Time-Resolved Photochemistry of Stiffened Stilbenes

Article abstract of DOI:10.1021/acs.jpcb.9b00784

Broadband transient absorption spectroscopy is used to study the photoisomerization of stiffened stilbenes in solution, specifically E/Z mixtures of bis(benzocyclobutylidene) (t4, c4) and (E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t6). Upon excitation to S₁, all evolve to perpendicular molecular conformation P, followed by decay to S₀, while the spectra and the kinetic behavior crucially depend on the size of the stiffening ring. In 4, contrary to all previously studied stilbenes, the trans and cis absorption and excited-state spectra are nearly indistinguishable, while the corresponding isomerization times are comparable: τ_i = 166 ps for t4 and τ_i = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile, respectively. Faster isomerization in polar solvents agrees with the zwitterionic character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S₁ → P evolution being directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P state is remarkably long-lived, τ_P = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S₁ energy surface.

Full text of DOI:10.1021/acs.jpcb.9b00784

Article
pubs.acs.org/JPCB
Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
Time-Resolved Photochemistry of Stiffened Stilbenes
Olivia A. Krohn,† Martin Quick,*,‡ Ilya N. Ioffe,*,§ Olga N. Mazaleva,§ Dieter Lenoir,∥ Heiner Detert,⊥
and Sergey A. Kovalenko‡
†Department of Physics and Department of Chemistry, University of Colorado, Boulder, Colorado 80309-0215, United States
‡
̈
Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany
§Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia
∥Helmholtz Center Mu
̈
nchen, Postfach 1129, 85664 Neuherberg, Germany
⊥Institute for Organic Chemistry, Johannes Gutenberg-University Mainz, Duesbergweg 10-14, Mainz 55099, Germany
S
* Supporting Information
ABSTRACT: Broadband transient absorption spectroscopy is used to study the
photoisomerization of stiffened stilbenes in solution, specifically E/Z mixtures of
bis(benzocyclobutylidene) (t4, c4) and (E)-1-(2,2-dimethyltetralinylidene)-2-2-
dimethyltetraline (t6). Upon excitation to S1, all evolve to perpendicular molecular
conformation P, followed by decay to S0, while the spectra and the kinetic behavior
crucially depend on the size of the stiffening ring. In 4, contrary to all previously
studied stilbenes, the trans and cis absorption and excited-state spectra are nearly
indistinguishable, while the corresponding isomerization times are comparable: τi = 166
ps for t4 and τi = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile,
respectively. Faster isomerization in polar solvents agrees with the zwitterionic
character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S1 → P evolution being
directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P
state is remarkably long-lived, τP = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis
photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and
substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S1
energy surface.
Scheme 1. Compounds under Study (t4, c4, t6) and
Previously Studied (t5, c5)
1. INTRODUCTION
The general trends in photodynamics of symmetric stilbenes
have been elucidated in several experimental1−13 and
theoretical12−20 studies. Normally, after initial excitation and
rapid relaxation of high-frequency vibrational modes, both the
cis and trans isomer undergo twisting around the central
ethylenic bond. Eventually, they arrive to the perpendicular,
spontaneously polarized domain of S1 (a.k.a. “phantom state”,
P) and then return therefrom to S0. However exact details of
the above evolution vary with the substitution. For example,
there may be a barrier for twisting to P, whose lifetime ranges
from femtoseconds4−6 to hundreds of picoseconds.7−11
between the experiment and the theory is better. Here, it is
worth noting that the P state lives too short to be captured, in
contrast with cis-stilbene c0 where the P state with a lifetime of
0.3 ps is well observed after the virtually barrier-less twisting.4
In the present paper, we extend our stationary and
femtosecond broadband transient absorption study to two
“cousin” molecules: (E/Z)-bis(benzocyclobutylidene) and
(E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t4,
c4, and t6, respectively). Smaller stiffening rings in t4 and c4
Stiffened stilbenes1,21−23 shown in Scheme 1 retain the π-
system topology of unsubstituted stilbene (t0) while
eliminating the rotational degrees of freedom of the phenyl
rings. Previously, we reported10 the photochemistry and
photodynamics of trans- and cis-1,1′-bis(indanylidene) (t5,
c5), the molecule with potential applications in molecular force
probing24 and light-driven nanomotors.14,25 Upon optical
excitation to S1, t5 isomerizes to P with a 5 ps time, over a
10 kJ/mol barrier in n-hexane;10 however, quantum chemical
calculations, with XMCQDPT2 multiconfigurational quasi-
degenerate second-order perturbation theory, show no barrier.
In c5, with its 12 kJ /mol twisting barrier, the agreement
Received: January 25, 2019
Revised: April 12, 2019
Published: April 19, 2019
© XXXX American Chemical Society
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diminish the steric interactions between the two halves of the
molecule, whereas the dimethyl substitution in t6 provokes
high sterical strain for the olefinic junction. Therefore, the type
of the stiffening ring crucially influences the lifetime of various
states in S1, as reflected in the transient photochemical
behavior.
