Ultrafast proton transfer dynamics of hydroxystilbene photoacids

Article abstract of DOI:10.1021/jp044942s

The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and 4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer singlet lifetimes and larger fluorescen

Full text of DOI:10.1021/jp044942s

J. Phys. Chem. A 2005, 109, 2443-2451
2443
Ultrafast Proton Transfer Dynamics of Hydroxystilbene Photoacids
Frederick D. Lewis,*,† Louise E. Sinks,† Wilfried Weigel,‡ Meledathu C. Sajimon,† and
Elizabeth M. Crompton†
Department of Chemistry, Northwestern UniVersity, EVanston, IL and Institute of Chemistry,
Humboldt-UniVersity of Berlin, D-10117 Berlin, Germany
ReceiVed: NoVember 4, 2004; In Final Form: January 15, 2005
The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and
4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer
singlet lifetimes and larger fluorescence quantum yields than do the 4-hydroxycyanostilbenes. The longer
lifetimes of 3-hydroxystilbene and its cyano derivatives are attributed to a “meta effect” on the stilbene torsional
barrier, similar to that previously observed for the aminostilbenes. The cyano substituent causes a marked
increase in both ground state and excited-state acidity of the hydroxystilbenes in aqueous solution. The dynamics
of excited-state proton transfer in methanol-water solution have been investigated by means of femtosecond
time-resolved transient absorption spectroscopy. Assignment of the transient absorption spectra is facilitated
by comparison to the spectra of the corresponding potassium salts of the conjugate bases and the methyl
ethers, which do not undergo excited-state proton transfer. The 4-cyanohydroxystilbenes undergo excited-
state proton transfer with rate constants of 5 × 1011 s-1. These rate constants are comparable to the fastest
that have been reported to date for a hydroxyaromatic photoacid and approach the theoretical limit for water-
mediated proton transfer. The isotope effect for proton transfer in deuterated methanol-water is 1.3 ( 0.2,
similar to the isotope effect for the dielectric response of water. The barrier for excited state double bond
torsion of the conjugate bases is small for 4-cyano-4-hydroxystilbene but large for 4-cyano-3-hydroxystilbene.
Thus the “meta effect” is observed for the singlet states of both the neutral and conjugate base.
Introduction
constant.6,7 Values of kH are derived from the fluorescence decay
of ROH* or the rise of the fluorescence of RO-*.8 The reliance
Electronic excitation of hydroxyaromatic molecules results
in a marked increase in their acidity.1 The isomeric naphthols
and their derivatives are prototypical photoacids whose excited-
state behavior have been the subject of numerous investigations.
The ground- and excited-state acidities of 1-naphthol in aqueous
solution are pKa and pKa* ) 9.2 and 0.4, respectively, and the
singlet state undergoes deprotonation with a rate constant of kH
on fluorescence precludes the study either of super photoacids
for which proton transfer is so fast that ROH* fluorescence is
too weak to permit accurate decay measurements or of photo-
acids with nonfluorescent conjugate bases. Transient absorption
spectroscopy with femtosecond (fs) time resolution has been
employed to study the dynamics of ESPT from the photoacid
8-hydroxypyrene-1,3,6-trisulfonate to acetate ion in water9,10 and
from 6-hydroxyquinoline and camptothecins containing the
6-hydroxyquinoline subunit to water.11,12
We recently reported the results of an investigation of the
excited-state behavior of 3- and 4-hydroxystilbene (3SOH and
4SOH, Chart 1) in several solvents.13,14 3SOH was found to
behave as a strong photoacid, displaying fluorescence from both
3SOH* and 3SO-* in aqueous solution with a Fo¨rster pKa* ∼
0.1. However, 4SOH exhibits fluorescence from 4SOH*, but
not from 4SO-* in aqueous solution. This substituent positional
effect was attributed to a lower barrier for CdC torsion in
4SOH* vs 3SOH*, which results in a very short singlet lifetime
for 4SOH*. Attempts to measure the fluorescence decay time
of 4SOH* with an apparatus having a 50 ps instrument function
were unsuccessful.
) 2.5 × 1010 s-1 2,3
.
Several derivatives of 1- and 2-naphthol
which possess electron-withdrawing cyano- or fluoroalkane-
sulfonyl substituents at the 5- or 6-position have greatly
enhanced excited-state acidities in water and also function as
photoacids in polar, nonaqueous solvents.4 For example, 5-cy-
ano-1-naphthol has estimated values of pKa* ) -1.2 and kH )
1.2 × 1011 s-1 in water and undergoes excited-state proton
transfer (ESPT) in alcohol solvents.5 The cyanonaphthols have
been referred to as “super” photoacids.
Essentially all measurements of photoacid pKa* and kH values
are based on analysis of the fluorescence properties of the neutral
hydroxyaromatic, ROH, and its conjugate base RO-. The former
property is most readily obtained using the Fo¨rster equation,
eq 1,
We report here the use of fs transient absorption spectroscopy
to investigate the dynamics of ESPT from the 4-cyanohydroxy-
stilbenes 43SNOH and 44SNOH and their 3-cyano isomers
33SNOH and 34SNOH in methanol-water (MW) solution. The
transient spectra of 3SOH and 4SOH have also been investigated
for the first time. The use of MW is necessitated by the low
solubility of the hydroxystilbenes in water. The transient spectra
of methoxy derivative 44SNOMe (Chart 1) and the conjugate
pKa* ) pKa - (hν1 - hν2)/2.3RT
(1)
where hν1 and hν2 are the zero-zero transitions for the acid
and its conjugate base, and pKa is the ground-state acidity
* Corresponding author. E-mail: lewis@chem.northwestern.edu.
