Solvation Dynamics Probed by Femtosecond Transient Absorption Spectroscopy:  Vibrational Cooling and Conformational Relaxation in S₁ trans-4,4‘-Diphenylstibene

Abstract

We present the femtosecond transient absorption spectra of trans-4,4‘-diphenylstibene (DPS) in dioxane, methylene chloride, and acetonitrile. The transient absorption spectra feature two bands that are assigned to different electronic transitions. We interpret the spectral changes in the transient absorption spectra in terms of vibrational cooling and conformational dynamics. Vibrational cooling is evidenced by changes in the integrated peak intensity of the transient absorption spectra. For the two transient absorption bands observed in S1 DPS, the vibrational cooling time constants correlate well with the thermal diffusivity of the solvent. The vibrational cooling rates for the two absorption bands are statistically different in all solvents. We attribute this difference to the relative energy exchange rates between the weighted average of the vibrational modes in the Franck−Condon region that contribute to the S1−S n transition and the weighted average of the vibrational modes that contribute to the S1−S m transition. On the basis of the present transient absorption data and previous transient Raman data, we suggest that the Franck−Condon region for the S1−S n transition has a significant contribution from the phenyl−phenyl portion of DPS; the S1−S m transition is likely more localized on the stilbene portion of DPS. The results from the conformational dynamics are consistent with this interpretation. Conformational dynamics are evidenced by peak position shifts in the transient absorption spectra. Only the S1−S n transition undergoes significant peak shifts with delay. We attribute the conformational change to the transition from a nonplanar (ground state) to planar (excited state) structure between the two adjacent phenyl rings in DPS.

Excited-state vibrational relaxation and deactivation dynamics of trans-4-(N,N-dimethylamino)-4′-nitrostilbene in nonpolar solvents studied by ultrafast time-resolved broadband fluorescence spectroscopy

“…The subsequent slower relaxations (t 2 and t 3 ) can be ascribed to a step-wise VC process similar to the sequential energy dissipation mechanism reported previously [4,10]. The $1.2 ps (t 2 ) component can be attributed to the energy transfer to the nearest solvent molecules, whereas the longer component (t 3 ) of $6-7 ps is the thermal equilibrium with the outer-sphere solvents.…”

“…Thus, the fast component (0.2 ps) can be ascribed to a slower IVR of some excited low-frequency levels that are not equilibrated after the initial ultrafast IVR. Sequential IVR have been reported for large molecules in liquid phases and can be understood on the basis of the tier model [5][6][7][8][9][10].…”

“…The former is an intramolecular relaxation of the initial non-uniform vibrational energy distribution, and the latter is an intermolecular energy transfer process leading to thermal equilibrium. Recent ultrafast time-resolved studies of VR have revealed that both IVR and VC may occur via sequential mechanisms in liquid solutions [4][5][6][7][8][9][10][11][12].…”

Time-resolved spectroscopy of the singlet excited state of betanin in aqueous and alcoholic solutions

“…33 Note that vibrational cooling is typically manifested by a narrowing of the hot S 1 or S 0 absorption bands, or fluorescence bands. 32,[34][35][36] The excess of vibrational energy in the bn S 1 state due the additional excitation energy at λ exc = 475 vs. 535 nm has a small influence on the transient absorption spectra and the deduced decay-associated spectra ( Figures S2 and S3). A substantial excess of the vibrational energy can be expected in the nascent bn S 1 state when using UV excitation (λ exc = 287 nm).…”

Unconventional Synthesis of a CuI Rotaxane with a Superacceptor Stopper: Ultrafast Excited‐State Dynamics and Near‐Infrared Luminescence

“…This "red-edge" or "hotband" absorption has been already observedi nm olecules having conicali ntersectionsa nd leads to an electronic relaxation that is more rapid than vibrational energyd issipation to the solvent. [66,[68][69][70] Further dedicated theoretical and experimental studies will elucidate whether T 1 or hot-S 0 best rationalizes the last excited-state deactivation process occurring in DCNQ acceptors. These two speciesa re associated with the S 1 excited state and ag eneric Xs tate, which can be ascribed to the T 1 or to the hot-S 0 state.…”