ρ(λ, t) = (ΔA − ΔA⊥)/(ΔA + 2ΔA⊥)
= (ΔA − ΔA⊥)/(3ΔA)
(1)
Multiple pump−probe scans (8−16) are performed with 0.02,
0.4, 2, or 8 ps time steps. The resulting averaged TA spectra
are time-corrected for the chirp of the continuum probe.26
2.3. Data Analysis. A global kinetic analysis over the full
TA spectra, with a sum of convoluted exponential time
functions, is used to derive the time constants and amplitudes
of the photoinduced relaxation processes. The joint sets of data
with different time steps are comprehensively analyzed to
ensure the resulting fits match each other. The results are
visualized with band integrals
2. EXPERIMENTAL AND COMPUTATIONAL SECTION
2.1. Probe Molecules. The synthesis of compounds 4 and
6 was described in detail in refs 21, 23. Briefly, cis- and trans-
(benzocyclobutenylidene)benzocyclobutene is obtained from
benzocyclobutenone via the McMurry reaction following the
21
̈
procedure given by Oelgemoller. Zinc powder (2.8 g, 8.4
λ2
mmol) was added to a cooled (−10−0 °C) solution of TiCl4
(1.62 mL, 15 mmol) in 150 mL anhydrous THF while stirring.
After 15 min of stirring, benzocylobutenone (2.12 g, 12 mol)
in 30 mL of abs THF was added dropwise, and the dark
mixture was stirred under reflux for 26 h. The cooled mixture
was added to 150 mL of 10% aqueous K2CO3 solution, and
this mixture was extracted with 3 × 100 mL of ether and 2 ×
100 mL of petroleum ether. The combined organic layers were
washed with water (2 × 100 mL) and brine (100 mL) and
dried over MgSO4, and the solvent was evaporated in vacuo.
Purification by chromatography (cyclohexane/acetone = 10:1)
gave a ∼2:1 mixture of trans and cis isomers (1.03 g, 24%).
The isomers were separated via repeated fractionating
crystallization from solutions in chloroform, and 2-propanol
was added until the solution became slightly turbid.
1
I(λ1, λ2, t) =
ΔA(λ, t)dλ/λ
∫
ln(λ2/λ1) λ1
(2)
calculated either directly from the experiment or from the
corresponding global fit. When the spectral range (λ1,λ2) is
carefully chosen, the band integrals improve the signal-to-noise
ratio and eliminate the contributions from the spectral shifts.
The integrals are fitted with exponentials to treat the evolution
under study.
In the present case, we allow the fitted kinetics to contain a
small (1−3%) contribution to ΔA from the anisotropy decay.
This contribution is probably due to depolarization of the
probe continuum by the CaF2 plate (where the continuum is
generated).
2.4. Calculations. Quantum chemical calculations are
performed with the Firefly QC package version 8.228 partially
based on the GAMESS (US) source code.29 Ground-state
geometry optimizations are done at the PBE0/Def2-TZVPP
level of theory (TD-PBE0 is also used for preliminary
reconnaissance of the S1 potential energy surface). The excited
state structures are optimized with the RI-XMCQDPT2
multistate multireference perturbation theory at the second
order30 with frozen chemical core. Unless specifically indicated,
no symmetry is used in the calculations. For t4 and c4, we
apply the larger basis set Def2-TZVP, while the smaller one,
Def2-SVP, was used for t6. The CASSCF active space, the
state-averaging scheme, and the XMCQDPT2 model space
had to be varied between some of the computations, and their
choice is described explicitly in each case. The intruder state
avoidance (ISA) parameter is set to 0.02 au.
The synthesis of E-I-(3,4-dihydro-2,2-dimethyl-1(2H)-naph-
talenyliden)-1,2,3,4-tetrahydro-2,2-dimethylnaphthalene is de-
scribed in ref 23. A solution of 2,2-dimethyl-1-tetralone (2.26
g, 13 mmol) in 15 mL of THF was added to a mixture of TiCl4
(1.6 mL) and Zn powder (2 g) in 110 mL of THF, as
described above for t4/c4. The stirred mixture was refluxed for
26 h, cooled, and hydrolyzed with 10% aq. K2CO3, and the
product was isolated by extraction with MTBE chromatog-
raphy (light benzene/ether = 2:1), yielding 1.07 g (56%)
colorless crystals, mp 193−194 °C.
Pure t4 and an E/Z mixture of 4 (∼1:1 ratio from NMR) are
measured upon 326 nm excitation. A second E/Z mixture
(∼1:1 ratio from NMR) is measured with TA, with 316 nm
excitation this time. For pure t4, stationary and transient
spectra should exhibit features from only the t4 isomer, while
the two mixtures should yield spectra with contributions of the
E and Z isomer. For compound 6, the cis isomer is unstable at
room temperature in solution and isomerizes to the more
stable trans conformation within a few minutes.
2.2. Transient Absorption (TA). The experimental setup
has been described elsewhere.10,26,27 In short, a Ti:Sa laser
produces 40 fs, 500 μJ, and 500 Hz pulses at 800 nm. The
majority of this, 420 μJ, pumps an OPA (optical parametric
amplifier) to produce 50 fs and 2 μJ pulses at 316 nm. This
pump beam has a spot size of 100 μm and excites the sample.
The rest of the fundamental is frequency-doubled and focused
to a 1 mm CaF2 plate to produce a supercontinuum probe.