† Northwestern University.
‡ Humboldt-University of Berlin.
10.1021/jp044942s CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/26/2005

2444 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Lewis et al.
CHART 1
1H), 7.06-7.14 (d, 1H), 7.38-7.46 (m, 3H), 7.48-7.52 (d, 1H),
7.66-7.71 (d, 1H), 7.75 (s, 1H); MS m/e: M+ ) 221.
Methods. Ground and excited-state dipole moments were
estimated and pKa values were determined as previously
described.14 UV-vis spectra were recorded on a Hewlett-
Packard 8452A or 8453 diode array spectrophotometer using a
1 cm path length quartz cell. Fluorescence spectra were
measured on a SPEX FluoroMax fluorometer. Fluorescence
quantum yields and ns decays were measured as previously
described.14 Subnanosecond decays were obtained with a
Ti:sapphire-pumped system with single-photon counting detec-
tion having an instrument response function of 50 ps.17 All
spectroscopic measurements were performed on solutions that
were purged with dry N2 for at least 25 min. ZINDO calculations
were performed using the ZINDO algorithm as implemented
in CAChe version 4.4.18
Transient absorption spectroscopy was carried out using a
femtosecond transient absorption system with an optical para-
metric amplifier (OPA), as described elsewhere.19 The UV light
was generated by tuning the OPA to 640 nm, and the resulting
light was sent down the track, and was then focused into a 1
mm BBO crystal (θ ) 40.5°, æ ) 0°) with a 100 mm f.l. lens.
The light was recollimated with a 200 mm f.l. lens, and then
focused into the sample using a 200 mm f.l. lens. The residual
640 nm OPA light was filtered from the UV light, and between
0.3 and 0.5 µJ per pulse of 320 nm light was produced at the
sample. The resulting pump light was vertically polarized, and
the white light probe (generated with CaF2) was set to the magic
angle with a calcite polarizer. The probe beam was chopped
according to the scheme in Lukas et al.20 Detection was
accomplished with a CCD array detector (Ocean Optics PC2000)
for simultaneous collection of spectral and kinetic data.21 The
total instrument response function for the pump-probe experi-
ments was 130 fs. Five seconds of averaging were typically
needed to obtain the transient spectrum at a given delay time.
Cuvettes with a 2 mm path length were used, and the samples
were irradiated with 0.3-0.5 µJ per pulse focused to a 200 µm
spot. The sample was stirred with a wire stirrer to prevent
thermal lensing and sample degradation. Absorption spectra
were recorded before and after laser excitation. Kinetic analyses
were performed at several wavelengths using a Levenberg-
Marquardt nonlinear least-squares fit to a general sum-of-
exponentials function with an added Gaussian to account for
the finite instrument response.
base 44SNO- provide reference analogues which are incapable
of undergoing ESPT. The 4-cyanohydroxystilbenes have rate
constants for ESPT of kH ∼ 5 × 1011 s-1, comparable to the
fastest that have been observed to date for a photoacid.11 Lower
rate constants are observed for 33SNOH and 3SOH, whereas
the occurrence of ESPT cannot be detected by transient
absorption spectroscopy for 34SNOH and 4SOH. The effects
of substituent position upon singlet lifetimes, ESPT dynamics,
and isomerization efficiency are discussed.
Experimental Section
Materials. The trans-cyanomethoxystilbenes 43SNOMe and
44SNOMe were synthesized by the Wittig reaction between
4-cyanobenzyl triphenylphosphonium bromide with the ap-
propriate anisaldehyde (meta or para).15 Yields were about 50%.
Products were characterized by NMR and GC/MS.
1
4-Cyano-3′-methoxystilbene (43SNOMe): H NMR (CDCl3,
400 MHz) δ 3.86 (s, 3H), 6.84-6.90 (dd, 1H), 7.02-7.11 (m,
2H), 7.11-7.15 (d, 1H), 7.16-7.22 (d, 1H), 7.28-7.34 (t, 1H),
7.55-7.61 (d, 2H), 7.62-7.67 (d, 2H); MS m/e: M+ ) 235.
1
4-Cyano-4′-methoxystilbene (44SNOMe): H NMR (CDCl3,
400 MHz) δ 3.84 (s, 3H), 6.88-6.99 (m, 3H), 7.13-7.20 (d,
1H), 7.45-7.50 (d, 2H), 7.53-7.58 (d, 2H), 7.58-7.65 (d, 2H);
MS m/e: M+ ) 235.
The cyanomethoxystilbenes were converted to the corre-
sponding cyano-hydroxystilbenes by the procedure of McOmie
et al.16 Yields were 30-60%. Products were characterized by
NMR and GC/MS.
Results and Discussion
Absorption and Emission Spectra. The absorption and
fluorescence spectra of 43SNOH and 44SNOH in MW and in
50% methanol/0.1 M KOH (MW-KOH) are shown in Figure
1. The appearance of the absorption spectra in MW and MW-
KOH are similar to those previously reported for 3SOH and
4SOH in water and aqueous base.13,14 Absorption spectra of
both 43- and 44-SNOH in 50% v/v methanol/water (MW)
display a strong long-wavelength absorption band. The absorp-
tion spectrum of 44SNOH in MW-KOH also displays a red-
shifted band. The absorption spectrum of 43SNOH in MW-
KOH is more complex (Figure 1a), consisting of a long-
wavelength shoulder and a strong band at shorter wavelength.
