Rotamer-Specific Photoisimerization of Difluorostilbenes from

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Rotamer-Specific Photoisimerization of Difluorostilbenes from Transient Absorption and Transient Raman Spectroscopy Martin Quick, Alexander L. Dobryakov, Ilya N. Ioffe, Falko Berndt, Rainer Mahrwald, Nikolaus P. Ernsting, and Sergey A. Kovalenko J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09283 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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The Journal of Physical Chemistry

Rotamer-Specific Photoisimerization of Difluorostilbenes from Transient Absorption and Transient Raman Spectroscopy M. Quick,*,1 A. L. Dobryakov,1 I. N. Ioffe,2 F. Berndt,1 R. Mahrwald,1 N. P. Ernsting,1 and S. A. Kovalenko1

1

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany

2

Department of Chemistry, Lomonosov Moscow State University, Moscow, Russia

E-mail:

[email protected]

Abstract Photoisomerization of 2,2′-, 3,3′- and 4,4′-difluorostilbene (F2, F3, F4, respectively) in nhexane, perfluoro-n-hexane and acetonitrile is studied with broadband transient absorption (TA) and femtosecond stimulated Raman (FSR) spectroscopy, and by DFT/TDDFT calculations. F2 and F3 possess three rotamers (rotational isomers) each, while F4 has one single conformation only. These differences are reflected in TA and FSR spectra. Thus, F4 reveals a mono-exponential decay of TA with τ1 =172 ps in n-hexane, as expected for a single species. For F2 and F3, the decays are bi-exponential in all solvents, corresponding to two distinctly discerned rotamers or rotamer-fractions. Specifically for F2 in n-hexane τ1 = 357 ps (83 %), τ2 = 62 ps (17 %), and for F3 in the same solvent τ1 = 222 ps (57 %), τ2 = 81 ps (43 %). The weights in brackets agree with 1 ACS Paragon Plus Environment

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theoretically estimated ground-state abundances of the rotamers. Furthermore, a global fit of the TA and FSR data allows us to extract the spectra of the pure rotamers. The Raman spectra of S0 and S1 are in qualitative agreement with calculations.

I. Introduction The photophysics and photochemistry of fluoro-substituted stilbenes were first reported by Fischer and coworkers and Luettke and Rauch.1 The authors studied the absorption and fluorescence spectra in a wide temperature range concerning fluorescence and isomerization yields of five selected fluorostilbenes. One of them, 1,1′-difluorostilbene, has recently been investigated by us2 with broadband transient absorption (TA) and femtosecond stimulated Raman (FSR) spectroscopy.3-6 The substitution at the ethylenic position eliminates the photoisomerization barrier, resulting in an ultrafast trans-to-cis torsion. Thus, in n-hexane the isomerization proceeds with 0.4 ps, compared to 84 ps for unsubstituted stilbene.2 In the present paper we extend our study to ring-fluorinated trans-stilbenes, 2,2′-, 3,3′-, and 4,4′-difluorostilbene (F2, F3, F4, respectively) as depicted in Scheme 1. A new interesting feature here (in addition to the photoisomerization) is the rotational isomerism (rotamerism) about the 1-7 or 1′-7′ single bonds. As can be seen, F2 and F3 possess three rotamers, while F4 has one single conformer only, quite like in parent stilbene. Generally, the rotamers should have different photoisomerization barriers, as can be checked by time-resolved spectroscopy.

F

C

C

C

C

F H

H

F2 a

F2 s'

F

F

H

H

H C

C

C

F

C

H

F

H

H

F

F

C

C

F3 s

H F

C

C

F2 s

F

H F

H

F3 a

F3 s'

H

H F

C

F4

H

C

F

2 ACS Paragon Plus Environment

F

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Scheme 1. F4 has a single rotamer (rotational isomer) while F2 and F3 possess three: symmetric s and s′ and asymmetric a. The steric hindrance depends on the F…H distance which is especially small in F2s′. Therefore the latter should be less populated than F2s or F2a. For F3, however, the steric hindrance is negligible; hence all the three rotamers may be populated.

The beginning of rotamer photochemistry goes back to the early 1960s, when Cherkasov7 explained the spectral anomalies of n-vinylanthracene by a mixture of its rotamers. Later on, Havinga and coworkers detected rotamer-specific photo-products also from hexa-1,3,5-triene.8 Also transient methods were applied, here by Park and Waldeck9, who observed a nonexponential decay in 3,3′-dimethylstilbene which they ascribed to the coexistence of different rotamers in the excited state. A large review article was drafted by Mazzucato and Momicchioli which constituted the state of the art until 1991.10 And more recently, Saltiel et al.11 and Karatsu et al.12 reported rotamer-specific adiabatic cis-trans photoisomerization of 1-(2-Anthryl)-2phenylethene. Coming back to Scheme 1, it is instructive to clarify two points. The first one concerns the interconversion between the rotamers. Earlier calculations10 showed that the relevant barriers in the S0 state range about ~20 kJ/mol, corresponding to an interconversion time of ~1 ns. This is much shorter than the diffusional timescale of chemical reactions in solution, thus explaining why the common (ground-state) chemistry does not differentiate the rotamers. However, the picture changes in the excited state. Upon S 0 → S 1 excitation, single bonds 1-7 and 1′-7′ strengthen, thus freezing the ground-state structure of the rotamer, while the ethylenic bond weakens allowing for a fast (~100 ps) photisomerization. This is the so-called principle of nonequilibration of excited rotamers (NEER)10 that seems to be valid for many conjugated systems, including those considered here. The second point deals with the choice of a substituent group. The fluorine atom has a relatively small van der Waals volume, 10 Ǻ3, compared to 28 Ǻ3 of the rather large methyl group. Accordingly, the steric hindrance is minimized by fluorines, so that a stable planar rotamer structure is preferred. Nonetheless, in F2s′ the fluorines are tightly spaced to the ethylenic hydrogens what destabilize this rotamer in favor of F2s. In case of F3 the influence on



steric hindrance should be negligible and both, F3s and F3s′, may be populated in similar amounts. The rest of the paper is organized as follows. Section II reports our quantum-chemical calculations in the S0 and S1 state of the rotamers. Experimental results are presented in section III, where we first discuss the symmetric F4 and then the rotamers of F2 and F3. A comparison of experimental results with quantum-chemical calculations is discussed in detail in Section IV.