The supercontinuum is spectrally filtered and split into a signal
and reference covering the spectral range of 275 to 690 nm.
TA spectra are measured at 21 °C with parallel ΔA∥(λ, t),
perpendicular ΔA⊥(λ, t), or magic-angle ΔA(λ, t) pump−
probe polarization. Transient anisotropy spectra ρ(λ, t) are
given as
3. RESULTS
3.1. Calculated Ground-State Geometry. In the ground
state, the four-membered stiffening rings in t4 and c4 are
planar, and due to the ∼90° valence angles, the halves of the
molecule are spaced farther apart than in the previously studied
t5 and c5. As a result, both t4 and c4 exhibit a perfect planarity
and mirror symmetry in the ground state (C2h and C2v,
respectively). The planarity of c4 constitutes a special case that
is very different from other nonplanar cis-stilbenes.
In contrast, the bulky dimethylated six-membered stiffening
rings in t6 do not allow planarity and give rise to several
sterically strained conformations of both t6 and c6. The most
stable conformation of t6 is characterized by the central
dihedral angle of 148.5° and ∼40° dihedral angles between the
phenyl rings and the central ethylenic bond that makes the two
rings nearly perpendicular (see the Supporting Information for
coordnates). Interestingly, the most stable conformer of c6 is
calculated to be only 27 kJ/mol higher than that of t6. The
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strong repulsion between the methylated stiffening rings in c6
is compensated by the stronger conjugation in the π system, in
contrast to other stilbenes where the trans isomers show a
higher degree of conjugation. The ground-state molecular
geometries of t4 and t6 are shown in Figure 1. The calculated
spectrum of t4 and c4, εc4 − εt4, when only nonadiabatic
trans−trans and trans−cis isomerization is considered. By
adding the smallest reasonable amount of εt4 to the TA
spectrum, the appropriately scaled extinction of c4 eventually
shows up. Moreover, the extinction spectrum of c4 can also be
derived by stationary means from the t4/c4 mixture (∼1:1
from NMR) by subtracting εt4 with scaling factor f. The scaling
factor f must be tuned until both c4 extinction spectra
(stationary and from TA) coincide. The maximum amplitude
within εt4 (∼70,000 cm−1) is ∼1.5 times larger than in εc4,
which is a trend that is often observed for trans- and cis-stilbene
and its derivatives. If another methylene unit is inserted to
result in 5,10 then a red shift of 1300 cm−1 is observed for the
S0 → S1 absorption band, which possibly points to the higher
conjugation for more relaxed valence angles. In addition, a
slight drop in the amplitude is caused by broadening of
vibrational bands, as compared to 4.
The vertical excitation energies in t4 and c4 are calculated at
the XMCQDPT2 level of theory with the complete π orbital
(14e,14o) active space, state-averaging over the six lowest
CASSCF roots to cover the relevant contributions to the S1,
and altogether 12 states in the model space (also see Section
3.4). Similar to t0,17 there are two appreciable transitions
associated with unequal contributions of the HOMO1LUMO1
electronic configuration. In 4, the absorbing states are S1 and
S3 that belong to the Bu representation in the C2h symmetric t4
and to the B1 representation in the C2v symmetric c4. The
vertical transition energies are calculated to be 4.06 eV for S0
→ S1 and 4.56 eV for S0 → S3 in both t4 and c4. The
calculated oscillator strengths are somewhat different, 0.74 for
S0 → S1 and 0.33 for S0 → S3 in t4 versus 0.61 and 0.26 in c4,
respectively. Also, almost equal are the estimates for the 0−0
transition obtained as the energy difference with the optimized
S1 (see below), 3.76 eV in t4 versus 3.77 eV in c4, the vertical
emission energy being slightly more different: 3.62 eV in t4
versus 3.56 eV in c4. Thus, in agreement with the experiment,
the absorption spectra of the two isomers coincide but could
be distinguished by spectral intensities.
Figure 1. Calculated ground-state geometries of t4 and t6.
geometries refer to the gas phase, and for the solid phase, X-ray
structures has been reported,21,22 but we will refer to our
calculated structures for our studies to be consistent.
3.2. Stationary Spectra. Extinction spectra of stilbenes 0,
4, 5, and t6 are displayed in Figure 2. Due to the higher
Comparing the calculated and measured spectra in Figure 2,
note the calculated 0−0 transition agrees well with the first
vibronic band. Unfortunately, determination of the center of
gravity of the spectra for comparison with the calculated
vertical transition is complicated by the overlapping S0 → S1
and S0 → S3 bands. Perhaps, the weaker transition to S3 is seen
as the bumps around 36,000 cm−1.
In t6, vibronic bands are not resolved anymore. The six-
membered ring allows for a larger structural variety with more
low-frequency degrees of freedom and thus leads to wider
spreading of the oscillator strength. A drop in extinction to
10,000 cm−1 M−1 goes along accordingly. Under illumination
of t6, its absorption spectrum shows no change, indicating that
eventually produced c6 isomerizes back to t6 within a minute.