The absorption spectra of 33- and 34SNOH are similar to those
of the 4-cyano isomers. Absorption maxima in several solvents
are reported in Table 1.
4-Cyano-3′-hydroxystilbene (43SNOH): 1H NMR (CDCl3,
400 MHz) δ 4.98 (s, 1H), 6.76-6.83 (dd, 1H), 7.01 (br s, 1H),
7.03-7.09 (d, 1H), 7.09-7.12 (d, 1H), 7.13-7.19 (d, 1H),
7.22-7.29 (t, 1H), 7.54-7.60 (d, 2H), 7.61-7.67 (d, 2H); MS
m/e: M+ ) 221.
4-Cyano-4′-hydroxystilbene (44SNOH): 1H NMR (CDCl3,
400 MHz) δ 5.12 (s, 1H), 6.83-6.88 (d, 2H), 6.90-6.98 (d,
1H), 7.10-7.19 (d, 1H), 7.40-7.47 (d, 2H), 7.52-7.58 (d, 2H),
7.58-7.65 (d, 2H); MS m/e: M+ ) 221.
3-Cyano-3′-hydroxystilbene (33SNOH): 1H NMR (CDCl3,
400 MHz) δ 4.86 (s, 1H), 6.75-6.81 (dd, 1H), 6.97-7.06 (m,
2H), 7.07-7.14 (m, 2H), 7.22-7.30 (t, 1H), 7.43-7.49 (t, 1H),
7.51-7.56 (d, 1H), 7.67-7.74 (d, 1H), 7.77 (s, 1H); MS m/e:
M+ ) 221.
The long-wavelength absorption bands of the cyanohy-
droxystilbenesareassignedtoessentiallypureπ,π*HOMOfLUMO
transitions on the basis of semiempirical ZINDO calcula-
3-Cyano-4′-hydroxystilbene (34SNOH): 1H NMR (CDCl3,
400 MHz) δ 5.13 (s, 1H), 6.82-6.88 (d, 2H), 6.88-6.97 (d,

Dynamics of Hydroxystilbene Photoacids
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2445
Figure 1. Normalized absorption (solid lines) and fluorescence spectra
(broken lines) of (a) 43SNOH and (b) 44SNOH in MW (black lines)
and MW-KOH (red lines).
TABLE 1: Spectral Data for 4′-Cyanohydroxystilbenes
Figure 2. ZINDO frontier orbitals (HOMO and LUMO) for (a)
solventa
43SNOH THF
MeOH
MW
λ
abs, nm λfl, nm
Φfl
τs, nsc
b
44SNOH and (b) 44SNO-.
320
320
322
401
417
401
0.40
0.22
0.02
1.3
TABLE 2: Formal Charges, Dipole Moments, and Acidities
of the Ground and Excited States
MW-KOH 322,373 420,501 <0.01
-qo(ROH)a -qo(RO-)a µg, Db
µec
9.09
6.56
pKad pKa*e
H2O
321
343
337
334
415
415
435
452
545
455
391
404
412
434
361
394
395
398
402
426
406
431
3SOHf
0.485
0.484
0.482
0.482
0.483
0.482
0.882
0.785
0.774
0.768
0.818
0.774
1.83
1.88
10.1 0.1
9.3 2.0
44SNOH THF
0.02
0.03
0.04
0.01
<0.05
MeOH
MW
MW-KOH 379
H2O
<0.05, 0.014d
4SOHf
43SNOH
44SNOH
33SNOH
34SNOH
6.68 11.3 (19.0) 9.1
5.20 10.6 (19.3) 8.5 0.6
6.85
g
0.23
0.42
<0.05
2.6
330
322
320
338
338
298
295
294
293
320
318
320
320
7.6 (17.0) 9.1
g
g
43SNOMe THF
0.25
0.02
3.37 15.0 (18.3) 8.6
MeOH
44SNOMe THF
MeOH
33SNOH THF
MeOH
MW
H2O
34SNOH THF
a Formal charges on oxygen obtained from ZINDO calculations for
the hydroxystilbenes (ROH) and their conjugate bases (RO-). b Ground-
state dipole moment calculated in CAChe using PM3 calculated
geometries. c Excited-state dipole moments calculated from solvato-
chromic data using the method of Liptay. Values for the FC and CT
(data in parentheses) excited states. d Experimental values in water
obtained spectrophotometrically. e Excited-state values in water obtained
using the Fo¨rster equation. f Data from ref 12. g Fluorescence too weak
to permit determination.
0.53
0.76
0.03
0.03
0.03
0.02
0.096
MeOH
MW
H2O
0.012d
charge transfer. The absorption band of 43SNO- is split into
two bands of different intensity as a consequence of symmetry
breaking by the meta substituent.24 ZINDO calculations for the
four lowest singlet states of the cyanohydroxystilbenes are
reported as Supporting Information. Formal charges on oxygen
obtained from ZINDO calculations for the ground states of the
hydroxystilbenes and their conjugate bases are reported in Table
2.