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II. Quantum-Chemical Calculations Electronic ground S0 and excited S1 states of F2, F3, F4 were studied at the DFT/TDDFT level of theory using the Firefly package13 partly based on the GAMESS(US) source code14, and with Gaussian09.15 The Firefly package was used for (TD-)PBE0/Def2-TZVPP calculations of the twisting barriers of rotamers in the S1 state and of non-resonant Raman activities in S0 and S1. The TDDFT calculations were performed with frozen chemical core. The Gaussian09 was employed to supply mPW2PLYP/Def2-TZVP data for the relative ground state stability of the rotamers in F2 and F3, in order to verify the qualitative agreement between the hybrid and double-hybrid exchange-correlation functionals. The task of accurately calculating relative rotameric abundances is complicated by low frequency modes due to rotation of the phenyl rings. Contributions of these modes to the thermodynamic functions can be comparable in magnitude to the energy difference between the rotamers, so possible inaccuracies may considerably distort the picture. Obviously, the phenyl rotational modes cannot be treated harmonically, but ambiguities regarding the exact shape of the S0 energy surface, and possible anharmonic coupling between vibrational modes16-18 suggest that even sophisticated approaches may still lack reliability. In view of that we restrict ourselves to comparison of the electronic energies with conventional harmonic ZPE correction. In the case of F2, PBE0 and mPW2PLYP predict slightly different exact stationary point geometries for the F2s rotamer. At the PBE0/Def2-TZVPP level, F2s has isoenergetic C2h and C2 minima (in the C2 case, phenyl rings are slightly rotated out of plane). At the mPW2PLYP/Def2TZVP level, a nonplanar Ci minimum is found instead of the C2h one, and the C2 and Ci minima are again isoenergetic. The F2s' rotamer is found to be planar with both exchange-correlation functionals. Asymmetric F2a shows no nontrivial symmetry; the geometry of its respective halves resembles those of F2s' and of nonplanar F2s. Considering symmetry numbers and chirality, the statistical factors to the rotameric abundance of F2s:F2a:F2s' thus are 3:4:1 at the PBE0 level, and 4:4:1 at the mPW2PLYP level. The calculated energies and abundances are reported in Table 1. We also checked the ground state barriers between the rotamers, and found them in agreement with earlier findings10 to be ~20 kJ/mol, thus suggesting that the femtosecond experiment indeed deals with an equilibrated ground-state rotamer mixture.



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In the case of F3, the lack of direct F…H interactions with ethylenic hydrogens results in higher geometric similarity and lower energy differences between the rotamers. F3s and F3s' are both found to be either C2h (PBE0) or C2-symmetric (mPW2PLYP), F3a being Cs or C1symmetric, respectively. The abundances are comparable, yet with somewhat higher content of F3a and F3s′. Table 1. Ground State Energies and Abundances of F2 and F3 PBE0/Def2-TZVPP

mPW2PLYP/Def2-TZVP

Rotamer

Erel+ZPE kJ/mol

Stat. factor

Abundance

Rotamer

Erel+ZPE kJ/mol

Stat. factor

Abundance

F2s (C2h +C2)

0

3

71 %

F2s (C2+Ci)

0.0

4

71%

F2a (C1)

3.3

4

25 %

F2a (C1)

2.8

4

23%

F2s' (C2h)

4.1

1

4%

F2s' (C2h)

2.7

1

6%

F3s (C2h)

1.8

1

17 %

F3s (C2)

1.3

1

19%

F3a (Cs)

1.0

2

47 %

F3a (C1)

0.7

2

49%

F3s' (C2h)

0.0

1

36 %

F3s' (C2)

0.0

1

32%

The photoisomerization barriers S 1 → P , from trans to perpendicular conformation P are calculated at TD-PBE0/Def2-TZVPP level and collected in Table 2.

Table 2. S 1 → P Isomerization Barriers Eb (TD-PBE0/Def2-TZVPP) stationary point symmetry in S1

Eb (kJ/mol)

F2s (C2h)

10.1

F2a (C1)

5.8

F2s' (C2)

2.2

F3s (C2)

6.5

F3a (C1, nearly Cs)

7.3

F3s' (C2h)

8.0


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F4 (C2)

8.0

Although the barriers are systematically underestimated, they still reveal the correct relative trends. In all rotamers, the optimized transition state geometry was highly similar, with twisting angle between 131° and 127° degrees (except for nearly 137° in F2s').

III. Experimental Results Experimental. Our TA setup with applications has been described elsewhere.2-4 The probe range is 275-690 nm, with a 0.1 ps pump-probe intensity cross-correlation time (fwhm), and timing precision of 0.02 ps. Solutions of F2, F3 and F4 (each with a 30 µM concentration) in n-hexane, acetonitrile and perfluoro-n-hexane (pfh) flow through a sample cell of a 0.4 mm internal thickness. TA spectra at the magic angle, ∆A(λ , t ) , are recorded upon S 0 → S1 optical excitation, λexc = 325 nm, or 310 nm for pfh. The spectra are also measured for parallel ∆A|| (λ , t ) , and perpendicular ∆A⊥ (λ , t ) pump-probe polarization. Transient anisotropy is calculated as

ρ (λ , t ) = ( ∆A|| − ∆A⊥ ) /( ∆A|| + 2 ∆A⊥ ) = ( ∆A|| − ∆A⊥ ) /(3∆A)

(1)

The FSR setup5,6 is similar to that for TA. The picosecond Raman pump, of ~10 cm-1 width and ~0.1 µJ energy, is tuned to λR = 621 nm. The polychromators are adjusted to cover a 1000 cm-1 probe range. Stokes Raman signals are recorded by chopping the Raman-pump beam, with actinic excitation at λac = 313 nm. In this registration scheme, signals at negative delays correspond to S0 Raman contributions from both solute and solvent. For positive delays these contributions are eliminated by subtracting the time-averaged signal at negative delays. The FSR spectra are recorded with parallel actinic/Raman/probe polarization. The magic-angle Raman signal ∆A(λ , t ) = (∆A|| + 2∆A⊥ ) / 3 is recalculated with the help of (1)

∆A(λ , t ) =

∆A|| (λ , t )

(2)

2 ρ (λ , t ) ⋅ f + 1

where ρ (λ , t ) is taken

from the TA data, and f ~ 0.4 is the maximum anisotropy (when

absorption and emission dipole are collinear), a factor which is adjusted to eliminate the anisotropy decay component in ∆A (see Fig. S0). 7 ACS Paragon Plus Environment


Ground-State Spectra. Fig. 1 displays extinction spectra ε(λ) together with normalized stimulated emission spectra E (λ ) = λ4 F (λ ) of F2, F3 and F4 in n-hexane. Recall that the λ4 factor converts the originally measured fluorescence quantum distribution F (λ ) into the corresponding spectrum for stimulated emission (SE). The spectra from F2 and F3 contain contributions from different rotamers, which cannot be easily discerned.

