Excited‐State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring‐to‐Linear Excitonic Behavioral Turning Point

Abstract

Excited-state dynamic planarization processes play ac rucial role in determining exciton size in cyclic systems,a s reported for p-conjugated linear oligomers.H erein, we report time-resolved fluorescence spectra and molecular dynamics simulations of p-conjugated cyclic oligothiophenes in which the number of subunits was chosen to showthe size-dependent dynamic planarization in the vicinity of ar ing-to-linear behavioral turning point. Analyses on the evolution of the total fluorescence intensity and the ratio between 0-1 to 0-0 vibronic bands suggest that excitons formed in ac yclic oligothiophene composed of six subunits fully delocalize over the cyclic carbon backbone,w hereas those formed in larger systems fail to achieve complete delocalization. With the aid of molecular dynamics simulations,i ti ss hown that distorted structures unfavorable for efficient exciton delocalization are more easily populated as the sizeo ft he cyclic system increases.

Unraveling Excited-Singlet-State Aromaticity via Vibrational Analysis

“…26,37 The Fourier transform IR (FTIR) and TRIR spectra of 1 and 2 were acquired (Figures 2 and S15) in the IR spectral range of 1,300-1,700 cm À1 , where the C=C stretching vibrational modes arise from the main conjugated rings; this region is the most sensitive to changes in the p-electronic structures. [38][39][40][41] Both FTIR spectra of 1 and 2 showed strong bands at 1,485 and 1,520 cm À1 and weak bands at 1,430 and 1,650 cm À1 . These common IR bands mainly arise from local vibrational modes of asymmetric meso-pentafluorophenyl (C 6 F 5 ) groups.…”

“…[32][33][34][35][36] Moreover, time-resolved vibrational spectroscopic techniques are increasingly recognized as effective tools for analyzing the structural changes in photoreactions, induced by changes in the electronic structures. [37][38][39] Because the IR activity of vibrational modes, particularly C=C stretching motions, is sensitive to molecular distortions as a result of the vibrational selection rule for a change in the dipole moment, 40,41 time-resolved IR (TRIR) spectroscopy allows us to monitor the changes in vibrational mode and conformational distortions between the ground and excited states associated with aromaticity changes.…”