In agreement with those observations, XMCQDPT2 opti-
mization of the t6-to-c6 minimum energy path in S0 with the
(10e,10o) active space and a single-state model space (thus
effectively MR-MP2) arrives at a less-stable conformer of c6,
which has the barrier for conversion to t6 of only 38 kJ/mol.
3.3. Photoisomerization Dynamics of t4 and c4.
Generally, t4 reveals similar transient features as t0, while c4
differs in behavior from c0. TA spectra ΔA(λ, t) of t4 and c4 in
n-hexane upon 326 nm excitation are displayed in Figure 3.
The photoinduced evolution is similar in acetonitrile, therefore
the following description also refers to that measurement.
Figure 2. Extinction spectra of various stilbenes: t0, c0, t4, c4, t5, c5,
and t6. A detailed description for the derivation of the c4 extinction
spectrum is given in the text and the caption of Figure S1.
planarity of c4 and rigidity of t4, the spectra of the E and Z
isomer, respectively, contain a sharp vibronic structure and,
above that, are visually hardly distinguishable. A feasible
separation into t4 and c4 extinction is performed on the late
transient absorption (TA) spectrum. Here, the bleach region
can principally be reconstructed by the difference extinction
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Figure 3. Transient absorption (TA) spectra of t4 and c4 in n-hexane upon photoexcitation at 326 nm. Bleach and SE are negative, and excited-
state absorption (ESA) is positive; arrows indicate the signal evolution. After Franck−Condon (FC) relaxation, both t4 and c4 isomerize to P with
specific time τi. A global analysis gives τi = 64 ps for c4 and 166 ps for t4 in n-hexane and τi = 45 ps (c4), 114 ps (t4) in acetonitrile. In the bottom
left, t4 extinction εt4 is added to reconstruct c4 extinction εc4. It follows that the c4 spectrum (in green) is blue-shifted by 1 nm with an amplitude
ratio c4/t4 = 1:1.5; the t4 → c4 isomerization yield being determined as Ytc = 20−30%. In the bottom right, the c4 → t4 yield is estimated as 5−
20%.
Recall that excited-state absorption (ESA) is positive while
stimulated emission (SE) and bleach enter the signal with a
negative sign.
Recall that, in c0, the S1 → P isomerization in n-hexane
proceeds with ∼1 ps over a low barrier,4,5 while in c5, a barrier
of 7.2 kJ/mol in acetonitrile (and 12.1 kJ/mol in n-hexane)
prevents ultrafast isomerization.10 Probably, the repulsion
between the phenyl rings is reduced with decreasing ring size
(from c5 to c4), which forces the rings further apart. As a
consequence, the isomerization barrier increases from 2 ps for
c5 to the observed time τ2 = 45 ps for c4.10 The biexponential
behavior in the E/Z mixture of 4 is visualized by the band
integral I(410,690) shown in Figure 4 (bottom left). The TA
signal (cyan) is well fitted by two exponentials ascribed to t4
(red) and c4 (blue).
The global analysis also gives species-associated spectra
(SAS) of t4 and c4. These are shown in the right panels of
Figure 4. The attribution follows from the separately measured
t4 and from a general trend that the S1 → P rearrangement is
usually faster for the cis isomer. Worthy of note is the spectral
resemblance between the SAS. The vibrational bands of the
bleach signals are well resolved and coincide spectrally, which
confirms the strict structural planarity in S0. The spectral
For t4, ESA rises at the early pump−probe delays, peaking at
516 nm in n-hexane (510 nm in acetonitrile). Franck−Condon
(FC) initial relaxation proceeds within the first 0.6 ps, as seen
by a redistribution of the oscillator strength in ESA and SE.
The ESA decays within 300 ps, indicating the S1 → P
isomerization to the perpendicular conformation P. One
exponential time constant is obtained by a global kinetic
analysis: τ1 = 166 ps in n-hexane and τ1 = 114 in acetonitrile.
These are consequently attributed to the trans isomer, in
agreement with our calculations (see next section). The TA
spectrum of pure c4 is isolated from the TA spectrum of co-
excited c4/t4 by subtracting pure t4. The S1 → P isomerization
time, however, is withdrawn from the mixture by using two
exponential time functions while fixing the time constant for
t4. In n-hexane, c4 isomerizes with τ2 = 64 ps, and in
acetonitrile, c4 isomerizes with τ2 = 45 ps.
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Figure 4. Results from a global analysis for pure t4 and c4/t4 mixture in n-hexane. Left: Band integral I(410,690) over the bleach band is shown
(cyan), with faster decay τ1 attributed to c4 (blue) and slower one τ2 to t4 (red). Right: Species-associated spectra (SAS) from the global fit result
in an E/Z ratio of ∼1:1 in n-hexane and acetonitrile in agreement with the NMR results (∼1:1).
similarity of t4 and c4 is further seen in their ESA bands shifted
by 300 cm−1 only.
pure c4 contains a more pronounced dispersive bleach, with
negative and positive parts being blue- and red-shifted. These
band shapes consequently follow from the derived extinction
spectra. Above 400 nm, the positive signal may indicate a
triplet state; an alternative dihydrophenanthrene (DHP)
relaxation channel common to many other stilbenes is
prohibited in 4 because the four-membered stiffening rings
keep the phenyl rings too far apart (see next section).
