Signature of Nonadiabatic Coupling in Excited-State Vibrational Modes

Abstract

Using analytical excited-state gradients, vibrational normal modes have been calculated at the minimum of the electronic excited-state potential energy surfaces for a set of extended conjugated molecules with different coupling between them. Molecular model systems composed of units of polyphenylene ethynylene (PPE), polyphenylenevinylene (PPV), and naphthacene/pentacene (NP) have been considered. In all cases except the NP model, the influence of the nonadiabatic coupling on the excited-state equilibrium normal modes is revealed as a unique highest frequency adiabatic vibrational mode that overlaps with the coupling vector. This feature is removed by using a locally diabatic representation in which the effect of NA interaction is removed. Comparison of the original adiabatic modes with a set of vibrational modes computed in the locally diabatic representation demonstrates that the effect of nonadiabaticity is confined to only a few modes. This suggests that the nonadiabatic character of a molecular system may be detected spectroscopically by identifying these unique state-specific high frequency vibrational modes.

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

“…Furthermore, upon non-adiabatic transition, the excess electronic energy is dispersed into the nuclear velocities in the direction of the NAC vector to enforce energy conservation. The direction of the NAC vector is highly significant and it represents the direction of the driving force acting along a unique normal mode direction throughout regions of strong coupling 46 , 47 . The fact that the direction of the NAC vector defines the flux of energy toward specific vibrations has been emphasized by Bittner et al 48 .…”

On the mechanism of vibrational control of light-induced charge transfer in donor–bridge–acceptor assemblies

“…The theoretical basis for the ability to influence ET by infrared light has been established in multiple frameworks [12][13][14][15][16] . Nevertheless, the application of vibrational excitation to manipulate photo-induced reactions in the condensed phase is challenging due to the expected rapid intramolecular vibrational redistribution (IVR).…”

Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

“…[41][42][43][44] Them anifestation of two distinct absorption bands allows access to the high-lying excited states of different wavefunction symmetry.I th as been demonstrated that the direction of derivative coupling at non-adiabatic regions determines the thermal activation of specific vibrational modes,which induces ultrafast reorganizations in bond lengths and torsional angles. [45][46][47][48] Since the symmetry of electronic wavefunctions at non-adiabatic regions defines the directions of derivative coupling,two optical transitions were employed to develop distinctive excited-state geometries in terms of torsional disorders via excited-state symmetrydependent electron-vibrational coupling. [29] Although the symmetry of electronic wavefunctions at non-adiabatic regions cannot be definitely determined owing to intrinsic structural disordering,q uasi-cyclic and symmetric structures of [n]CPPs may maintain quasi-(anti)symmetric characters of excited-state wavefunctions at non-adiabatic regions.H ereafter, we employ the term "(anti)symmetric-like" to roughly characterize the excited-states.A fter the completion of nonadiabatic transitions from the two high-lying excited states to the lowest excited state,e xciton (de)localization was comparatively investigated until the energy minimum structure of S 1 state was achieved with the aid of slow torsional motions, which is called torsional relaxation.…”