Twisted Intramolecular Charge Transfer State of a "Push-Pull" Emitter.

Sebok Lee,Myungsam Jen,Yoonsoo Pang

doi:10.3390/ijms21217999

Abstract

The excited state Raman spectra of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in the locally-excited (LE) and the intramolecular charge transfer (ICT) states have been separately measured by time-resolved stimulated Raman spectroscopy. In a polar dimethylsulfoxide solution, the ultrafast ICT of DCM with a time constant of 1.0 ps was observed in addition to the vibrational relaxation in the ICT state of 4–7 ps. On the other hand, the energy of the ICT state of DCM becomes higher than that of the LE state in a less polar chloroform solution, where the initially-photoexcited ICT state with the LE state shows the ultrafast internal conversion to the LE state with a time constant of 300 fs. The excited-state Raman spectra of the LE and ICT state of DCM showed several major vibrational modes of DCM in the LE and ICT conformer states coexisting in the excited state. Comparing to the time-dependent density functional theory simulations and the experimental results of similar push-pull type molecules, a twisted geometry of the dimethylamino group is suggested for the structure of DCM in the S1/ICT state.

Highlights

Intramolecular charge transfer (ICT) process has been of great interest in chemistry and related disciplines for many decades [1,2,3,4,5,6,7,8,9]
We showed that the ultrafast conversion (0.3 ps) from the CT to LE conformer states in weakly polar CHCl3 solution in addition to the conversion from the LE to intramolecular charge transfer (ICT) state (1.0 ps) in polar DMSO solution which has been numerously observed in time-resolved absorption and emission measurements [12,13,22,25,34,35]
The ground state geometries were optimized with the fixed dihedral angles of DMA or DMAP group at the B3LYP/6–311G(d,p) level with the polarized continuum model (PCM) for DMSO and CHCl3, and the vertical transition energies were calculated by the single point time-dependent density functional theory (TDDFT) simulations at each optimized ground state geometry

Summary

Introduction

Intramolecular charge transfer (ICT) process has been of great interest in chemistry and related disciplines for many decades [1,2,3,4,5,6,7,8,9]. Due to the distinct photophysical aspects, DCM has been used in numerous applications including laser dyes [15,16], OLED emitters [17,18], molecular photo-switches [19,20,21,22]. The planar structures of DCM in both the ground state and the excited state were proposed by time-dependent density functional theory (TDDFT) simulations with the CAM-B3LYP functional [30]. Another TDDFT study at the mPW1PBE/6-31G(d) level of theory suggested the TICT state with a twisted DMAP group for the emitting S1 state of DCM in ethanol [29]. Even with the TDDFT simulations considered as the most efficient computation method for the excited states, the choice of approximate exchange-correlational functional is very important for the accuracy of the simulation results [31]

Methods

Results

Discussion

Conclusion