To calculate energy barrier Eb for S1 → P or P → S0 half-
torsion, the following estimate is applied
The E/Z ratio in S0 can be deduced from the S1 SAS in
Figure 4, when the derived extinction spectra of t4 and c4 are
taken as a basis. Recall that the oscillator strength of the S1−S0
transition is 1.5 times larger for t4. Due to the slight spectral
shift between both isomers, the ratio εt4/εc4 at λexc = 326 nm is
approximately 2:1. The calculated band integrals over the SAS
bleach yield a ratio t4/c4 of ∼3:1 (in both solvents) which, in
the case of equal E and Z S0 abundance, directly reflects the
product of both the larger oscillator strength of t4 and its two
times favored pumping into S1: 3/1 = 1.5/1 and 2/1. This
finding is in agreement with the NMR results where an S0 ratio
of ∼1:1 is determined. Note that this shift is not necessarily
recognized in the bleach region of the SAS since the overlap
with SE is strong for the 0−0 transition.
Returning back to the TA spectra in Figure 3, note that the
bleach recovers with the same time constants as the ESA decay
(τ1 = 166 ps, τ2 = 64 ps for n-hexane and τ1 = 114 ps, τ2 = 45
ps for acetonitrile), indicating that the P → S0 transition
proceeds much faster than the S1 → P half-torsion. Late
spectra at the bottom are recorded at 2 ns when the relaxation
is complete. The residual bleach is small being ∼5% of that at
the early time (top). This suggests that either the t4 → c4 yield
is small or the yield is substantial but the extinctions of t4 and
c4 are similar. Following the latter interpretation, the residual
bleach cannot be perfectly reconstructed solely with the t4
extinction spectrum. In fact, little dispersive features hint for
the c4 extinction spectrum being slightly blue-shifted. Adding
the fewest possible amounts of εt4 to the late TA spectrum
eventually exposes the appropriately scaled c4 extinction
spectrum, indeed being blue-shifted and with a maximum
amplitude of 60% compared to t4. Knowing the extinction
spectrum of c4 allows an alternative way of derivation.
Therefore, the scaled εt4 is subtracted from the measured
joint extinction of t4 and c4 until the difference spectrum
matches εc4 from TA. The late TA spectrum left over from
Eb = kT ln(τ/τ0)
(3)
i
with kT = 2.44 kJ/mol at T = 21 °C. Here, τ0 = 0.1 ps is taken
for barrier-less torsion (of the phenyl group). This value agrees
with the isomerization rate of the parent stilbene,4,5 and it is
assumed to be similar in the studied compounds. Such
calculated isomerization barriers are collected in Tables 1 and
2.
3.4. Calculation Results on t4 and c4. XMCQDPT2
optimization of the excited t4 and c4 is carried out with a
(10e,10o) CASSCF active space that included 10 π orbitals
with the two lowermost occupied and the two uppermost
Table 1. Rotational Diffusion Time τR, S1 → P
Isomerization Time τi, Barrier Eb1, and Trans-to-Cis
Isomerization Yield Ytc in Various Stilbenes
stilbene
solvent
τR (ps)
τi (ps)
Eb1 (kJ/mol)
Ytc
t0
n-hexane
14
17
14
16
18
22
16
22
84
38
166
114
5
16.7
14.9
18.1
17.2
9.7
0.5013
acetonitrile
n-hexane
0.50
t4
t5
t6
acetonitrile
n-hexane
0.3310
0.33
acetonitrile
n-hexane
2
0.1
0.1
7.4
0.0
0.25
acetonitrile
0.0
0.25
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separated by 3 kJ/mol only. One of them is shown in Figure 5.
Note that the sum of the three valence angles at the
Table 2. S1 → P Isomerization Time τi, Barrier Eb1, and Cis-
to-Trans IsomerizationYield Yct and Yc cl in Various
Stilbenes
Eb1
(kJ/mol)
stilbene
solvent
τi (ps)
Yct
Yc cl
c0
n-hexane
0.23 (16%), 0.91
5.2
0.50
0.054
(84%)4
acetonitrile 0.23 (27%),
0.48 (73%),4 0.755
3.9, 5.0
c4
c5
n-hexane
64
acetonitrile 45
n-hexane
1310
acetonitrile 210
15.8
14.9
12.1
7.4
0.33
0.33
0.0510
vacant ones left out. The HOMO1LUMO1 configuration that
dominates the S1 state is typically distributed over two or three
B symmetric CASSCF roots (if C2 symmetric geometry is
considered). In our symmetry-less calculations, we use state-
averaging over the six lowest CASSCF roots to cover the
relevant contributions. The XMCQDPT2 model space
includes the 12 states.
Figure 5. Calculated P-state geometry (S1/S0 intersection) of 4. The
relevant angles and bond length are indicated.
Similar to S0, photoexcited t4 is found to be perfectly planar
in S1. The C2 symmetric saddle point for twisting toward the P
state is located at the central C(Ph)−C(Et)−C(Et)−C(Ph)
dihedral angle of 138° (trans = 180°), and the barrier height is
calculated to be 8 kJ/mol, a certain underestimation with
respect to the experiment. Single-point recalculations with the
(14e,14o) active space and with all 14 π orbitals result in an
even lower value of 5 kJ/mol. The use of the C2 symmetry and
expansion of the model space to 30 B(Bu) symmetric states
(with three-state averaging) produces only a marginally better
estimate of 9 kJ/mol. It is worth noting that a similar
underestimation was previously observed in stilbene17 and thus
probably reflects the accuracy of the approach.
