Soc. 1993, 115, 2441.
(6) Il’ichev, Y. V.; Ku¨hnle, W.; Zachariasse, K. A. Chem. Phys. 1996,
211, 441.
(7) Lapouyade and Rettig5 have proposed the involvement of a relaxed
locally excited state in addition to the CT and P* (“three state” model).
The “two state” model of Go¨rner4 and Zachariasse6 is sufficient to explain
our results.
(10) (a) Semiempirical INDO/S-SCF-CI (ZINDO) calculations10b for 2m
and 2p indicate that both isomers have lowest energy HOMO f LUMO,
π f π* transitions with similar energies (334 and 340 nm for 2m and 2p,
respectively). However, this transition is calculated to have more extensive
configuration interaction and a lower oscillator strength for 2m vs those of
2p. (b) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1970, 53, 21.
(8) Fluorescence spectra are provided as Supporting Information.
(9) (a) Lippert, V. E. Z. Naturforsch. 1955, 10a, 541. (b) Mataga, N.;
Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1955, 28, 690.
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Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3835
fluorescence quantum yield decreases with increasing solvent
polarity, and no fluorescence is detected in methanol solution.
(d) The photoisomerization quantum yield for 1m is much
smaller than that for 1p in both benzene and acetonitrile solution.
Similar differences in photochemical behavior are observed
for the meta vs para isomers of m-(aminostyryl)pyridine (2) and
2-(aminostyryl)naphthalene (3). The absorption maxima of 2m
and 3m are at shorter wavelength than those of their para
isomers (Figure 1); however, their fluorescence maxima are at
longer wavelength and display larger solvent-induced shifts in
polar solvents. Compared to their para isomers, 2m and 3m
have significantly longer decay times, larger fluorescence
quantum yields, and smaller photoisomerization quantum yields.
The dual exponential decay observed for 2m and 3m is
attributed to the decay of two conformers which may differ in
either aryl vinyl or anilino vinyl conformation. The absence
of excitation wavelength dependence of the emission spectra
indicates that the two conformer have similar spectra. Emission
from a single conformer or two conformers with similar decay
times could account for the observation of single exponential
decay from 1m. Dual exponential fluorescence decay might
also be expected for 2p and 3p; however, a decay component
comparable to or shorter than those reported in Table 1 would
not be resolved by our lifetime apparatus. The fluorescence
decay times of 2m and 3m are shorter in methanol than in
acetonitrile, but their fluorescence is not completely quenched
as it is for 1m. Upon irradiation in methanol (or methanol-d4)
solution, 3m is regioselectively converted to the solvent adduct
4 in >80% yield (eq 1).11 The assignment of structure 4 is
based upon mass spectral and 1H NMR data. Methanol adduct
formation is not observed for 2m, 3m, or any of the para
isomers.
formation of the adduct 6 (eq 2). Their result is consistent with
5
(2)
6
nucleophilic addition of methanol to a polarized or zwitterionic
singlet state in which the positive charge is stabilized by the
p-methoxy substituent. The formation of 4 from 3m can be
rationalized by attack of methanol on a CT state in which the
positive charge is localized on the aniline ring and the negative
charge on the vinyl naphthalene (eq 1). Methanol addition to
styrene anion radicals is known to occur via protonation of the
â carbon followed by nucleophilic capture of the resulting
benzylic cation.14 The efficient quenching of 1m and 2m in
methanol and the absence of adduct formation may result from
protonation of the ester carbonyl or pyridyl lone pair, both of
which should be highly basic in the CT excited state.15
A model which might explain the pronounced difference in
behavior between the meta and para isomers of 1-3 is that the
CT states of the meta isomers are twisted and those of the para
isomers are planar.16 Twisting about the anilino-styrene bond
would result in localization of the positive charge on aniline
and the negative charge on styrene, in accord with the observed
regiochemistry of methanol addition to 3m. More extensive
twisting for the meta vs para isomers could account for the larger
singet state dipole moments meta isomers and the observation
methanol quenching of the meta but not the para isomers. It
could also result in a larger barrier for twisting about the central
bond which in turn could account for the significantly longer
singlet lifetimes, higher fluorescence quantum yields, and lower
isomerization quantum yields for the meta isomers. Further
studies designed to test this model are in progress.