The fluorescence spectra of the cyanohydroxystilbenes in
cyclohexane solution display vibronic structure (data not shown),
similar to that of trans-stilbene and the isomeric amino-
stilbenes.24 In THF and more polar solvents, they appear as a
broad, structureless band, as shown in Figure 1 for 43- and
44SNOH in MW solution. In MW-KOH solution, the fluo-
rescence spectra are shifted to longer wavelength and are
a MW ) 50% v/v methanol-water. MW-KOH ) 50% methanol,
50% 0.1 M aqueous KOH. b Fluorescence quantum yields determined
on deoxygenated solutions at room temperature. c Fluorescence decay
times determined on deoxygenated solutions at room temperature.
Values of <0.05 indicate decay times too short for measurement with
a laser-based lifetime apparatus having an instrument response function
of 50 picoseconds. d Values determined by transient absorption spec-
troscopy.
tions.22,23 The HOMO is ethylene-localized and the LUMO is
cyanostyrene-localized, as shown in Figure 2a for 44SNOH,
providing a moderate degree of charge transfer to this transition.
The red-shifted long-wavelength band of 44SNO- in MW-
KOH is also assigned to a HOMOfLUMO transition (Figure
2b). In this case the HOMO is phenoxide-localized and the
LUMO benzonitrile-localized, resulting in a high degree of

2446 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Lewis et al.
SCHEME 1
attributed to the conjugate bases 43SNO- and 44SNO-. A weak
long-wavelength shoulder attributed to the conjugate base
44SNO- is observed in the fluorescence spectrum of 44SNOH
in MW, but not in the spectrum of 43SNOH. Weak conjugate
base fluorescence may result from either inefficient formation
of SNO-* or a low fluorescence quantum yield (Scheme 1).
Absorption and fluorescence spectral data for the cyanohy-
droxystilbenes and their methoxy analogues in several solvents
are summarized in Table 1. The spectra of the alcohols and
their methoxy analogues are similar both in band shape and
absorption maxima and display only modest solvent-induced
shifts (except in the case of 44SNOH in MW-KOH). Spectra
of the alcohols in water and aqueous base are similar to those
in MW and MW-KOH.
Fluorescence quantum yields (Φf) and singlet lifetimes (τs)
for the cyanohydroxystilbenes and their methoxy analogues in
several solvents are reported in Table 1. The meta-substituted
isomers 43SNOH, 43SNOMe, and 33SNOH have larger values
of Φf and longer values of τs than do the para-hydroxy-
substituted isomers in THF or MeOH solution. This trend has
been previously noted for 3SOH and 4SOH as well as for the
corresponding aminostilbenes.13,14,24 The longer lifetimes of
meta- vs para-substituted stilbenes possessing strong, electron-
donating substituents is attributed to the ability of the meta
substituent to selectively stabilize the planar singlet vs twisted
singlet, resulting in an increased barrier for CdC torsion and
an increased singlet lifetime.
Dipole Moments and Ground-State Acidities. Ground and
excited-state dipole moments and acidities for the cyanohy-
droxystilbenes are summarized in Table 2 along with published
data for the hydroxystilbenes.14 Ground-state dipole moments
were calculated in CAChe using PM3 optimized geometries.18
The increase in µg upon addition of a 4-cyano substituent is
similar to that previously reported for the aminostilbenes.25 The
dipole moments of the Franck-Condon (µFC) and charge-
transfer excited states (µCT) were calculated from the solvent
dependence of the absorption and fluorescence maxima, re-
spectively, using standard methods, as previously described for
the aminostilbenes (see Supporting Information).25 The resulting
values are similar to those reported for the aminostilbenes and
are indicative of a moderately polar LE state, in accord with
the ZINDO description of the vertical singlet state (Figure 2a),
and a highly polar CT state.22 More accurate analysis of specific
solvent effects on the fluorescence spectra of photoacids is
possible using a Kamlet-Taft analysis.14,26 However, the values
reported in Table 2 suffice to establish the enhanced CT
character of the cyanohydroxystilbenes when compared to the
hydroxystilbenes.
Figure 3. Time-dependent absorbance of the hydroxystilbenes in MW
solution monitored at their absorption maxima with monochromatic
irradiation at their absorption maxima in MW solution.
in an increase in acidity of ca. 1 pKa unit for both the
cyanohydroxystilbenes vs 3SOH and 4SOH, respectively (Table
2). This increase in acidity is smaller than that for 6-cyano-2-
naphthol vs 2-naphthol (pKa ) 10.5 and 8.8, respectively).27
The excited-state acidity of 44SNOH in aqueous solution was
estimated using the Fo¨rster equation (eq 1). The value of pKa*
for 44SNOH* indicates an increase in excited-state acidity by
1.4 pKa units upon introduction of the 4-cyano substituent. This
increase is also smaller than that for 6-cyano-2-naphthol vs
2-naphthol (pKa* ) 0.2 and 2.8, respectively).8 The very weak
fluorescence of the other conjugate bases precludes determina-
tion of their pKa* values by either a Fo¨rster cycle or more
accurate methods such as fluorescence titration7 and singular
value decomposition with self-modeling.13
Irradiation in Methanol-Water Solution. Murohoshi et
al.28 have reported that the trans isomers of 3SOH and 4SOH
have high quantum yields for conversion to a mixture of the
cis isomer and stilbene-water adduct in 1:1 MeCN/H2O (Φ )
0.36 and 0.49, respectively). We have investigated the steady-
state irradiation of 3- and 4SOH, 44- and 43SNOH, and
44SNOMe in MW solution. The time-dependent changes in
long-wavelength absorbance is shown in Figure 3. Both 3- and
4SOH display an initial rapid decrease in absorbance attributed,
by analogy to the results of Murohoski et al.,28 to trans-cis
isomerization and a slower bleaching attributed to solvent
addition and phenanthrene formation. 43SNOH is relatively
stable upon irradiation, indicative of inefficient trans-cis isomer-
ization. In contrast 44SNOH and 44SNOMe undergo rapid
reaction to a constant absorbance, attributed to the formation
of a steady-state mixtures of trans and cis isomers, which do
not undergo further reaction.