Absorption and SE spectra of F2, F3, F4 in n-hexane 20

F2

10 -1

normalized stimulated emission (SE)

-1

Extinction coefficient ε (1000 cm M )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

F3

10

20

F4 10

225

250

275

300

325

350

375

400

425

450

wavelength λ (nm)

Fig. 1.

Extinction spectra ε(λ) and normalized SE spectra E (λ ) = λ4 F (λ ) in

n-hexane, where F (λ ) are fluorescence spectra.

Fig. 2 shows S0 Raman spectra of F2, F3 and F4. Like in parent stilbene strong Raman activity is seen above 1000 cm-1. The intense lines can be attributed to C-C- and C=C stretching modes.19,20 Signals at lower frequencies are very weak. The Raman spectrum of F4 was previously calculated by Negri and Orlandi.21 Our calculations for different rotamers are depicted with vertical bars in Fig. 2, at right. A qualitative agreement with the experiment is


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apparent, yet, not fully satisfying since the S0 Raman spectra of the individual rotamers are not directly accessible.

S0 FSR spectra, and calculated spectra of rotamers

S0 FSR spectra in n-hexane 0.0

0.0

-0.4

-0.1

F2

-0.2

-0.8 differential absorbance ∆A (mOD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

-0.4

F3 -0.8 0.0

calculated spectra s a s'

F2

0.0

-0.1

F3 -0.2

0.0

-0.1

-0.4

F4 F4

-0.2

-0.8

200

400

600

800

1000 1200 1400 1600

200

400

-1

600

800

1000 1200 1400 1600 -1

Stokes wavenumber, ν (cm )

Stokes wavenumber, ν (cm )

Fig. 2. Ground-state S0 Raman spectra of F2, F3 and F4 in n-hexane from FSR spectroscopy5,6 with Raman excitation λR = 621 nm (at left) and the same extended spectra together with the calculated spectra of rotamers (at right).

Spectral Evolution of F4 in n-hexane. To get an overview of the general transient evolution of fluorostilbenes, it is convenient to start with F4 which exists as a single rotamer only. Its TA spectra and kinetics in n-hexane are shown in Fig. 3. 9 ACS Paragon Plus Environment


F4, TA spectra in n-hexane, λ exc = 325 nm 120

40 0

ESA(FC)

delay t (ps) -0.08 -0.04 0 0.04 0.08

80

F4, TA decay kinetics in n-hexane

I(275,315) global fit

-10 -15

bleach

SE(FC) 120

ESA

0.1 0.5

80

FCrelaxation τFC = 0.2 ps

40 0 SE 120

576

2, 50 ... 400 ps in 50 ps steps

80

-20

-5

SE I(325,390) global fit

-10 -15

nearly monoexponential decays with τ1 = 172 ps

-20 40 30

isomerization τ1 = 172 ps

40

bleach

-5

differential absorbance ∆A [mOD]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ESA 20

∆A(tinf) x50

I(410,690) global fit

10

0

300

350

400

450

500

550

600

650

0

100

probe wavelength λ [nm]

200

300

400

500

600

700

pump-probe delay t [ps]

Fig. 3. TA spectra ∆A(λ , t ) of F4 in n-hexane upon S 0 → S1 optical excitation at T = 21 °C (at left). Bleach and SE are negative, ESA is positive. Early Franck-Condon (FC) spectra develop (on top) within the instrument response and relax with τFC = 0.2 ps (middle). The overall decay with τ1 = 172 ps (bottom) is due to the trans-tocis isomerization consisting of two steps: the S1 → P twist to the perpendicular conformation P, followed by the P → S0 relaxation. Cis- and trans-products are seen in the bleach region at late time (400 ps, in green). The

associated spectrum to the Heaviside function contains spectral pattern whose evolution exceed our time-window (∆A(tinf), in brown). Traces of dihydrophenantrene (DHP) and triplets are indicated within. Decay kinetics of bleach, SE and ESA are expressed via band integrals I ( λ1 , λ 2 ) of Eq. (3) (at right). The global fit describes the kinetics as nearly mono-exponential with τ1 = 172 ps. A third exponential time-function with τf = 2.4 ps is added


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to improve the fit at early time. This component contributes with only 3 % and describes a small rise in ESA, probably due to changing oscillator-strengths during the relaxation in S1.

Let us consider the transient spectra at left. Early Franck-Condon (FC) spectra about timezero develop within the pump-probe cross-correlation (top). Negative signals originate from bleach and stimulated emission (SE), while excited-state absorption (ESA) is positive. Arrows indicate the direction of signal evolution. Relaxation away from the FC region occurs with τFC = 0.2 ps (middle). It is seen by vanishing SE structure (around 325 nm) and as a small red shift of ESA. Not directly recognizable in the TA spectra is a small intensity-gain of ESA (by 3 %) with τf = 2.4 ps. The corresponding decay associated spectrum (DAS) exhibits a negative sign in this region describing a rise of optical density. Such a phenomenon can be attributed to an increase in oscillator-strength of the SnS1 transition as part of relaxation in S1. Afterwards, the bleach, SE and ESA decay nearly mono-exponentially with τ1 = 172 ps (bottom). The isomerization products, i.e. cis- and trans-isomers, are evident in the bleach region, λ < 330 nm at infinite delay times. In addition, there are traces of triplets and possibly dihydrophenanthrene (DHP) in the range of 400 - 500 nm. The latter might be formed from the cis-configuration that is populated via an adiabatic trans-cis isomerization. Generally, the spectral bands and their evolution are very similar to those of trans-stilbene in n-hexane,22,23 although the decay time in the present case is nearly twice as long. This decay is associated with thermally-activated

