Anomalous photophysics of bis(hydroxystyryl)benzenes: A twist on the para/meta dichotomy

Article abstract of DOI:10.1021/ol8006925

(Chemical Equation Presented) The dianions of two isomeric bis(hydroxystyryl)benzenes show dramatically different photophysical properties.

Full text of DOI:10.1021/ol8006925

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2429-2432
Anomalous Photophysics of
Bis(hydroxystyryl)benzenes: A Twist on
the Para/Meta Dichotomy
Kyril M. Solntsev, Psaras L. McGrier, Christoph J. Fahrni,* Laren M. Tolbert,* and
Uwe H. F. Bunz*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta,
Georgia 30332
uwe.bunz@chemistry.gatech.edu; laren.tolbert@chemistry.gatech.edu
Received March 26, 2008
ABSTRACT
The dianions of two isomeric bis(hydroxystyryl)benzenes show dramatically different photophysical properties.
Stilbenes and distyrylbenzenes represent the first two oligo-
mers leading to poly(p-phenylenevinylene) (PPV) and thus
are important model compounds for conjugated polymers.1
As a consequence, there is a significant fundamental interest
in the excited-state behavior of these materials. Curiously,
trans-stilbene (TS) is poorly fluorescent, the result of its well-
known propensity for isomerization in the excited state.2 In
contrast, trans,trans-distyrylbenzene (TTSB) and certain
derivatives resist isomerization and are strongly fluorescent.3,4
Recently, Lewis et al.5 investigated the photoinduced pro-
cesses in 4-hydroxystilbene (2), 3-hydroxystilbene (4), and
several of their derivatives. The authors observed strikingly
different behavior upon excitation, concluding that 4 is a
strong photoacid in water, while 2 does not undergo efficient
ESPT (excited-state proton transfer) because of much faster
photoisomerization. As a result, both compounds exhibit very
weak fluorescence in aqueous solutions.
Given the stronger emissive properties of TTSB vs TS,
we speculated that hydroxyarenes based upon the former
might show interesting excited-state proton transfer (ESPT)
properties. In this paper, we demonstrate that the excited-
state properties of the homologues 1 and 3 are different both
from each other and from those of 2 and of 4, coinciding
with the differences between TS and TTSB in some respects
but not in others. Specifically, we show that neither 1 nor 3
is a photoacid. Surprisingly, there is no published literature
on the spectroscopy and photoinduced phenomena of either
1 or 3. In methanol, both 1 and 3 display intensive, single-
peak blue fluorescence with a Φfl of 0.37 and 0.62,
(1) Benfaremo, N.; Sandman, D. J.; Tripathy, S.; Kumar, J.; Yang, K.;
Rubner, M. F.; Lyons, C. Macromolecules 1998, 31, 3595–3599.
(2) For leading references, see: (a) Saltiel, J.; Sun, Y. P. J. Phys. Chem.
1989, 93, 6246–50. (b) Meier, H. Angew. Chem., Int. Ed. 1992, 31, 1399–
1440. (c) Waldeck, D. H. J. Mol. Liq. 1993, 57, 127–148.
(3) Sandros, K.; Sundahl, M.; Wennerstro¨m, O.; Norinder, U. J. Am.
Chem. Soc. 1990, 112, 3082–3086
(4) Fengqiang, Z.; Motoyoshiya, J.; Nakamura, J.; Nishii, Y.; Aoyama,
H. Photochem. Photobiol. 2006, 82, 1645–1650
.
.
(5) (a) Lewis, F. D.; Crompton, E. M. J. Am. Chem. Soc. 2003, 125,
4044–4045. (b) Crompton, E. M.; Lewis, F. D. Photochem. Photobiol. Sci.
2004, 3, 660–668. (c) Lewis, F. D.; Sinks, L. E.; Weigel, W.; Sajimon,
M. C.; Crompton, E. M. J. Phys. Chem. A 2005, 109, 2443–2451.
10.1021/ol8006925 CCC: $40.75
Published on Web 05/14/2008
2008 American Chemical Society