Transient Absorption Spectroscopy of 44SNOH, 44SNO-,
and 44SNOMe. Transient absorption spectra were obtained
using a Ti:sapphire based laser system with a CCD array
detector for simultaneous collection of spectral and kinetic
data.21 The total instrument response function for the pump-
probe experiments is 130 fs. Samples were stirred and their
absorption spectra recorded before and after measurement of
their transient spectra. We are confident that the transients
observed can be attributed to the singlet states of the trans
isomers and their conjugate bases rather than their nonfluores-
cent cis isomers or methanol adducts. Assignment of the
transient spectra of 44SNOH and the other hydroxystilbenes is
Ground state acidities were determined titrametrically in
aqueous solution. The introduction of a cyano substituent results

Dynamics of Hydroxystilbene Photoacids
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2447
TABLE 3: Dynamics of Singlet State Formation, Relaxation,
and Decay and Conjugate Base Formation and Decaya
τrs (500)b
τ
vr (530)c
τ
ds (480)d
τ
rcb (420)e
τ
dcb (420)f
3SOH
4SOH
0.15
0.16
0.14
0.16
0.16
0.16
0.26
0.22
2.3
2.1
29
17
37
490
43SNOH
44SNOH
4SNODi
44SNO-
43SNOMe
44SNOMe
33SNOH
34SNOH
1.1g
1.7g
1.9g
240, 340h
246
33
2.8
1.7
2.1
25
30, 27h
36, 21h
1.8
2.4
2.1
2.6
1.8
4.6
19
12
a Kinetics in picoseconds determined at wavelengths indicated in
parentheses (except as noted) in MW solution (except as noted). b Rise
time for 500 nm transient. c Vibrational relaxation time. d Decay of the
f
singlet state. e Rise time for the conjugate base. Decay of the conjugate
base. g Decay times determined at 510 nm. h Rise time for the 540 nm
emissive band. i Data for MeOD/D2O.
Figure 4. Transient absorption spectra for (a) 44SNOMe and (b)
44SNO- in MW solution.
complicated by the overlapping absorption bands of the pho-
toacid and its conjugate base. However, comparison of their
transient spectra with those for 44SNOMe and 44SNO-, which
cannot undergo ESPT permits assignment of the more complex
spectra of 44SNOH. Our assignments of the transient absorption
spectra of the hydroxystilbenes are similar to those reported by
Cohen et al.10 for proton transfer between the photoacid
8-hydroxypyrene-1,3,6-trisulfonate and the base acetate ion in
water and to those reported by Poizat et al.11 for proton transfer
from 6-hydroxyquinoline to water.
The transient spectra of 44SNOMe display initial formation
of a band with a maximum near 500 nm within ca. 1 ps,
followed by a blue shift in the band maximum to 480 nm within
the next several ps (Figure 4a), accompanied by band-narrowing.
Single-wavelength kinetics determined at 10 nm intervals across
the band establish that the 500 nm band has a rise time of 0.26
ps, slightly longer than the instrument response function, and
that band-narrowing occurs with a decay time of 2.1 ps. Finally,
the decay of the 480 nm band is dominated by a 33 ps decay
component. This short decay time is consistent with the upper
bound of 50 ps for the lifetime of 44SNOMe in THF estimated
from attempted fluorescence decay measurements (Table 1).
Band narrowing of the initially formed transient is attributed
to vibrational relaxation of the stilbene FC excited state. Stilbene
singlet excited states are known to have more planar energy
minima than the corresponding ground states.29 Vibrational
relaxation of stilbene FC singlet states has been investigated
by means of a number of time-resolved methods, including
transient absorption, Raman, anti-Stokes Raman scattering
spectroscopy, and time-resolved emission.30-32 These studies
provide relaxation times of ca. 2 ps, similar to the band
narrowing for 44SNOMe (Table 3). It is interesting to note that
neither trans-stilbene30 nor its 4,4-dicarboxamide derivative33
display shifts in their transient absorption band maxima upon
vibrational relaxation in nonaqueous solution. However, the
fluorescence of 4-(dimethylamino)-4′-cyanostilbene and other
push-pull substituted stilbenes undergo a blue shift attributed
to relaxation from the FC to CT singlet state with a decay time
Figure 5. Transient absorption spectra for (a) 44SNOH and (b)
43SNOH in MW solution.
of ca. 5-7 ps in ethanol solution.32,34 It is likely that relaxation
from the initially formed FC singlet state to the CT singlet state
of the cyanohydroxystilbenes is responsible for the band shift
observed in MW solution.