The excited c4, on the contrary, slightly deviates from the
ground-state planarity, its central dihedral angle becoming
10.6°. The saddle point at 43.1° is calculated to be 4 kJ/mol
above, so there is a consistent underestimation of the twisting
barrier on both the cis and trans sides. Unlike c0 and c5,10
cyclization of c4 into a DHP-like molecule is unlikely as the
relevant C···C distance is larger (3.9 Å vs 3.0 Å in c5) and the
π-orbital axes of the respective two carbon atoms are nearly
perpendicular to the interatomic vector.
Beyond the twisting barrier, within P, optimization reveals
pyramidalization of one of the central C(Et) carbon atoms
together with considerable stretching of the respective C(Et)−
CH2 bond. In order to properly account for the elongation
effect, the active space is expanded to (12e,12o) including the
σ-bonding and σ*-antibonding orbitals of the elongated bond.
Interestingly, when still close to the twisting barrier, the
processing pyramidalization required inclusion of the seventh
CASSCF root into state averaging in order to encompass the
major contributions to S1. However, further evolution into the
phantom state allows reduction to only two-state averaging and
limiting of the XMCQDPT2 model space to six states only. It
is found that P does not have any stationary point in S1 as its
minima coincide with the S1/S0 intersection sites, ∼0.3 eV
below the stationary points of t4 and c4 in S1. Thus, in
agreement with the experiment, the P state must instantly relax
to S0.
pyramidalized center is ∼293°, and the C(Et)−CH2 bond is
elongated to 1.85 Å. While in the course of geometry
optimization, the dipole moment of P exceeds 10 D (and a
quasi-lone pair forms at the pyramidalized site) but drops to 4
D in the direct vicinity of the crossing points.17
3.5. Photoisomerization of t6. TA spectra of t6 are
recorded under the same experimental conditions (Figure 6).
At the earliest delays, as the spectrum builds up, the SE band
already starts shifting to the red (top). This shift indicates that
the planar trans conformation is highly unstable in S1.
In both solvents, the early S1 → P torsion progresses with
<0.1 ps, indicating a barrier-less process. Band integrals are
calculated over ESA and P absorption to visualize this
evolution, (Figure 7). A strong and ultrafast red shift of SE
should be ascribed to the decreasing S1 → S0 transition
frequency in the course of the wavepacket motion toward P.
The SE band shifts from 400 to 550 nm (by 7000 cm−1) and
decays within 0.1 ps, while the red ESA band concurrently
decays. Simultaneously, an ESA band from the P state emerges
and peaks at 350 nm. The identification as P-ESA follows from
the comparison with other stilbenes.4,6−11 The P state lives
extraordinarily long in n-hexane, τP = 1840 ps, while in
acetonitrile, the lifetime shortens to τP = 35 ps. A comparison
of the P lifetimes in other stilbenes is given in Table 3.
Late bleach spectra, when all the population returns to S0,
allow an estimate of the trans-to-cis isomerization yield Ytc by
comparing with the early bleach at t = 0.1 ps.6 For acetonitrile,
this gives Ytc = 20%. Then, the c6 extinction spectrum can be
reconstructed as shown by the orange lines in Figure 6
(bottom). In addition, a broad positive signal for λ > 450 nm
indicates a phenanthrene-like photoproduct via ring closure in
adiabatically formed c6. In n-hexane, a 20% photoisomeriza-
tion yield is found again. Here, the full recovery of S0 exceeds
our time window of 2 ns. By enlarging the probe optical path
manually, a TA spectrum at 8 ns was measured and used to
estimate Ytc. Note that no positive band emerges at λ > 450
nm; hence, no ring closure takes place in n-hexane. The
photoisomerization barriers calculated with eq 3 are collected
in Table 1.
The two crossing points located can be distinguished by the
direction in which the pyramidalizing C(Et) carbon points to.
Apart from that, they share a similar structure and are
3.6. Calculation Results on t6. The XMCQDPT2
calculations, with a (10e,10o) active space, seven-state
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Figure 6. TA spectra of t6 in acetonitrile (left) and in n-hexane (right) upon photoexcitation at 316 nm. The initial evolution is ultrafast, τ < 0.1 ps
and includes the ESA decay, SE red shift and decay, and P rise at 350 nm (second frames). The P lifetime is extraodinarily long in n-hexane, τP =
1840 ps, while it is much shorter in acetonitrile, τP = 32 ps (third frames). At the bottom, late spectra (olive) allow derivation of the isomerization
yield Ytc = 20% and the cis spectra (orange).
averaging, and seven states in the model space, show that
photoexcited t6 relaxes to P without a barrier, in agreement
with the experiment. Unlike for 4, P here has a proper
stationary point located at 1.45 eV below the vertical
excitation. However, its geometry is significantly different
from those calculated in t0 and t510,17 and from the crossing
points in t4 and c4. While there is a pronounced
desymmetrization between the halves of the molecule and
the dipole moment exceeds 13 D, there is almost no
pyramidalization in the P state. On the contrary, the S1/S0
crossing point is pyramidalized (see the Supporting Informa-
tion) as in other stilbenes. The S1/S0 minimum energy crossing
point is calculated to be 50 kJ/mol higher than the minimum
of the P state. Compared to the experimental P → S0 barrier of
24 kJ/mol, this is a twofold overestimate due to, perhaps, a
relatively scarce basis set. However, this increased relaxation
barrier illustrates a trend that is discussed below.