respectively. Upon preparative photolysis with 355 nm light
for 1 h, 3% of the compound 1 interconverted into its
cis,trans-isomer, while 3 remained unchanged. Similar to
hydroxystilbenes, the bis-para isomer 1 has somewhat red-
shifted spectroscopic features in absorption and emission
compared to the meta derivative 3 (Table 1). Both 1 and 3
To obtain more information about the deprotonation-
dependent properties of the two distyrylbenzenes 1 and 3
we analyzed the obtained absorption data (1 and 3) using
the principal component analysis program SPECFIT.7 Figure
2 shows the results. The pKa values for 1 are pKa1 ) 10.1
Table 1. Thermodynamic and Photophysical Properties of 1 and
3 in MeOH/H2O (2:1 v/v) at 298 K
compd
1
3
pKa1, pKa2
species
10.1 ( 0.1, 12.0 ( 0.1 10.6 ( 0.1, 11.2 ( 0.3
H2A, HA-, A2-
362, 388, 393
H2A, HA-, A2-
355, 359, 363
absorption
maxima (nm)
emission
maxima (nm)
Φfl
428, 575, 533
412, quenched
0.34, n/d, 0.26
0.91, n/d, 1.0
0.46 < 0.001
1.41
τ (ns)
are soluble in methanol but begin to aggregate in solutions
with more than 50 vol % of water. To obtain pKa values, all
measurements were performed in a 2:1 methanol/water (v/
v) mixture,6 in which both compounds were soluble without
apparent aggregation. Figure 1 shows the absorption and
Figure 2. Deconvoluted absorption spectra (left) and species distribu-
tion diagram (right) for compounds 3 (row a) and 1 (row b).
((0.1) and pKa2 ) 12.0 ((0.1), while those for 3 are pKa1
) 10.6 ((0.1) and pKa2 ) 11.2 ((0.3). The absorption data
show nicely that the first deprotonation of 1 is easier than
that of 3, however, the second pKa of 1 is almost 0.8 units
higher than that of 3. The difference must be due to the
conjugative, mesomeric interactions effective in 1 and its
anions, as opposed to the purely electrostatic interactions in
3, that render its two pKa’s more alike.
The pKa1* of 1 estimated using a modified Fo¨rster
method,8 however, is 1.9, and unlike in more condensed
hydroxyarenes, such moderate thermodynamic photoacid-
ity does not lead to detectable ESPT in neutral methanol/
water solutions that can compete effectively with the 0.91
ns decay.9 Therefore, for 1 we do not see any appreciable
ESPT, and the pKa’s determined from the fluorescence
pH-titration simply reflects the ground-state acid-base
equilibrium.
From Figure 2, it is clear that upon the first deprotonation
of 1 an absorption spectrum results that is close in appearance
to that of the dianion 12-. However, in emission, the
monoanion 1- emission is red-shifted (575 nm) from both
the neutral compound (428 nm) and the dianion 12- (533
nm). In contrast to the emission, the monoanion of 1 exhibits
a blue-shifted absorption from that of 12-. We assume that
Figure 1. Absorption (left column) and emission (right column)
spectra of 1 (top row) and 3 (bottom row) in the 2:1 mixture of
MeOH/H2O (v/v) at different pH values.
(6) Canals, I.; Oumada, F. Z.; Roses, M.; Bosch, E. J. Chromatogr. A
2001, 911, 191–202.
emission spectra of 1 and 3. Upon addition of KOH, the
absorption maximum of 1 shifts from 362 nm in the neutral
compound to 393 nm in the bis-deprotonated form of 1 (12-)
while λmax of 3 shifted from 355 to 363 nm.
(7) Binstead, R. A.; Zuberbu¨hler, A. D.; Jung, B. SPECFIT V. 3.0.40;
Spectrum Software Associates: Marlborough, MA, 2007.
(8) Grabowski, Z. R.; Grabowska, A. Z. Phys. Chem. N.F. 1976, 101,
197–208.
(9) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19–27.
2430
Org. Lett., Vol. 10, No. 12, 2008