The transient spectra of 44SNOH in MW-KOH shown in
Figure 4b are assigned to the formation and decay of the singlet
state of the conjugate base 44SNO-*. The 500 nm transient is
formed with an instrument-limited rise time and relaxes to the
485 nm transient with a decay time of 2.9 ps, values similar to
that for 44SNOMe. In addition, there is a broad emissive band
centered near 550 nm, which forms rapidly (ca. 1 ps) and decays
with a rise time similar to the decay time of the 480 nm band
(Table 3). These long-lived transients are assigned to the
absorption and fluorescence of 44SNO-* and have lifetimes
similar to the fluorescence decay of 44SNOH in MW-KOH
(Table 1).
We turn next to the transient spectra of 44SNOH (Figure 5a).
The most prominent feature of its spectra is the formation of
the 500 nm transient with an instrument-limited rise time,
followed by band narrowing with a shift to 480 nm, with a time

2448 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Lewis et al.
methanol, respectively, attributed to nonradiative decay (via
CdC torsion) of 44SNOH*, which is weakly fluorescent in
methanol (Table 1). Thus we conclude that 44SNOH* fails to
undergo ESPT to an appreciable extent in either methanol or
9:1 MW.
Transient Absorption Spectra of 43SNOH, 33SNOH, and
34SNOH. The transient absorption spectra of 43SNOH in MW
shown in Figure 5b differ from those of 44SNOH in displaying
a broader absorption band, no long-wavelength emissive
component, and a stronger short-wavelength emissive band. The
absence of long-wavelength emission is consistent with the
fluorescence spectra of 43SNOH* in MW which shows no
component of 43SNO- emission (Figure 1a). The broader 480
nm band for 43SNO-* vs 44SNO-* may be a consequence of
the presence of two rotational isomers for 43SNOH and its
conjugate base or simply the loss of symmetry in meta- vs para-
substituted stilbenes. The 2.8 ps rise time of the 420 nm shoulder
of the broad absorption band can be assigned to the formation
of 43SNO-*, and the 25 ps decay time can be assigned to the
disappearance of 43SNO-*. The dynamics of the formation and
decay of 43SNOH* and 43SNO-* are summarized in Table 3.
The transient spectra of 43SNOMe (not shown) are somewhat
broader than those of 44SNOMe (Figure 4a), but have similar
rise and decay times for the formation and narrowing of the
500 nm band (Table 2). The 250 ps decay time for the 480 nm
transient is similar to the fluorescence decay time of 43SNOMe*
in THF solution (Table 1) and is assigned to decay of the singlet
state.
Figure 6. Transient decay of 44SNOH in MW solution monitored at
(a) 541 and (b) 421 nm.
The transient absorption spectra of 33SNOH (provided as
Supporting Information) closely resemble those of 43SNOH
(Figure 5b). The rise time for formation of 33SNO-* is
somewhat longer than that for 43SNO-* (τrcb ) 4.6 vs 2.8 ps)
and its decay time is slightly shorter (τdcb ) 19 vs 25 ps). The
transient absorption spectra of 34SNOH differ from those of
44SNOH in that they display no emissive components and a
single absorption band that decays with a lifetime of 12 ps with
little change in band shape. They more nearly resemble the
transient spectra of 44SNOMe, which does not undergo ESPT.
On this basis, the transient spectra of 34SNOH are assigned to
the singlet state 34SNOH* rather than to the conjugate base.
This assignment is consistent with the exceptionally short
fluorescence decay time of 34SNOH* in THF solution (Table
1).
constant of 1.7 ps obtained from the 510 nm decay. Since the
singlet states of both 44SNOMe and 44SNO- have transient
absorption bands near 480 nm, this band cannot be assigned
with certainty to either the singlet or its conjugate base. The
band shift is accompanied by the appearance of a weak emissive
band near 540 nm, assigned to the fluorescence of 44SNO-*.
The transient kinetics for the formation and decay of this
emissive band are shown in Figure 6a. The decay and rise are
fit to 1.4 and 27 ps, respectively (Table 3).
In addition to these long-wavelength components, an emissive
component with a minimum at 430 nm is formed within 1 ps.
This component is assigned to the fluorescence of 44SNOH*,
which has a steady-state fluorescence maximum at 435 nm in
methanol solution (Table 1). This emissive band is replaced by
a 420 nm absorption band, which has a rise time of 1.7 ps
(Figure 6b). The similarity of this rise time to the 1.4 ps 510
nm decay and the appearance of the 44SNO-* fluorescence
band permit assignment of the 420 and 480 nm absorption bands
to the absorption spectrum of 44SNO-*. Both the 420 nm
absorption and 540 nm emission bands have decay times of ca.
30 ps. The rise and decay times assigned to 44SNO-* formed
upon irradiation of singlet 44SNOH in MW are summarized in
Table 3.
The effects of solvent deuteration and solvent composition
on the transient absorption spectra of 44SNOH have been briefly
investigated. The transient absorption spectra of 44SNOH in
CH3OD/D2O (MW-d) are similar to those in MW. Decay times
for the singlet and its conjugate base in MW-d are reported in
Table 2 and are discussed later. Both in pure methanol and 9:1
methanol-water (v/v) the transient spectra for 44SNOH are
similar to those for 44SNOMe in 1:1 MW (Figure 4a). A 510
nm absorption band is formed with an instrument-limited rise
time and undergoes a blue shift to 480 nm with a decay time of
ca. 2 ps, attributed to vibrational cooling. The 480 band has
decay times of 17 and 14 ps in 9:1 methanol/water and pure
Transient Absorption Spectroscopy of 3SOH and 4SOH.