S1 → P intramolecular torsion, when the molecule acquires the perpendicular conformation P. The subsequent P → S 0 step, that completes the isomerization in the ground state, should be ultrafast (< 1 ps) and is not resolved here. Consistently, the bleach recovery proceeds with the same τ1 = 172 ps, without delay. Note, that the P → S 0 step can be well resolved when the

S1 → P reaction is ultrafast.2,15,16 The temporal evolution of bleach, SE and ESA is displayed in Fig. 3, at right. Here the experimental trace is expressed by band integrals

I (λ1 , λ2 ; t ) =

1

λ2

ln(λ2 / λ1 ) λ∫1

∆A(λ , t )dλ / λ

(3)

where (λ1,λ2) is the spectral region of interest. Solid lines represent a global fit [ a 1 ( λ ) exp( − t / τ 1 ) + a 0 ( λ ) exp( − t / τ f )] that is nearly mono-exponential in the present case



yielding the main parameters: a1 = 0.97 and τ1 = 172 ps. Such mono-exponential behavior is indeed expected for a single species.

FSR Spectra of F4 in n-hexane. Time-resolved excited-state Raman spectra of F4 in n-hexane are shown in Fig. 4. They are recorded on the Stokes side of Raman excitation (λR = 621 nm) upon optical pumping with λac = 313 nm. Early FC spectra (from -0.08 to 0.08 ps) develop on top, followed by FC relaxation in the middle and an overall decay at the bottom with τ1 = 140 ps. The deviation from the TA decay, τ1 = 172 ps, is probably due to the recalculation (2) of the magic-angle Raman signal from parallel polarization.


Page 12 of 29

Page 13 of 29

F4, S1 Raman spectra in n-hexane 0.0

-0.4

delay t (ps) -0.08 -0.04 0 0.04 0.08

-0.8

-1.2

differential absorbance ∆A (mOD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 -0.4 0.1 ... 0.5 ps in 0.1 ps step

-0.8

FC relaxation τFC = 0.5 ps

-1.2

0.0 -0.2 -0.4

2, 50 ... 400 ps in 50 ps step

-0.6

Isomerization τ = 140 ps

-0.8 -1.0 200

400

600

800

1000

1200

1400

1600

-1

Stokes wavenumber ν (cm )

Fig. 4. S1 Raman spectra of F4 in n-hexane upon actinic excitation at λexc = 313 nm and Raman pumping with λR = 621 nm. An early decay with τFC = 0.5 ps (middle) reflects the FC relaxation and the following mono-exponential decay with τ1 = 140 ps (bottom) corresponds to the trans-cis photoisomerization.

Spectral Evolution of F2 and F3 in n-hexane. TA spectra of F2 and F3 are displayed in Fig. 5. Top frames again show a very early spectral development, while the FC relaxation is shown in the middle and the bottom displays the overall decay.



F3, TA spectra in n-hexane

F2, TA spectra in n-hexane 100

40 20

60 40 20

bleach

0

ESA(FC)

delay t (ps) -0.08 -0.04 0 0.04 0.08

80

bleach

-20

-20

567

SE(FC)

100

0.1 0.5

80 60 FCrelaxation τFC = 0.4 ps

40 20 0 -20

ESA

SE

100

2, 50 ... 400 ps in 50 ps step

80

120

573

SE(FC)

100

0.1 0.5

80 60

FCrelaxation τFC = 0.2 ps

40 20 0 -20

ESA

SE

100 2, 50 ... 400 ps in 50 ps step

80

60

60

biexponential isomerization 40 τ =357 ps (83%) 1

20

100

-0.08 -0.04 0 0.04 0.08

60

0

ESA(FC)

delay t (ps)

80

differential absorbance ∆A [mOD]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

τ2= 62 ps (17%)

20

0

biexponential isomerization τ1=222 ps (57%) τ2 =81 ps (43%)

0 ∆A(tinf) x50

-20 300

350

400

450

500

550

600

650

∆A(tinf) x50

-20 300

350


400

450

500

550

600

650


Fig. 5. TA spectra of F2 and F3 in n-hexane upon S 0 → S1 optical excitation at 325 nm (T = 21 °C). Both, F2 and F3, possess three rotamers suggesting tri-exponential decays for both molecules. Yet, the experimental decays are bi-exponentially indicating the significant contribution of only two rotamers in each case.

By a first view into the spectra, the apparent locations of the ESA bands at early delay times are 567 nm for F2 and 573 nm for F3. However, since both fluorostilbenes exist divided into several rotamers these numbers cannot be attributed specifically. Now, let us move on to the spectral decomposition into the different rotamers via a proper processing of the TA data. Like for F4, a global fit with multiple exponential time-functions (plus offset) is applied. This time, the final decay occurs bi-exponentially in both cases; actually corresponding to two rotamer14 ACS Paragon Plus Environment

Page 15 of 29

fractions instead of three. To visualize this finding band-integrals are calculated over bleach, SE and ESA bands for pure data and global fits, respectively, and are shown together in Fig. 6.

F2, decay kinetics in n-hexane

F3, decay kinetics in n-hexane

-4

-5

-8

-10

bleach I(275,315)

-12 -16 -4

band integral 2exp fit

-8

SE I(325,390)

-12

τ1= 357 ps (83%)

-16

bleach I(275,315)

-15

τ2= 62 ps (17%)

-20 32


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5

band integral 2exp fit

-10

SE I(325,390)

-15

τ1=222 ps (57%) τ2= 81 ps (43%)

-20 30

24 20

ESA I(410,690)

16

ESA I(410,690)

10

8

0

200

400

600

800 1000 1200 1400

0

100 200 300 400 500 600 700 800 900

pump-probe delay t (ps)

pump-probe delay t (ps)

Fig. 6. Bleach, SE and ESA kinetics of F2 and F3 in n-hexane. The signals are fitted globally and mainly expressed by two exponential time-functions [ a1 ( λ ) exp( − t / τ 1 ) + a 2 ( λ ) exp( − t / τ 2 )] . Again, a third exponential time-function with τ = 3.2 ps is added to improve the fit at early delay-times for both, F2 and F3. The corresponding DAS indicate a rise of ESA by 5 % which is why an attribution to a third decaying rotamer can be ruled out. The bi-exponentiality reflects two discerned rotamers instead of the three expected ones.