the anion experiences an intramolecular charge transfer
stabilization in the excited-state as the monodeprotonated
species is formally a donor-acceptor system, leading to the
observed red-shifted emission.
The same experiment, i.e. deprotonation of 3 (λmax 355
nm) to 32- leads to a broadening of the absorption and a
slight red shift to 363 nm with a red-shifted absorption edge.
The emission of neutral 3 is centered at 412 nm. Upon
deprotonation its fluorescence is not shifted but quenched.
A reliable determination of pKa* for 3 is problematic due to
the complete absence of anion fluorescence.
Figure 4. .
LUMO (top) and HOMO (bottom) of 32-
These observations, i.e., the quenching of the fluorescence
of 3 upon deprotonation and the large red-shift of the
fluorescence of 1 upon exposure to aqueous base are in stark
contrast to the effects visible upon deprotonation of 2 and
4.5 On the one hand, 1 shows a much larger red–shift upon
deprotonation and its dianion 12- is highly fluorescent. On
the other hand, dianion 32- is weakly fluorescent but its
absorption spectrum does not show a significant shift upon
exposure to base, similar to the observation for other
m-hydroxybenzylidene derivatives (m-hydroxystilbene5a and
m-hydroxybenzylideneimidazolinone10). It is tempting to
conclude that the reason for the fluorescence quenching
involves twisting about the formal double bond. However,
Sandros3 and Motoyoshiya4 have observed that distyrylben-
zenes undergo adiabatic one-way cis/trans isomerization,
producing emission spectra corresponding to the E/E forms
only. More recently, time-resolved studies indicate the
formation of an intermediate but largely planar excited
state.11 Thus, we conclude that twisting leading to quenching
does not occur. In the case of stilbene, twisting leads to such
decay pathways.
the delocalized phenolate moieties. According to these
calculations, the HOMO-LUMO gap decreases upon depro-
tonation from 3.27 to 2.48 eV.
Table 2. Gas-Phase Computational Data for Compounds
1and 3a
compd
1
3
species
H2A
A2-
H2A
A2-
S1 (eV)
3.18
2.48
3.25
1.91
c
c
7Bgf7Au
S6 (eV)
(2.33)b
(2.39)b
(2.21)b
(0.061)b
3.13
6Bgf7Au
(1.74)b
exp (eV)d
3.42 (S1) 3.15 (S1) 3.49 (S1) 3.04 (S1)e
3.42 (S6)
HOMO (eV) (7Bg)
LUMO (eV) (7Au)
-5.22
-1.96
0.73
3.21
2.48
-5.50
-2.18
3.32
0.44
2.69
2.26
HOMO-LUMO Gap (eV) 3.27
a TDDFT/B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(2d,2p) level of
theory. b Oscillator strength in parantheses. c Major component of the CI
description. d Experimental vertical absorption energy. e From excitation
spectrum.
In order to investigate this phenomenon further, we
performed quantum chemical calculations (B3LYP/6-
311+G(2d,2p)// B3LYP//6-311+G(2d,2p)) upon 1, 3, and
their respective dianions 12- and 32-. Figures 3 and 4 and
In the case of 3, the situation is dramatically different.
The HOMO and the HOMO-1 are almost degenerate and
localized on the two phenolate rings. In valence bond terms,
the two phenolates are disjoint12 and are electronically only
weakly coupled, while the LUMO is extended over the whole
π-system but has larger coefficients in the central ring.13
Given the poor orbital overlap, the HOMO-LUMO transi-
tion leading to the lowest singlet excited-state S1 is expected
to exhibit a negligible oscillator strength despite the fact that
a Bg f Au transition is symmetry allowed in the C2h point
group.
A closer inspection of the excited-state manifold obtained
from time-dependent density functional theory (TD-DFT)
calculations indeed revealed a strong S1 oscillator strength
for neutral 1 and 3 as well as 12- but not the meta-substituted
dianion 32- (Table 2). The latter is prefentially excited into
Figure 3. .
LUMO (top) and HOMO (bottom) of 12-
Table 2 show the most salient results. While the neutral
compounds 1 and 3 show frontier molecular orbitals (FMO)
that are similar to those calculated for distyrylbenzene (see
the Supporting Information), the FMOs for 12- show larger
amplitudes in the two peripheral rings, as a consequence of
(11) Hsu, F.-C.; Hayashi, M.; Wang, H.-W.; Lin, S. H.; Wang, J.-K. J.
Phys. Chem. 2007, 111, 759–763.
(12) (a) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99,
4587–4594. (b) Properly speaking, “disjoint” refers to diradicals, but the
description applies to radical anions and dianions as well.
(13) (a) Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2005, 127,
4124–4125. (b) Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem.
Soc. 2006, 128, 11872–11881.
(10) Solntsev, K. M.; Poizat, P.; Dong, J.; Rehault, J.; Lou, Y.; Burda,
C.; Tolbert, L. M. J. Phys. Chem. B 2008, 112, 2700–2711.
Org. Lett., Vol. 10, No. 12, 2008
2431