While the trans and cis sites of S1 in the Franck−Condon
geometry are not affected by the solvent polarity, the potential
of the P state lowers in acetonitrile. As a consequence, the
intersection between the trans (or cis) isomer and P potential
shifts toward the stationary point of the untwisted config-
uration, and the isomerization barrier Eb to P reduces. The
subsequent transition to S0 follows nearly instantaneously.
Recall that, for c0, the S1 → P isomerization barrier is strongly
reduced (Eb = 4 kJ/mol in the experiment or no barrier in the
computations), resulting in a fast S1 → P transition with
solvent-dependent τi = 0.4−0.9 ps. For c4, this is not the case;
the S1 → P barrier remains high, contrary to what happens in
other stilbenes.
The dependence of the P lifetime on solvent polarity is
apparent in t6 and other substituted stilbenes6−8 and can be
explained in Figure 8 (bottom). The aforementioned lowering
of the diabatic P potential in polar acetonitrile affects the S1/S0
crossing (pyramidalized) that approaches the stationary point
within P (nonpyramidalized), resulting in the barrier lowering.
Next, compound t4 represents the smallest homologue in
the series of bridged stilbene derivatives. Due to an increase in
the S1 torsional barrier of ∼2−3 kJ/mol (Table 1), the
progression to P is delayed by a factor of 2−3 in n-hexane (τi =
166 ps) and acetonitrile (τi = 114 ps). The increase in Eb can
be ascribed to the reduced steric interactions compared to t0,
where the repulsion between phenyl and ethylenic hydrogens
disfavors the planar trans configuration.
4. DISCUSSION
First, we compared the TA results on 4 and t6 with parent
stilbenes 0 and 5. For t0 in n-hexane, the S1 → P torsion
occurs with τi = 84 ps over barrier Eb = 16.7 kJ/mol. In
acetonitrile, the barrier is reduced to Eb = 14.9 kJ/mol as the P
state is zwitterionic and stabilized in polar solvents. The case of
solvent-induced stabilization for t0 and t4 is schematically
approached by diabatic potentials, as shown in Figure 8 (top).
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Figure 7. Evolution kinetics of t6 in n-hexane (left) and acetonitrile (right). Top panels show that an ultrafast S1 → P transition is recognized by S1
decay (blue) and P development (red). Red-shifted SE is seen as a negative signal about t = 0.2 ps. The P → S0 decay is shown at the bottom, with
τP = 1840 ps in n-hexane and 35 ps in acetonitrile.
Table 3. Lifetime τP of Perpendicular State P and P → S0
Barrier Eb2 in Stilbenes
Switching to trans stiff t5 with two bridging methylene
units,10 here, more flexible five-membered rings sterically
interact with the opposite phenyl rings, giving rise to a broader
extinction spectrum. The steric effects reduce the S1 → P
torsional barrier to Eb = 10 kJ/mol in n-hexane and 7 kJ/mol in
acetonitrile (Table 1), and the photoisomerization time τi
decreases to 5 and 2 ps, respectively.
The last representative t6 further follows this trend. In both
solvents, torsion to P proceeds without a barrier, reflecting the
planar conformation to be highly unstable in S1. The very
Figure 8. Diabatic energy surfaces to understand the experimental
proximity of the phenyl rings to the tert-butylene units causes
strong steric repulsion, yielding a broad distribution of adopted
dihedral angles distinct from 180° (planarity). This large
structural variety is reflected in a broadened extinction
spectrum, which also involves an amplitude drop down. As
the twisted configuration must already be largely attained in S0,
torsion in S1 should proceed very fast, and the formation of P
becomes visible in the transient spectra. The corresponding
ESA band is identified by comparing to other stilbenes.4,6−8
The ring size, likewise, affects the stability of the excited cis
conformation. In the almost planar excited c4, where the
results. For t4 (top), the S1 → P barrier Eb1 is relatively high lowers in
acetonitrile compared to n-hexane, while the P → S0 barrier Eb2 is
small enough to assure the same bleach recovery time as the S1 → P
time τi. On the contrary, for t6 (bottom), the S1 → P barrier Eb1 is
negligible, whereas the P → S0 barrier Eb2 is substantial and depends
on solvent polarity (lowers in acetonitrile relative to n-hexane).
opposite phenyl rings interact only marginally, the torsional
lifetimes (τi = 64 ps in n-hexane and 45 ps in acetonitrile) are
comparable to those of t4 (τi = 166 ps in n-hexane and 114 ps
in acetonitrile). Another consequence of the structural rigidity
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and planarity is that the S0 and S1 absorption spectra of c4 and
t4 become similar in shape. However, due to a slight spectral
shift between both (∼1 nm), addition of a reasonable amount
of εt4 to the late TA bleach eventually produces the c4
extinction spectrum. Unfortunately, the accuracy of this
approach reduces with increasing spectral resemblance of the
isomers. The E/Z ratio in S0 calculated with S1 spectra gives
1:1 in both acetonitrile and n-hexane, in good agreement with
the NMR results (1:1). In less planar c5, the S1 → P barrier
lowers (see Table 2), and in t6, no stable cis isomer is
produced at room temperature in S0.
substitution motifs in stilbenes, one can tune the photo-
chemical behavior.