The fit yields for F2: τ1 = 357 ps (83 %), τ2 = 62 ps (17 %), and for F3 it does: τ1 = 222 ps (57 %), τ2 = 81 ps (43 %). The relative pre-factors are given in brackets and represent the relative abundances of the rotamers (see F for procedure). First of all, a third time-constant could not be 15 ACS Paragon Plus Environment


detected and has probably disappeared in instrumental noise. The reason for the absence differs according to which fluorostilbene is regarded. For F2, an explanation is given by the low abundance of the energetically unfavorable F2s' rotamer (4 – 6 %). Even though a third time-constant with τf = 3.2 ps is indeed obtained the corresponding DAS suggests an intensity rise of ESA by 5 % instead of a decay. Furthermore, according to the quantum-chemical calculation an isomerization barrier in S1 (Eb = 2.2 kJ mol-1) corresponds to a decay-time of 15 ps, not 3.2 ps. The predicted large gap between rotamers F2s (71 %) and F2a (23 – 25 %) is also reproduced in the actual measurement. A preliminary assignment of isomerization rate to a specific rotamer (and symmetry) shall be done at this point: asymmetric F2a isomerizes faster (τ2 = 62 ps) than symmetric F2s (τ1 = 357 ps). At this point, two observations should be summarized in order to argue any further. First, the symmetric fluoro-stilbene F4 has a life-time in n-hexane that is twice as large as unsubstituted stilbene (172 ps vs. 80 ps). Second, while the symmetric F2s also lives longer, it is the asymmetric F2a on the contrary that decays even faster. We assume that fluorination that leads to a symmetric rotamer stabilizes the trans-configuration while asymmetry results in destabilization. For F3, it can be concluded that a missing third rotamer might be ascribed to very similar decay constants of F3s and F3s'. According to our calculation their abundances in S0 are significant and close. Also, their similar isomerization barriers reason for close-lying timeconstants. Both abundances together represent a bit more than half of the total amount of rotamers. Consequently, the photoisomerization of asymmetric F3a proceeds faster (τ2 = 81 ps) than of equally evolving F3s and F3s’ (τ1 = 222 ps). In any case, these results demonstrate that TA measurements do allow one to discern rotamers via their different decay times. The global fit results from measurements in three solvents are collected in Table 3.


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rotamer-Associated TA Spectra. The rotamer-associated TA spectra (or species-associated spectra; SAS) are directly given by the decay-associated spectra of the global fit. They are shown in Fig. 7, for F2 on top and for F3 at the bottom. For comparison, the spectrum of F4 is also displayed here at the bottom.

Rotamer-associated S1 spectra 555

1

568

F2 s (83%) F2 a (17%) ESA

0 SE 570 576

1

F3 s (57%) F3 a (43%) F4 ESA 0 SE 320 360 400 440 480 520 560 600 640 680 probe wavelength λ (nm)

Fig. 7. Discerned rotamer spectra of F2 and F3 in the S1 state. The F4 spectrum is also shown for comparison. The rotamer abundances (in brackets) are obtained by the global fit.

Starting with F3, the general spectral shape of the two SAS is similar to F4. There is a shift of 6 nm (180 cm-1) in the ESA region between the two SAS of F3 that marks a second distinctive feature, besides the different time-constants. It is rather small since all rotamers of F3 are structurally similar. The SE bands from the two rotamers coincide spectrally confirming that no distinguishable fluorescence bands are observed with excitation-dependent fluorescence measurements (see Fig. S2). 17 ACS Paragon Plus Environment


The shift between the ESA bands in case of F2 is 13 nm (410 cm-1), larger than for F3, reflecting the stronger difference between the symmetric and asymmetric rotamer structures. To derive the relative amounts given in E a band integral is calculated over the ESA band of each rotamer. In general, this integral is proportional to the overall transition probability between S1-Sn and therefore reflects the S1 population. We assume that the oscillator-strengths of the S1-Sn transition are similar alongside rotamers of the same fluorostilbene. Relative abundances are obtained when band-integrals of F2- or F3-related rotamers are brought into proportion. As outlined earlier these relative amounts can be demised to the ground-state because of the NEER principle.

FSR Spectra of F2 and F3. All determined time-constants and abundances from TA ought to be roughly reproduced with FSR spectroscopy. Measurement conditions are the exact same as for F4. The evolution of the S1 Raman spectra is depicted in Fig. 8. In both fluorostilbenes the initial sub-picosecond relaxation (not shown) is followed by a bi-exponential decay of S1 Raman signals. Resultant time-constants from a global fit agree well with those from TA. For F2, the fit yields τ1 = 357 ps (81 %), τ2 = 62 ps (19 %), and for F3 it does: τ1 = 222 ps (47 %), τ2 = 81 ps (53 %). It is this consistency in time-constants which further allows the attribution of S1 Raman base-spectra to corresponding rotamers, as it is done in the captions of Fig. 9. Brackets contain the relative abundances that are estimated with FSR measurements (see H for procedure). As usual, the quality of the fit is expressed by band-integrals that are calculated for the entire spectrum (50 – 1650 cm-1) and for the fit-functions themselves (see Fig. S8). Unlike TA, the coexistence of the two rotamers is already recognized here when the early spectrum (at 1 ps) is compared with the spectrum at 150 ps. See for example the faster vanishing low-frequency signal from the double-peak at 240 cm-1. However, such an obvious picture is not present for F2 since one rotamer dominates the spectrum with ~80 %.