S6, while all the lower states exhibit neglible oscillator
strengths. Although the quantum chemical calculations offer
only estimates for gas phase vertical excitation energies at
0 K, the dominant lowest energy transitions scale linearly
with the solution phase experimental data (correlation
coefficient 0.994, mean unsigned error 0.01 eV). In agree-
ment with the experiment, the calculations predict a strong
bathochromic shift upon deprotonation of 1, but only a small
shift for 3 due to excitation into S6 rather S1 (Table 2). The
TD-DFT results furthermore indicate a possible nonradiative
deactivation pathway through a lower lying triplet 3(n-π*)
state (Figure 5). According to the El-Sayed rule,14 intersys-
efficient nonradiative deactivation channel from S6 through
T3 and T4. Because the calculations indicate that the two
1
3(n-π*) states lie above the lowest energy (π-π*) state,
this nonradiative pathway should not be accessible upon
excitation into S1. Excitation at the red-edge in the absorption
spectrum of 32- revealed indeed a weak emission band
centered around 541 nm, which was not visible with
excitation at the absorption maximum (Supporting Informa-
tion). The corresponding excitation trace acquired at 541 nm
peaked at 407 nm and lacked the major higher energy band
visible in the absorption spectrum, thus confirming that
excitation into S6 results in nonradiative deactivation without
detectible emission from S1.
In conclusion, we have demonstrated that the two bis(hy-
droxystyryl)benzenes 1 and 3 show photophysical properties
that are distinct from each other and also distinct from the
smaller 3- and 4-hydroxystilbenes 2 and 4. It is remarkable
that the dianion of 1 is highly fluorescent, while the dianion
of its isomer 3 is completely nonfluorescent. The large
quantum yield of fluorescence of 1, and its dianion presum-
ably reflects a planarized and quite rigid excited-state with
quinoidal resonance contributions,3 while the quenching of
the dianion of 3 may be explained by the presence of an
3
intermediate (n-π*) state combined with a poor Franck-
Condon overlap between the HOMO and LUMO of this
double phenolate. We recently observed similar phenomena
in the case of p- vs m-dihydroxycruciforms.17,18 Overall, we
find it remarkable that a consanguine group of styryl-based
phenols 1-4 display such disparatesand fundamentally
interestingsphotoinduced effects, not easily predicted by
simply examining the structural motifs involved. Such
effects, when understood, help illuminate the rather unusual
properties of the related cruciforms18 and may aid in the
design of other conjugated fluorophores.19
Figure 5.
Excited-state manifold for dianion 32- based on TD-
DFT calculations (B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(2d,2p)).
Upon excitation into S6, nonradiative deactivation may occur through
rapid intersystem crossing (ISC) to the 3(n-π*) states T3 and T4. The
surface plots to the right illustrate the π-π* and n-π* nature of S1,
S3, and S6 with the corresponding electron detachment (blue) and
attachment (red) densities.
Acknowledgment. We thank the Center for Computational
Molecular Science and Technology (Gatech), the National
Science Foundation (CHE-0456892, L.M.T., K.M.S.; CHE-
0750275 U.H.F.B., P.L.M.; CRIF CHE-0443564, C.J.F.), and
the National Institutes for Health (DK 68096, C.J.F.) for
financial support.
tem crossing from 1(π-π*) to 3(n-π*) is rapid and typically
results in fluorescence quenching due to an increased
nonradiative deactivation rate.15 As illustrated with the
electron detachment-attachment densities16 in Figure 5, the
triplet states T3 and T4 together with their parent states S3
and S4 exhibit n-π* character involving excitation of a
nonbonding oxygen lone-pair electron, thus offering an
Supporting Information Available: Synthetic details for
1 and 3, a description of the experimental procedures, and
titration curves. This material is available free of charge via
the Internet at http://pubs.acs.org.
(14) (a) Lower, S. K.; El-Sayed, M. A. Chem. ReV. 1966, 66, 199–241.
(b) El-Sayed, M. A. Acc. Chem. Res. 1968, 1, 8–16.
OL8006925
(15) (a) Brederek, K.; Forster, T.; Oesterlin, H. G. Luminescence of
Organic and Inorganic Materials; Kallman, H. P., Sprunch, G. M.; Eds.;
Wiley: New York, 1962; p. 161. (b) Young, V, Jr.; Quiring, H. L.; Sykes,
A. G. J. Am. Chem. Soc. 1997, 119, 12477–12480. (c) Leray, I.; Lefevre,
J. P.; Delouis, J. F.; Delaire, J.; Valeur, B. Chem.sEur. J. 2001, 7, 4590–
4598. (d) Zhou, Z.; Fahrni, C. J. J. Am. Chem. Soc. 2004, 126, 8862–8863.
(16) Head-Gordon, M.; Grana, A. M.; Maurice, D.; White, C. A. J. Phys.
Chem. 1995, 99, 14261–14270.
(17) IUPAC names: 4,4′-(1E,1′E)-2,2′-(2,5-bis((4-tert-butylphenyl)ethy-
nyl)-1,4-phenylene)bis(ethene-2,1-diyl)diphenol and 3,3′-(1E,1′E)-2,2′-(2,5-
bis((4-tert-butylphenyl)ethynyl)-1,4-phenylene)bis(ethene-2,1-diyl)diphe-
nol.
(18) McGrier, P. L.; Solntsev, K. M.; Scho¨nhaber, J.; Brombosz, S. M.;
Tolbert, L. M.; Bunz, U. H. F. Chem. Commun. 2007, 2127–2129
(19) Hudson, B.; Kohler, B. Synth. Met. 1984, 9, 241–53.
.
2432
Org. Lett., Vol. 10, No. 12, 2008