Ultrafast Intramolecular and Solvation Dynamics in 4,7-Bis (4,5-dibutylbenzo[1,2-b:4,3-b′]bisthiophene[1,2-b:4,3-b′]bisthiophen-2-yl)-2,1,3-benzothiadiazole

Abstract

We report a combined approach of stationary and time-resolved fluorescence measurements and ultraviolet–visible (UV–vis) transient absorption spectroscopy (TAS) along with ab initio calculations, which provide an overall picture of the dynamics occurring after excitation in a push–pull molecule, namely, 4,7-bis (4,5-dibutylbenzo[1,2-b:4,3-b′]bisthiophene[1,2-b:4,3-b′]bisthiophen-2-yl)-2,1,3-benzothiadiazole. The analysis of the emission spectra in solvents of different polarities reveals the presence of three conformers whose structures differ in the orientation of the 4,5-dibutylbenzo-bisthiophene groups and in their planarity with respect to the benzothiadiazole acceptor group. The Kawski method allows us to estimate the ground- and first-excited state dipole moments (μg and μe) for the three conformers. We find values of μe similar for the three conformers and higher than the relative μg values as can be expected from a push–pull molecule undergoing a light-induced charge-transfer (CT) transition. UV–vis TAS in different solvents highlights the instantaneous (within our instrumental resolution) formation of a locally excited S1 state (accompanied by a big change in the dipole moment with respect to S0), which undergoes a rapid intramolecular CT (ICT) assisted by molecule planarization [planar ICT (PICT)]. The strong dipole–dipole interactions with the polarized solvent molecules stabilize the S1 CT state that decays principally through fluorescence emission. Both PICT and solvation dynamics are responsible for the big Stokes’ shift characterizing the molecule, particularly in polar solvents. The fluorescence lifetimes are substantially longer in polar solvents, and also fluorescence quantum yields are higher in polar solvents. We conclude that the radiative relaxation time increases when molecular planarization of the S1 emissive state takes place, and this condition is favored in polar solvents where local dipole–dipole interactions support the structural stabilization of the CT emissive state. In the poly(methyl methacrylate) matrix, the structural and solvation dynamics are strongly inhibited, leading to reduction of nonradiative processes and to shortening of the fluorescence relaxation time.