The p-dimethylamino substituent has been widely used as
the electron-donor in donor-acceptor-substituted π-conjugated
systems which are of interest due to their applications as
nonlinear optical materials,17 laser dyes,18 fluorescence probes,19
and fluorescence brighteners.20 Our results suggest that long
fluorescence lifetimes and high fluorescence quantum yields may
be a general characteristic of m-amino-substituted trans-1,2-
diarylolefins. Preliminary results indicate that a m-amino effect
is also observed for cis-1,2-diarylolefins. As such, the m-amino
effect may prove of value in the design of new fluorescent
materials.
(1)
The behavior of the para isomers of 1-3 is similar to that of
DCS. The smaller dipole moment of 1p (and of 2p or 3p) vs
DCS (21 D6) is consistent with its weaker donor (NH2 vs NMe2)
and acceptor (CO2Me vs CN) substituents. The lifetimes of
all three para isomers are short (<0.4 ns), their fluorescence
quantum yields are small (<0.08), and their isomerization
quantum yields are moderate to large (>0.17) both in nonpolar
and polar solvents. The parent stilbene and styrylpyridine also
have short lifetimes, whereas the parent 2-styrylnaphthalene has
a longer lifetime (5.2 and 27.6 ns for the two conformers12).
Thus, the behavior of 3p resembles that of the donor-acceptor
stilbenes rather than that of its parent.
Acknowledgment. Financial support for this research has been
provided by the National Science Foundation.
Supporting Information Available: An expanded version of Table
1 with data for additional solvents, the fluorescence spectra of 3m and
3p showing the typical solvent-dependent fluorescence behavior of both
m- and p-aminostilbenes, and the fluorescence spectra of 2m in
cyclohexane showing the absence of excitation wavelength-independent
fluorescence behavior (3 pages). See any current masthead page for
ordering and Internet access instructions.
JA964465U
A clue to the possible origin of the unusual behavior of the
meta isomers of 1-3 is provided by the regioselectivity of
methanol addition to 3m (eq 1). Woning et al.13 observed that
irradiation of p-methoxystilbene 5 in methanol results in
(14) (a) Hixson, S. S. J. Am. Chem. Soc. 1972, 94, 2505. (b) Maroulis,
A. J.; Arnold, D. R. Synthesis 1979, 819. (c) Mizuno, K.; Nakanishi, I.;
Ichinose, N.; Otsuji, Y. Chem. Lett. 1989, 1095. (d) Kojima, M.; Ishida,
A.; Takamuku, S.; Wada, Y.; Yanagida, S. Chem. Lett. 1994, 1897.
(15) (a) Mazzucato, G. F.; Masetti, F. J. Phys. Chem. 1973, 77, 601. (b)
Kovi, P. J.; Capomacchia, A. C.; Schulman, S. G. Spectrosc. Lett. 1973, 6,
7.
(11) The formation of compound 4 was confirmed by 1H NMR and
MS: 1H NMR (CDCl3) δ 7.78-7.84 (m, 3H), 7.66 (s, 1H), 7.42-7.49 (m,
3H), 7.36 (s, 1H), 7.01 (t, J ) 7.5 Hz, 1H), 6.49-6.56 (m, 2H), 4.48 (dd,
J ) 5.8 and 7.6 Hz), 3.21 (s, 3H), 3.60 (bs, 2H), 3.11 (dd, J ) 7.6 and 13.9
Hz, 1H), 2.89 (dd, J ) 5.7 and 13.8 Hz, 1H); MS (m/e) 277 (M+, 5), 171
(16) The twisted model was originally proposed by Lapouyade and
Rettig5 for the para aminostilbenes such as DCS.
(17) Marder, S. R.; Perry, J. W. Science 1994, 263, 1706.
(18) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
(19) (a) Hammond, P. R. Opt. Commun. 1979, 29. 331. (b) Drake, J.
M.; Lesiecki, M. L.; Camaioni, D. M. Chem. Phys. Lett. 1985, 113, 530.
(c) Taylor, J. R. Opt. Commun. 1986, 57. 117.
1
(M+ - C6H4(NH2)(CH2), 100). The corresponding H NMR and MS data
for the compound formed from irradiation of 3m in CD3OD conform to
the structure of deuterated 4.
(12) Mazzucato, U.; Momicchioli, F. Chem. ReV. 1991, 91, 1679.
(13) Woning, J.; Oudenampsen, A.; Laarhoven, W. H. J. Chem. Soc.,
Perkin Trans. 2 1989, 2147.
(20) Pape, M. Pure Appl. Chem. 1975, 41, 535.
(21) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: London, 1970; p 128.