Page 18 of 29

Page 19 of 29

F3 in n-hexane, FSR spectra in the S1 state

F2 in n-hexane, FSR spectra in the S1 state 0.0

0.0

-0.1

-0.2

1, 24 ... 120 ps in 30 ps steps

-0.2

-0.3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fast decay τ2 = 60 ps

-0.4

0.0

-0.1

150 ... 350 ps in 50 ps steps slow decay τ1 = 360 ps

-0.2

200

400

600

800

1000

-0.4

1, 30 ... 150 ps in 30 ps steps

-0.6

fast decay τ2 = 80 ps

-0.8 0.0

-0.1

150 ... 400 ps in 50 ps steps slow decay τ1 = 220 ps

-0.2

1200

1400

1600

200

400


600

800

1000

1200

1400

1600


-1

-1

Fig. 8. S1 Raman spectra of F2 and F3 in n-hexane, upon actinic λexc = 325 nm and Raman excitation λR = 621 nm. The signal decays bi-exponentially, with the same time constants as in the TA measurements (Figs 5,6). Early decays (τ2, on top) are due to the asymmetric rotamers, F2a or F3a, and longer decays (τ1, at the bottom) are from the symmetric ones: F2s or F3s.

Rotamer-Associated FSR Spectra. To derive the species-associated Raman spectra (SARS) together with another estimate for the corresponding relative amount of the rotamers let us consider the fit results. Once again, the two decay-associated spectra that correspond to time-constants τ1 and τ2 coincide with the two SAS of the rotamers; but they are abbreviated SARS here. The SARS of F2 and F3, respectively, are normalized by the division through their respective mole fraction xi (relative amount from TA) and are presented in the top panels of Fig. 9. Calculated S1 Raman spectra are shown in the bottom. Note, that the intensities of lines from the low frequency region are systematically underestimated. The reason is that the calculations were performed with nonresonant Raman transitions.



We start with F3 at right, because the relative abundances of the discerned rotamer-fractions are similar. The fast decaying rotamer is depicted in red and the slower one in dark yellow. At first appearance differences are noticeable at various positions, for example at 240 cm-1 where the previously mentioned low-frequency signal from the double-peak is located. By kinetic analysis this signal can be discerned from the up-shifted signal (at 280 cm-1), and both are assigned to different rotamers. Another distinctive feature is seen for the bunch of signals that is located around 1200 cm1

. For the slower rotamer (dark yellow) these signals are better resolved than for the faster one. In

the left panels the SARS of F2 are shown in blue (faster rotamer) and cyan (slower rotamer). Note, that division by the mole-fraction xi has a stronger impact on the resulting relative ratio after normalization (blue: increase by factor ~5). Therefore, care should be taken for the less abundant rotamer F2a when it comes to the verification of certain lines. However, distinct spectral characteristics are also noticeable here, e.g. around 200 cm-1 and 500 cm-1. F3 in n-hexane, S1 Raman spectra of rotamers

F2 in n-hexane, S1 Raman spectra of rotamers 0.0


0.0

-0.1 -0.2 -0.3

Experiment -0.4

F2 s (83%) F2 a (17%)

-0.5

200

400

600

800

-0.2 -0.4

Experiment

-0.6

F3 s + F3 s' (57%) F3 a (43%)

-0.8 -1.0

1000 1200 1400 1600

200

400

600

800

1000 1200 1400 1600

0.0

0.0

-0.2

-0.2

x 25

-0.4 -0.6

arbitrary units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

Calculations F2 s (71%) F2 a (24%)

-0.8

x 50

-0.4 -0.6

Calculations

-0.8

F3 s + F3 s' (51%) F3 a (49%)

-1.0

-1.0

200

400

600

800

1000 1200 1400 1600

200

400


600

800

1000 1200 1400 1600


-1

-1

Fig. 9. Rotamer-associated Raman spectra (SARS) of F2 and F3 in the S1 state (top panels). The rotamer spectra are derived with the help of a global fit from the spectra shown in Fig. 8. In brackets are written the 20 ACS Paragon Plus Environment

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rotamer populations from TA by which the originally derived SARS are divided for normalization. The calculated rotamer spectra are shown at the bottom.

To derive the relative amount of the rotamers also from the SARS of F2 and F3 band-integrals are calculated over the complete spectral range 50 – 1650 cm-1. Starting with F3, the values are 0.53 for the faster rotamer (red, 0.43 in TA) and 0.47 for the slower one (dark yellow, 57 in TA). As one can see, the trend is now the opposite for F3, in favor of the faster rotamer. The SAS of F3 in Fig. 7 suggests that rotamer F3a exploits stronger Raman enhancement since the ESA-peak is shifted towards the Raman-wavelength of 621 nm. This enhancement in the signal generation over-compensates the fewer population in S1 and results in the present relative ratio. In case of F2, a value of 0.19 is found for the faster rotamer (blue, 0.17 in TA) and 0.81 is obtained for the slower one (cyan, 0.83 in TA). Here no significant change occurs because no preference in signal enhancement is apparent for any of these two rotamers (see Fig. 7).

Comparison of S1 Raman spectra of rotamers 0

Experiment

F2 a F3 a -1

200

400

600

800

1000 1200 1400 1600

0

Experiment

F2 s F3 s -1

200

400

600

800

1000 1200 1400 1600

Stokes wavenumber ν (cm ) -1



Fig. 10. Comparison of S1 Raman spectra of F2 and F3 n-hexane for asymmetric (top) and symmetric rotamers (bottom).

IV. Discussion and Conclusion Discerning rotamers via a global analysis. One of the main results of the present study is that different rotamers of ring-fluorinated stilbenes can be distinguished in the photoisomerization reaction by their different photoisomerization paths. This is valid both for TA and FSR spectra, each of which can be decomposed into the rotamer contributions (Figs. 7, 9). The decomposition is accomplished by a global multi-exponential fit, in which each component corresponds to a particular rotamer with its own specific photisomerization rate (Fig. 6). The procedure works of course only when the photoisomerization kinetics is mono-exponential for each rotamer. The latter is indeed true for symmetric F4 (Fig. 3) and F0 (parent stilbene),22 and is also consistent with the present results on F2 and F3. For these molecules, however, we discerned only two rotamer (-fractions) instead of the expected three. For F2 this generic result may be due to the low abundance of F2s' (4 - 6%, Table 1), whereas for F3 the abundances of F3s and F3s' are comparable. We therefore have to conclude that their isomerization rates are too close to be distinguished in the current experiment.