We previously reported that 3SOH* undergoes deprotonation
in aqueous solution to yield the fluorescent conjugate base
3SO-*, but that 4SOH does not.13,14 We suggested that this
difference in excited-state behavior is a consequence of the much
longer singlet lifetime for 3SOH vs 4SOH. The longer singlet
lifetime of 3SOH is a function of a larger barrier for singlet
state CdC torsion, a result of the “meta effect”.24 We were
unable to determine the lifetime of 3SOH* by means of time-
resolved fluorescence using an apparatus with an instrument
response function of 50 ps, placing a lower bound on the value
of kH < 2 × 1010 s-1, similar to the value for 1-naphthol.2
The transient spectra for 3SOH (Figure 7a) are similar to those
for 43SNOH (Figure 5b). Formation of the 530 nm transient
occurs with an instrument-limited rise time. The red edge of
this band has a decay time of 2.0 ps, attributed to vibrational
cooling of the FC singlet state, which is accompanied by a shift
in the absorption maximum to 480 nm. The decay of the 480
nm band is best fit by a dual exponential with decay times of
29 and 490 ps. The blue edge of the absorption band has a rise
time of 37 ps, similar to the short-lived component of the 480

Dynamics of Hydroxystilbene Photoacids
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2449
the super photoacid 5-cyano-1-naphthol. They report that the
dissociation lifetime increases with increasing methanol content
from ca. 10 ps in 50% methanol to ca. 25 ps in 90% methanol
and 390 ps in pure methanol. A similar decrease in the rate of
ESPT for 44SNOH would account for our failure to observe
the formation of 44SNO-* in either 90% methanol or pure
methanol. The 14 ps singlet decay time of 44SNOH in methanol
(Table 1) presumably is too fast to permit competing ESPT to
occur. Pines et al.5 report that the isotope effect for ESPT
depends on the solvent composition, increasing from ca. 1.5 in
water to 2.5 in methanol. The rise times for 44SNO-* formation
in MW vs deuterated MW (Table 3) provide an isotope effect
of ca. 1.3 ( 0.2. This value is similar to the isotope effect for
the dielectric response of water.5
Agmon et al.35 have investigated the photoacidity of cyanon-
aphthols using AM1 calculations to determine the Mulliken
charges on oxygen and the ring carbons of the acid and its
conjugate base. They find that the strongest acids have the lowest
charge densities on oxygen in the singlet state of the conjugate
base. The exceptionally high kinetic acidity of 44SNOH can
similarly be attributed to charge delocalization in the relaxed
singlet state of its conjugate base, which can be described as a
phenoxidefbenzonitrile CT state (Figure 2b).
The 30 ps decay time for the conjugate base 44SNO-* is
significantly faster than that for 3SO- (Table 4) or for the
conjugate base of the super photoacid 5-cyano-1-naphthol
(τcb ) 110 ps).5 It seems unlikely that the short decay time of
44SNO-* is a consequence of quenching by reprotonation,
which would require rates that are much faster than those
reported for weaker photoacids. Huppert et al.36 have observed
small positive solvent isotope effects for geminate reprotonation
of the conjugate base of several cyano-substituted 2-naphthols
in H2O vs D2O. Reprotonation can occur via both reversible
and irreversible mechanisms.37 We observe a small positive
isotope effect for the decay of SNO-* transient absorption;
however, a small negative isotope effect is obtained from the
rise time for its fluorescence (Table 4). Thus the decay kinetics
provide ambiguous evidence for quenching by reprotonation.
The rapid bleaching of 44SNOH absorbance upon irradiation
(Figure 3) suggests that 44SNO-* undergoes rapid CdC torsion,
as is the case for the singlet state of 4-aminostilbene.24
Figure 7. Transient absorption spectra for (a) 3SOH and (b) 4SOH in
MW solution.
nm decay, both of which are attributed to ESPT with formation
of 3SOH-*. The long-lived 480 nm decay component is similar
to the major (500 ps) component of the fluorescence decay of
3SOH-* determined in aqueous base.14
The 500 nm transient of 4SOH (Figure 7b) has an instrument-
limited rise time and a decay time of 17 ps. The signal amplitude
is much weaker than that for 44SNOH. The low signal amplitude
and short decay time can be attributed to rapid decay of 4SOH*
and/or 4SO-* via CdC torsion. The assignment of the 500 nm
transient to 4SOH* rather than to its conjugate base is consistent
with the short fluorescence decay time of 4SOH in THF solution
(ca. 30 ps) and its low fluorescence quantum yield in aqueous
solution.14 The dynamics of the formation and decay of singlet
4SOH, 3SOH, and 3SO- are reported in Table 3.
The excited-state behavior of 44SNOH in MW is summarized
in Scheme 1. The low fluorescence quantum yield for 44SNOH*
indicates that its lifetime is determined by the rate constant for
transfer of a proton to water (kH). Similarly, the low fluorescence
quantum yield for 44SNO-* indicates that its lifetime is
determined by the rate constant for nonradiative decay via
internal conversion or CdC torsion (kic and kt, respectively).
