Twisted intramolecular charge transfer of nitroaromatic push–pull chromophores

Abstract

The structural changes during the intramolecular charge transfer (ICT) of nitroaromatic chromophores, 4-dimethylamino-4′-nitrobiphenyl (DNBP) and 4-dimethylamino-4′-nitrostilbene (DNS) were investigated by femtosecond stimulated Raman spectroscopy (FSRS) with both high spectral and temporal resolutions. The kinetically resolved Raman spectra of DNBP and DNS in the locally-excited and charge-transferred states of the S1 state appear distinct, especially in the skeletal vibrational modes of biphenyl and stilbene including ν8a and νC=C. The ν8a of two phenyls and the νC=C of the central ethylene group (only for stilbene), which are strongly coupled in the planar geometries, are broken with the twist of nitrophenyl group with the ICT. Time-resolved vibrational spectroscopy measurements and the time-dependent density functional theory simulations support the ultrafast ICT dynamics of 220–480 fs with the twist of nitrophenyl group occurring in the S1 state of the nitroaromatic chromophores. While the ICT of DNBP occurs via a barrier-less pathway, the ICT coordinates of DNS are strongly coupled to several low-frequency out-of-phase deformation modes relevant to the twist of the nitrophenyl group.

Ultrafast Spectroscopies of Nitrophenols and Nitrophenolates in Solution: From Electronic Dynamics and Vibrational Structures to Photochemical and Environmental Implications

“…Previous calculations showed that hydrogen transfer from the aci -tautomer back to o -nitrophenol (2NP) is effectively barrierless in the electronic ground state [ 14 ], so conversion to the aci -tautomer and relaxation to the ground state would allow for efficient conversion back to the original molecule, especially in methanol because the lengthened HGS relaxation may allow this step to occur. The definitive determination of whether ESIPT occurs in either solvent could inspire future experimental and/or computational work, possibly on excited-state FSRS characterization [ 51 , 86 , 88 ], which underlies the importance of fundamental mechanisms and potential applications for these environmentally relevant molecules to achieve desirable outcomes and societal benefits. In essence, we envision the systematic characterization of nitrophenols and nitrophenolates in water and alcohol (methanol) in this comprehensive work to lay a solid foundation for future steady-state and time-resolved (particularly ultrafast) spectroscopies of these environmentally relevant small molecules in condensed phase, including both excited-state FSRS [ 54 , 75 , 78 ] and its time-domain analogue (impulsive Raman or ISRS (impulsive stimulated Raman scattering) [ 89 , 90 , 91 ]).…”

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

“…32,36 Photoexcitation of push-pull NSBs into their S 1 state is believed to produce an intramolecular CT state, whose deactivation dynamics dictates the fluorescence behaviors and is complicated by the competition among several major pathways, including (1) barrier crossing along the central ethylenic bond torsion to a roughly perpendicular configuration ( 1 p*), (2) formation of the twisted intramolecular charge-transfer (TICT) states with respect to one or more twisting coordinates, and (3) ISC to the triplet-state manifold followed by isomerization and/or other nonadiabatic transitions. 32,[36][37][38][39][40][41][42][43] The dipole moments and fluorescence properties of various stationary states involved in these pathways are believed to be quite different. Consequently, the solvent polarity can modulate the energy profile of deactivation pathways, resulting in strong solvent-dependent fluorescence and photochemical behaviors.…”

Synthesis, crystal structures and semiconductor properties of 2-(thiopyran-4-ylidene)-1,3-benzodithioles with an aryl substituent

“…3,27 On the other hand, the introduction of the CN double bond to a position with a large LUMO coefficient may result in a small HOMO-LUMO energy gap and enhance intramolecular chargetransfer (ICT) transition. [28][29][30] The reduction of the HOMO-LUMO energy gap is favorable for light-based organic semiconductor applications. As an example, phthalimide derivatives were found to be effective photo-responsive n-type organic semiconductors because of ICT contribution.…”