Associating experimental observations to rotamers. The actual assignment of SAS and timeconstants to symmetric and asymmetric rotamers follows from a comparison between experimental and calculated relative abundances (see section E). The asymmetric rotamers of F2 and F3 photoisomerize faster than the symmetric ones. Since the decay behavior from TA should be reproduced in the FSR-measurements the further attribution of S1 Raman base-spectra is straight forward.

Attributing Raman signals to calculated frequencies. At this point it would be interesting to compare the experimentally derived SARS with the calculated frequencies in order to find agreements. Therefore, S1 Raman spectra of F2 and F3, respectively, are shown in Fig. 9. Results from measurements are in the top panels while calculated Raman frequencies are presented within a spectrum in form of broadened lines in the bottom panels.

Raman signals of F3. We start with F3, on the right. Because the measured SARS of the symmetric rotamer presumably consists of F3s and F3s’, respectively, the calculated spectra of 22 ACS Paragon Plus Environment

Page 22 of 29

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the symmetric rotamers are also shown as a linear combination of these two rotamers (weighted with abundances from Table 1). The first thing noticed is a strong similarity between the symmetric and the asymmetric rotamer in both, the recorded and calculated spectra. This goes alongside with structural resemblances as all three rotamers are almost equally planar. A closer view on the top panel still reveals spectral differences. The F3a signals at 246 and 137 cm-1 are located in between lines from symmetric F3 at 278 and 115 cm-1. Going to the bottom, a similar pattern is present when F3a signals at 245 and 174 cm-1 are neighbored on the low- and highfrequency sides. Another distinct feature is apparent in the range 400 – 500 cm-1. While for F3a only one single line is observed, a double-peak shows up for F3. The same picture emerges in the calculated spectra. One final example is illustrated for the region around 1200 cm-1. On a first view, lines appear to be much better resolved for F3 than for F3a, but they actually originate from a larger amount of lines, since a combination of F3s and F3s’ is shown.

Raman signals of F2. A comparison with calculations confirms this observation. Let us now examine the S1 Raman spectra of F2 on the left. This time, the rotamers show rather distinct features in both, the measured and calculated spectra. For the measured spectra it must be stated, that normalization (division by mole-fraction) strongly affects the SARS of the less abundant rotamer F2a. With a factor of ~ 5 not only lines will be magnified but also the noise that could possibly be misinterpreted. Therefore, an attribution of observed signals to calculated frequencies is not too reliable. On the other hand, for F2s, there are plenty of good agreements that will be outlined in the following. Below 1000 cm-1 there are four Raman lines that coincide in the experiment and in the calculation: 856 (878), 678 (612), 508 (501) and 188 cm-1 (202 cm1

); where calculated frequencies are shown in brackets. In the high-frequency region one finds a

signal cluster around 1200 cm-1 on top and its prediction shown in the bottom panel, as well as the C=C stretching mode at 1565 cm-1. At this point, it appears to be appropriate to comment the discussed results regarding the limits of reasonable interpretation. First of all, FSR signals on the excited state usually appear with intensities of only a few tens of µOD. When exponential timefunctions are applied to the noisy data the trueness of the extracted SARS might be sophisticated by low signal strengths. Spectral pattern of the dominant species can mix into the SARS of the less abundant compound. This is the case for F2a which exists by less than 20 %. The attribution of S1 Raman signals to calculated frequencies therefore lacks in reliability. However, the other side of the coin is that the attribution to specific rotamers becomes unambiguous because 23 ACS Paragon Plus Environment


Page 24 of 29

experiment and calculation confirm and add to each other to the same clear trend. For F3 the opposite case is present. While symmetric and asymmetric rotamers appear with similar amounts (easier photophysical separation), there is no clear attribution by relative abundances, but the combination of FSR spectroscopy together with quantum chemical calculation now becomes a helpful tool to this task.

The stabilizing effect of fluorine on symmetric F2, F3 and F4. Table 3 collects TA results on F2, F3, F4 and F0 in n-hexane, perfluoro-n-hexane and acetonitrile. Consider F4 in n-hexane. Its photoisomerization time, τ = 172 ps, is two times longer than that of stilbene, 84 ps. When going to F3 and F2 we see for the slower components that the decay times increase even more: τ1 = 222 ps and τ1 = 357 ps, respectively. This trend allows one to draw the same conclusion as in the previous paragraph. The larger decay time τ1 must be associated with symmetric rotamers F3s and F2s (we do not distinguish between s and s' anymore), because the molecular structure of symmetric F4 correlates with that of F3s or F2s. Second, the ring-fluorination appears to stabilize the excited state of symmetric fluorostilbene and the stabilization increases when going from the fluorination at position 4 to 3 and then to position 2. The shorter time τ2 is ascribed to asymmetric F3a and F2a. Here the trend is just opposite: for F2a τ2 = 62 ps, shorter than τ2 = 81 ps for F3a. In other words F2a is more destabilized than F3a, probably due to the interaction with the ethylenic hydrogens. This trend of stability might also be reflected in the SARS in Fig. 10. Here, S1 Raman spectra of the same symmetry are shown together in one panel. It appears that there is better coincidence for line-positions in case of symmetric rotamers than for asymmetric ones. This might hint that the symmetric rotamers have a comprehensively similar structure that stabilizes the S1 state, quiet independent from the F-position.

Steric and electronic influences on rotamer evolution. The photoisomerization barrier Ej (for the twist from planar to perpendicular state P) can be estimated from the isomerization time τj using the Arrhenius relation

τ i = τ 0 exp(E j / kT )

(4)

where τ0 is the barrierless reaction time. For stilbenes a good estimate is τ0 = 0.1 ps.2 The barriers E i = kT ln(τ i / τ 0 ) calculated with this τ0 are collected in Table 3 together with the TDDFT data. Comparing them against the calculated TD-PBE0 barrier heights, one can see that 24 ACS Paragon Plus Environment

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the latter are apparently systematically underestimated. According to our tests, that seems to be a common problem of many widely used exchange-correlation functionals. In fact, the barrier heights in question are, perhaps, too small for reliable quantitative reproducibility at the TDDFT level. In parent stilbene, a remarkably good barrier estimate of 0.15 eV has been previously delivered by spin-flip TDDFT with the BHHLYP exchange correlation functional,24 but calculations of such type often suffer from undesirable spin contamination effects. Considering our computational estimates from the viewpoint of relative trends rather than absolute values, one can see that they corroborate some of the experimental observations. Thus, the difference between the calculated barrier heights for F2s and F2a agrees with their ca. 5.8-fold lifetime ratio. A pronounced increase in the calculated barrier height from F2s to F2a, and to the missing F2s' is likely due to essential differences in the steric F…H interactions with central ethylenic hydrogens.