Rapid and efficient isomerization of 44SNOH in MW (Figure
3) is consistent with CdC torsion as the dominant decay
pathway for 44SNO-*. The 30 ps lifetime for 44SNO-*
provides a value of kt ∼ 3 × 1010 s-1. The efficient isomerization
and 33 ps lifetime for 44SNOMe* suggests that deprotonation
has little effect on the barrier for CdC torsion.
Dynamics of Conjugate Base Formation and Decay. The
1.7 ps rise time for the 420 nm transient attributed to formation
of 44SNO-* from 44SNOH* is slightly faster than the value
of 2.2 ps recently reported by Poizat et al.11 for the 6-hy-
droxyquinolinium ion in acidic solution. Pines et al.5 have
reported that deprotonation of singlet 5-cyano-1-naphthol in
water occurs with lifetimes of ca. 8 and 10 ps in water and
MW (1/1 by volume), respectively. They suggest that the time
scale for solvent-mediated proton-transfer in water can be
bracketed by the Debye and Onsager relaxation times (τD ∼ 8
ps and τL ∼ 0.5 ps, respectively). Our values of τrcb for 44SNOH
(Table 3) lies in the middle of this range.
The other 4-hydroxystilbenes, 4SOH and 34SNOH, also form
short-lived 480 nm transients in MW (17 and 12 ps, respectively)
but fail to display emission from their conjugate bases in either
their steady state or transient spectra. We previously attributed
the absence of steady-state emission from 4SO-* to the failure
of 4SOH* ESPT to compete with CdC torsion.13,14 However,
it is also possible that 4SOH* does in fact undergo ESPT, but
that 4SO-* undergoes CdC torsion more rapidly than it is
formed, thus precluding the observation of either transient
absorption or fluorescence from 4SO-*.
Pines et al.5 have investigated the effects of MW solvent
composition and deuteration upon the dynamics of ESPT for

2450 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Lewis et al.
The dynamics conjugate base formation and decay for
43SNOH are strikingly similar to those for 44SNOH (Table
3). The weaker fluorescence from 43SNOH* in MW and
43SNO-* in MW-KOH when compared to 44SNOH* and
44SNO-* is a consequence of lower fluorescence rate constants
for the 3- vs 4-hydroxystilbenes.
Unlike 44SNOH, 43SNOH does not undergo rapid photo-
isomerization in MW (Figure 3). Thus the dominant decay
pathway for 43SNO-* must be internal conversion to its ground
state (Scheme 1) with a rate constant kic ∼ 4 × 1010 s-1. Rapid
internal conversion has been reported for a number of push-
pull substituted stilbenes.25 The transient dynamics for 33SNOH
are similar to those for 43SNOH, suggesting similar rate
constants for the formation and nonradiative decay of its
conjugate base.
Acknowledgment. Funding for this project was provided
by grants from the National Science Foundation (CHE-0100596
and CHE-0400663).
Supporting Information Available: ZINDO-calculated
singlet states and Lippert-Mataga Plots for the cyanohydroxy-
stilbenes and transient absorption data for the 3-cyanohydroxy-
stilbenes. This material is available free of charge via the Internet
at http://pubs.acs.org.
References and Notes
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The rate constants for both the formation and decay of 3SO-*
are appreciably slower than those for 43- and 33SNO-* (Table
3), permitting the observation of fluorescence from both 3SOH*
and 3SO-* in aqueous solution. The observation of efficient
photoisomerization for 3SOH in MW (Figure 3) indicates that
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Concluding Remarks
The use of fs transient absorption spectroscopy has permitted
investigation of the dynamics of proton transfer for the hydroxy-
stilbenes and their cyano derivatives in methanol-water mixed
solvents. Assignment of their transient absorption spectra is
supported by the use of models for the singlet and its conjugate
base. The short decay times of these photoacids and weak
fluorescence of their conjugate bases renders impractical the
study of their dynamics by transient fluorescence. The ESPT
rates for the cyanohydroxystilbenes 43SNOH and 44SNOH are
comparable to the fastest reported to date11 and the solvent
deuterium isotope effect the smallest reported to date for a
photoacid in water or methanol-water.5 Both the rate constants
for ESPT and the isotope effect approach the theoretical limits
for water-mediated proton transfer.
The initial objective of our studies of the hydroxystilbenes
was to determine the generality of the “meta effect” for
substituted stilbenes. Amine, methoxy, and hydroxy substituents,
all of which are strongly mesomeric, are found to markedly
increase the stilbene lifetime when located in the meta, but not
in the para position.13,14,24 This manifestation of the generalized
“meta effect”, as initially formulated by Zimmerman,38 is
attributed to selective stabilization of the planar stilbene singlet
state vs the twisted singlet, resulting in an enhanced barrier for
CdC torsion. The present results establish that the behavior of
both the cyanohydroxystilbenes and their conjugate bases are
subject to the meta effect. In nonaqueous solution the singlet
states of 43SNOH and 33SNOH have relatively long singlet
lifetimes and high fluorescence quantum yields, indicative of
high CdC torsional barriers; whereas both 44SNOH and
34SNOH have short singlet lifetimes and low fluorescence
quantum yields, indicative of low CdC torsional barriers. The
excited conjugate base 43SNO-* decays via internal conversion,
whereas 44SNO-* decays via CdC torsion. Thus CdC torsional
barriers for the conjugate bases parallel those of the neutrals.
As previously observed in our studies of the cyanoamino-
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