A rather technical aspect is addressed by the estimate of the
S0 population ratio of co-excited t4 and c4. Due to the
structural rigidity and planarity, their extinction spectra share
very similar shapes but are not identical. Conveniently, it
follows that the S0−S1 spectrum of c4 can be reconstructed
with the help of late TA spectra. Accordingly, upon excitation,
both isomers have their S1 states populated with known
probabilities, and the bleach bands of the corresponding
species reflect their S0 population. Alternatively, for purely
measured t4, the separation of trans from cis follows from their
distinct isomerization rate to P, with cis being accelerated by a
factor of 2 to 3. A global kinetic analysis allows to discern the
characteristic TA traces and to catch the corresponding
species-associated spectra.
The computational results of the present and previous works
suggest an explanation for the trends in the lifetime of the P
state. It turns out that the S1/S0 minimum energy crossing
point, through which the relaxation to S0 proceeds, is always
highly pyramidalized, so that the S0 potential energy surface
steeply approaches S1 along the pyramidalization coordinate.
The stationary point of the P state in S1 also shows a clear
tendency to pyramidalize, but it may be counterbalanced by
the steric strains brought about by the narrowing valence
angles at the pyramidalized site. Those strains become more
severe when the substituents are bulkier. In that case, the
stationary point should be less pyramidalized and thus farther
in terms of both energy and geometric changes from the
barrier at the S1/S0 crossing point. Indeed, the four-membered
ring in 4 keeps one of the valence angles below 90°, thus
providing more room for pyramidalization through narrowing
of the two other valence angles (see Figure 5). On the
contrary, in 6, the valence angles in the six-membered
stiffening rings remain at ∼115−120°, effectively preventing
the pyramidalization, even though the charge separation in the
molecule is as strong as in other stilbenes. Furthermore, as
seen from Table 3, simple alkyl substituents in nonstiffened
compounds deliver the same effect, and the P lifetime directly
correlates with their size. Thus, substitution at the ethylenic
sites provides convenient means for controlling the P lifetime.
However, at the same time, bulkier substituents likely reduce
the S1 lifetime through steric interaction. To decouple the two
effects, one may try a combination of ethylenic and ring
substitution that can alter the electronic structure of the trans
and cis domains in S1 and effectively increase the lifetime.9
Very interesting results are obtained for t6. First, the S1 → P
torsion here is fully barrier-less, similar to stilbenes with
olefinic substitutions.6−8 Second, the P state is very long-lived,
1840 ps in n-hexane, which is currently the longest lifetime of
the perpendicular state in stilbenes.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcb.9b00784.
■
S
Additional data on the results from stationary and
transient absorption, NMR measurements determining
the relative mole fractions of t4 and c4 in the mixtures,
and optimized coordinates for geometries from quantum
chemical calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: quickmaq@chemie.hu-berlin.de (M.Q.).
*E-mail: ioffe@phys.chem.msu.ru (I.N.I.).
■
ORCID
Martin Quick: 0000-0002-9029-8802
Sergey A. Kovalenko: 0000-0003-4278-9305
Notes
The authors declare no competing financial interest.
5. CONCLUSIONS
ACKNOWLEDGMENTS
Substitution plays an important role in the evolution of
stilbenes. We have recently demonstrated9 that the phenyl ring
substitution results in alteration of the electronic structure of
S1 in the vicinity of the Franck−Condon geometries. The
present data help one to rationalize the effects of substitution
at the central ethylenic sites. Comparison of stiffened stilbenes
t4, c4, and t6 with previously studied parent stilbene t0 and c0,
stiff stilbenes t5 and c5, and several other substituted stilbenes
reveals a dual steric effect of both the stiffening rings and
monovalent substituents. On the one hand, bulkier groups and
larger stiffening rings destabilize the excited trans and cis
conformation due to stronger steric repulsions between the
halves of the molecule and thus reduce its lifetime. On the
other hand, they hamper pyramidalization of the ethylenic
centers in the P state, which is a necessary prerequisite for
reaching the ground state through the S1/S0 crossing and
hence increases the P lifetime. Those details of the P-state
behavior also affect the trans-to-cis photoisomerization yield
that requires a separate study. Thus, by combining various
■
We thank the Deutsche Forschungsgemeinschaft for the
support. The research was carried out using the equipment
of the shared research facilities of HPC computing resources at
Lomonosov Moscow State University. O.K. thanks the
National Science Foundation for the support under grant
CHE 1566435.
DEDICATION
■
Dedicated to the memory of “Doc” Leo A. Paquette (1934−
2019).31,32
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