RESUMEN
Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(-|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S0 â S1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative sign indicates vibrational coherences in S1 or higher Sn, whereas the positive sign reveals coherences in S0 or Sn via S1 â Sn â Sm (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans-trans, cis-trans (ct), and cis-cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S1 or Sn. When approaching higher resonances Sn â Sm, some Raman bands switch their sign from negative to positive, thus, indicating new coherences in Sn. The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.
RESUMEN
The photoisomerization path and dynamics of trans-trans (ttD), cis-trans (ctD), and cis-cis (ccD) 1,4-diphenyl-1,3-butadiene (DPB) in solution are studied with broadband transient absorption (TA) spectroscopy and quantum chemical calculations. For ttD in n-hexane, 2-photon-excited TA spectra indicate that the 2Ag state is located above 1-photon allowed 1Bu (S1) by â¼1000 cm-1. Following S0 â S1 optical excitation, the isomerization occurs via torsion about a butadiene double bond to perpendicular molecular configuration P. The P-state is detected in ccD with an excited-state absorption band at 390 nm. This P-band develops during S1 â P half-torsion with time of 0.15 ps, followed further by P â S0 half-torsion and simultaneous decay with 1.6 ps in acetonitrile and 5 ps in n-hexane. In addition, two oscillation cycles between P and S1 population are observed before equilibration in n-hexane. For ctD, an indication of rising and decaying P is found in acetonitrile. The vast majority of ctD species photoisomerizes to ttD and not to ccD, in agreement a with calculated low torsional barrier about the cis double bond and high barrier about the trans double bond. Photoisomerization yield Y and time τi depend drastically on the solvent polarity. Thus, in n-hexane, the isomerization ttD â ctD has yield Ytt,ct = 0.1 and time τi = 829 ps, while in acetonitrile, Ytt,ct = 0.4 and τi = 27 ps. The 30-fold acceleration of the isomerization in acetonitrile clearly reflects a highly polar character of P, consistent with a dipole moment µP > 9.6 D. The results for DPB are discussed in comparison to stilbene.
RESUMEN
The rotamerism and photoisomerization of trans- and cis-1,2-di-(2-naphthyl)ethylene (tN and cN) are studied with stationary and transient absorption spectroscopies assisted by quantum chemical calculations. Absorption and emission spectra of rotamers (rotational isomers) tN-S (C2h-symmetric), tN-A (C1), and tN-S' (C2) are derived with a 53:47 ratio of tN-S to tN-A. Upon photoexcitation, the equilibration of the rotamers in S0 (rotamerization) is observed in the bleach region with characteristic time τrotamer ≈ 0.5 ns. With excitation at 364 nm, the S0 equilibrium shifts because, mainly, tN-A is bleached and the rotamerization becomes traceable, whereas with excitation at 345 nm, the equilibrium is preserved and the bleach spectrum remains unchanged. It is just long-lived (â¼2 ns) S1 that allows for monitoring the rotamer dynamics in S0. Replacement of the stilbene phenyl rings with larger naphthyls increases the S1 â P torsional barrier E1act toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with τR ≈ 3.7 ns becomes the main deactivation channel, and accordingly, the measured decays show nearly no dependence on the solvent viscosity. The cis-to-trans photoisomerization occurs via two paths: adiabatic cS1 â P â tS1 (20%) and more common nonadiabatic cS1 â P â S0 (80%). The barrier cS1 â P in the cis-isomer is reduced in polar solvents because of a zwitterionic character of P. The P-state is directly detected with the cN isomer in acetonitrile by an excited-state absorption band at 400 nm developing with 0.7 ps and decaying with 1.6 ps. Two dihydrophenanthrene (DHP)-like products result from photoexcited cN. The metastable DHP-A builds up transiently from cN-A, and its spectrum at about 550 nm matches the published DHP absorption. The stable DHP-S' accumulates under stationary illumination and is formed either from excited cN-S' or metastable DHP-A.
RESUMEN
We present the synthesis and crystal structure of a new chlorinated non-IPR isomer of C76 fullerene, 18387C76Cl30. The new chloride is formed from IPR-D2-C76via a sequence of chlorination-promoted Stone-Wales rearrangements concurrently with the previously known non-IPR compound 18917C76Cl24. A considerable difference between the two simultaneously forming non-IPR compounds suggests that higher fullerenes can exhibit a much higher versatility of skeletal transformations than the presently known compounds. Our theoretical analysis identifies yet further favorable Stone-Wales pathways in C76 and predicts several stable non-IPR and non-classical (i.e. heptagon-containing) C76 chlorides that may await experimental isolation.
RESUMEN
Photoisomerization of 2,2'-, 3,3'-, and 4,4'-difluorostilbene (F2, F3, F4, respectively) in n-hexane, 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 monoexponential decay of TA with τ1 = 172 ps in n-hexane, as expected for a single species. For F2 and F3, the decays are biexponential in all solvents, corresponding to two distinctly discerned rotamers or rotamer fractions. Specifically, for F2 in n-hexane, τ1 = 357 ps (83%) and τ2 = 62 ps (17%), and for F3 in the same solvent, τ1 = 222 ps (57%), and τ2 = 81 ps (43%). The weights in brackets agree with 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.
