Intramolecular charge transfers (ICTs) of push-pull emitters between the electron donor and accepting groups connected by π-conjugated backbones are of great interest due to the many applications, including solar cells and artificial photosynthesis. The structural changes of chromophores during the ICT have long been investigated experimentally and theoretically. However, the structures in the ICT states, such as the "twisted" or "planar" geometry of electron donor or acceptor groups proposed in many theoretical studies, have not been verified experimentally via time-resolved vibrational spectroscopy. In this work, femtosecond stimulated Raman spectroscopy (FSRS) with both high temporal (&lt;50 fs) and spectral resolutions (&lt;10 cm-1) are used to obtain experimental evidence for the structural changes upon the ICT of push-pull emitters with the nitro group as the electron acceptor, 4-dimethylamino-4′-nitrobiphenyl (DNBP) and 4-dimethylamino-4′-nitrostilbene (DNS). Twist of the electron-donating or accepting group accompanying the ICT process leads to the changes in the skeletal stretchings of two phenyls and central ethylene (DNS only). The planar or twisted molecular geometries of the backbones, biphenyl (for DNBP) or stilbene (for DNS), respectively, are observed via the vibrational couplings between two ν8a and νC=C (DNS only) in the excited-state Raman spectra. The potential surface of DNS toward the ICT state appears strongly coupled to low-frequency vibrational modes of in-plane and out-of-plane deformations leading to the rotation of electron donor or acceptor group. The coherent oscillations resulting from the anharmonic coupling of the potential surfaces are only observed in the major vibrational modes of DNS. These coherent oscillations are absent in the ICT of DNBP, which is estimated to occur via a barrier-less potential surface by many theoretical studies.

Structural changes of push-pull emitters during intramolecular charge transfer

ACS Fall 2021

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