Page 26 of 29

Table 3. Global Fit of TA data (at 21 °C) solvent

η

τR

τ1

a1

τ2

a2

F0

he

0.31

14

84

1

16.1

-

-

F2

he

0.31

14

357

0.83

19.0 (10)

62

0.17

14.7 (6)

5 (3.5)

F3

he

0.31

15

222

0.57

17.8 (8)

81

0.43

15.4 (7)

1.3 (1)

F4

he

0.31

16

173

1

17.2 (8)

-

-

F0

ac

0.36

17

38

1

14.0

-

-

F2

ac

0.36

18

86

0.73

15.5

32

F3

ac

0.36

20

90

0.61

15.6

F4

ac

0.36

22

88

1

F0

pfh

0.66

21

51

F2

pfh

0.66

24

F3

pfh

0.66

F4

pfh

0.66

E1

E2

-

a1 /a2

-

-

-

-

0.27

13.1

2.7

37

0.39

13.4

1.6

15.6

-

-

-

-

1

14.9

-

-

-

-

277

0.72

18.4

41

0.28

13.7

2.6

24

139

0.59

16.7

31

0.41

13.0

1.4

27

147

1

16.8

-

-

-

-

F0 is unsubstituted trans-stilbene; he, ac, pfh denote n-hexane, acetonitrile and perfluoro-n-hexane; η – viscosity in cP; τR – rotational diffusion time (in ps); τ1, τ2 – long and short decay constant (in ps) from global fit [ a1 exp( −t / τ 1 ) + a 2 exp( −t / τ 2 )] . Isomerization barriers E j = kT ln(τ j / τ 0 ) are in kJ/mol; τ0 = 0.1 ps, kT = 2.44 kJ/mol at T = 294 K. Calculation results are in brackets.


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Considering the rotamers of F3, however, one cannot find any obvious cause for the 2.7fold lifetime ratio, and the computations cannot reproduce it as well. Unlike F2, the rotamers are similar in terms of steric interactions. Possibly, the differences in F3 are due to effects of intramolecular induction interactions of the C-F dipoles with the π-system. In acetonitrile, the photoisomerization proceeds approximately two times faster than in nhexane. The effect is generally valid for many other stilbenes and is explained by the polar nature of the P state.2,22,23 Interestingly, in perluoro-n-hexane (pfh) the reaction is yet again faster than in n-hexane despite pfh is a strictly nonpolar solvent. The anomalous behavior may be induced

by

specific

solute-solvent

interactions

which

can

directly

activate

the

photisomerization.25

Acknowledgement We thank the Deutsche Forschungsgemeinschaft for financial support (grant ER 154/10-4). The computational research is partly carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University.

Associated content Supporting Information

Recalculation of the Raman signal into magic angle polarization (Fig. S1), excitationwavelength dependent fluorescence spectra (Fig. S2) and TA spectra/kinetics in other solvents are provided (Figs. 3 – 9). An S0 Raman spectrum (Fig. S10) and assignments of rotameric Raman signals in S0 and S1 are summarized in Tables S1 – S6.



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2

Dobryakov, A. L.; Quick, M.; Richter, C.; Knie, C.; Ioffe, I. N.; Granovsky, A. A.; Mahrwald, R.; Ernsting, N. P.; Kovalenko, S. A. Photoisomerization Pathways and Raman Activity od 1,1′Difluorostilbene, J. Chem. Phys. 2017, 146, 044501. 3

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4

Moreno, J.; Dobryakov, A. L.; Ioffe, I. N.; Granovsky, A. A.; Hecht, S.; Kovalenko, S. A. Broadband Transient Absorption Spectroscopy with 1- and 2-Photon Excitation J. Chem. Phys. 2015, 143, 024311. 5

Kovalenko, S. A.; Dobryakov, A. L.; Ernsting, N. P. An Efficient Setup for Femtosecond Stimulated Raman Spectroscopy, Rev. Sci. Instum. 2011, 82, 063102. 6

Dobryakov, A. L.; Ioffe, I.; Granovsky, A. A.; Ernsting, N. P.; Kovalenko, S. A. Femtosecond Raman Spectra of Cis- and Trans-Stilbene with Isotopomers in Solution, J. Chem. Phys. 2012, 137, 244505. 7

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9

Park, N. S.; Waldeck, D. H. On the Dimensionality of Stilbene Isomerization, Chem. Phys. Lett. 1990, 168, 379-384. 10

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Saltiel, J.; Zhang, Y.; Sears, Jr., D. F. Highly Efficient Conformer-Specific Adiabatic Cis-Trans Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene in S1, J. Am. Chem. Soc. 1996, 118, 2811-2817. 12

Karatsu, T.; Itoh, H.; Nishigaki, A.; Fukui, K.; Kitamura, A.; Matsuo, S.; Misawa, H. Picosecond TimeResolved Fluorescence Spectroscopy of (Z)-1-(2-Anthryl)-2-Phenylethene and Its Model Compounds: Understanding the Photochemistry by Distinguishing between the s-Cis and s-Trans Rotamers, J. Phys. Chem. A 2000, 104, 6993-7001. 13

Granovsky, A. A. Firefly v. 8.2, http://classic.chem.msu.su/gran/firefly/index.html

14

Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. et al General Atomic and Molecular Electronic Structure System, J. Comput. Chem. 1993, 14, 1347-1363. 15

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B. et al. Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009.

16

Kwasniewski, S. P.; Claes, L.; François, J.-P.; Deleuze, M. S. High Level Theoretical Study of the Structure and Rotational Barriers of Trans-Stilbene, J. Chem. Phys. 2003, 118, 7823-7836.


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17

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Rotamer-Specific Photoisimerization of Difluorostilbenes from

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