RESUMEN
The photoisomerization of 1,1'-difluorostilbene, following S0âS1 optical excitation in solution, was studied with femtosecond broadband transient absorption and stimulated Raman spectroscopy, and by quantum-chemical calculations. In n-hexane, trans-to-cis (tâc) isomerization starts with Franck-Condon relaxation (τ1t = 0.07 ps) followed by nearly barrierless torsion around the ethylenic bond (τ2t ≈ 0.3 ps) to a perpendicular conformation P. About 50% of the excited molecules are trapped in P, while others reach the S1(cis) conformation adiabatically. For the opposite cis-to-trans (cât) path, the dynamics in n-hexane (τ1c = 0.04 ps, τ2c = 0.7 ps) suggest a 5 kJ/mol barrier between the relaxed S1(cis) and P states. The subsequent P decay with τ3 = 0.4 ps is followed by much slower ground-state recovery (τ4 ≈ 3 ps), indicating an intermediate state X. The tâP and câP torsion depend on solvent viscosity and polarity, whereas the PâXâS0 relaxation and residual torsion is viscosity-independent but still polarity-dependent. Photoisomerization yields are derived from the transient absorption data and compared to those from actinometric measurements. Low-frequency oscillations in the transient signal are assigned to nuclei motions. Transient and stationary stimulated Raman spectra are compared to calculations. Early Franck-Condon Raman spectra differ from those of the quasistationary trans or cis S1 state. The photoisomerization behavior of stilbene and vinyl-substituted derivatives is compared and the general features are discussed.
RESUMEN
We report on a bistable azobenzene derivative with sufficiently high 2-photon absorption to induce its photochemical isomerization and measurable excited state dynamics. Broadband transient absorption spectra were recorded and compared upon 1-photon (331 nm) and 2-photon (640 nm) excitation of the S0 â S2 transition. The spectra are different at early (t â¼ 1 ps) and late (t â¼ 100 ps) time but show similar photoisomerization behavior on a 10 ps time scale. With 2-photon excitation, strong population transfer S2 â Sn occurs due to resonance absorption of a third pump photon. Subsequent internal conversion Sn â S1 results in a very hot S1 population causing extra-broadening of the transient spectra. The resonance pump absorption is common with nonlinear excitation and should be taken into account when considering photochemical applications. The 2-photon excitation cross-section σ((2)) at 640 nm was measured to be 7 GM for the specific tetra-ortho-fluorinated azobenzene derivative and 1 GM for unsubstituted parent azobenzene. The direct 2-photon induced trans-to-cis isomerization, described herein, provides an unprecedented potential for spatially addressing P-type (bistable) azobenzene photoswitches in 3D.
RESUMEN
1-photon (382 nm) and 2-photon (752 nm) excitations to the S1 state are applied to record and compare transient absorption spectra of a push-pull triphenylamine (TrP) dye in solution. After 1-photon excitation, ultrafast vibrational and structural molecular relaxations are detected on a 0.1 ps time scale in nonpolar hexane, while in polar acetonitrile, the spectral evolution is dominated by dipolar solvation. Upon 2-photon excitation, transient spectra in hexane reveal an unexpected growth of stimulated emission (SE) and excited-state absorption (ESA) bands. The behavior is explained by strong population transfer S1 â Sn due to resonant absorption of a third pump photon. Subsequent Sn â S1 internal conversion (with τ1 = 1 ps) prepares a very hot S1 state which cools down with τ2 = 13 ps. The pump pulse energy dependence proves the 2-photon origin of the bleach signal. At the same time, SE and ESA are strongly affected by higher-order pump absorptions that should be taken into account in nonlinear fluorescence applications. The 2-photon excitation cross sections σ(2) = 32 â 10(-50) cm(4) s at 752 nm are evaluated from the bleach signal.
RESUMEN
The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) â S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.
RESUMEN
We show that femtosecond stimulated Raman spectroscopy can record excited-state spectra in the absence of actinic excitation, if the Raman pump is in resonance with an electronic transition. The approach is illustrated by recording S1 and S0 spectra of trans-azobenzene in n-hexane. The S1 spectra were also measured conventionally, upon nπ* (S0 â S1) actinic excitation. The results are discussed and compared to earlier reports.
RESUMEN
Photoinduced isomerization of 1,1'-bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S1 isomerization proceeds over a 600 and 400 cm(-1) barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3-0.4 ps) viscosity-independent and slower (2-26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm(-1), respectively. Raman S0 and S1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm(-1) barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.
RESUMEN
We report the detailed XMCQDPT2/cc-pVTZ study of trans-cis photoisomerization in one of the core systems of both experimental and computational photochemistry-the stilbene molecule. For the first time, the potential energy surface (PES) of the S1 state has been directly optimized and scanned using a multistate multiconfiguration second-order perturbation theory. We characterize the trans-stilbene, pyramidalized (phantom), and DHP-cis-stilbene geometric domains of the S1 state and describe their stationary points including the transition states between them, as well as S1/S0 intersections. Also reported are the minima and the activation barriers in the ground state. Our calculations correctly predict the kinetic isotope effect due to H/D exchange at ethylenic hydrogens, the dynamic behavior of excited cis-stilbene, and trans-cis branching ratio after relaxation to S0 through a rather unsymmetric conical intersection. In general, the XMCQDPT2 results confirm the qualitative adequacy of the TDDFT (especially SF-TDDFT) picture of the excited stilbene but also reveal quantitative discrepancies that deserve further exploration.