Solvation-Dependent Excited-State Dynamics of Donor-Acceptor Molecules with Hybridized Local and Charge Transfer Character

Recently, an organic synthetic strategy based on hybridized local and charge transfer (HLCT) character has been attracting much attention because of its potential for designing high-efficiency organic light-emitting diode materials. In this work, two novel molecules, N,N-diphenyl-4-phenol-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl) biphenyl-4-amine (TPA-PPI-OH) and N,N-diphenyl-4'-(1-phenyl-1H- phenanthro[9,10-d]-imidazol-2-yl)-[1,1'-biphenyl]-4-amine (TPA-PPI), were investigated by quantum chemical calculations, steady-state spectroscopy, and femtosecond transient absorption spectroscopy (fs-TA) to explore the nature of HLCT. Computational results and steady-state spectra suggest that the lowest excited state is dominated by local excitation (LE) character in low-polar toluene (TOL), whereas the charge transfer (CT) character plays the main role in high-polar acetonitrile (ACN) for both TPA-PPI-OH and TPA-PPI. Relative to TPA-PPI, TPA-PPI-OH shows less sensitivity to solvent polarity with higher quantum yields because of the more planar geometric structure, fabricated by inserting an additional intramolecular hydrogen bond (H-bond) to enhance the inflexibility of the molecule. Ultrafast fs-TA clearly shows the conversion of excited states from LE to CT with the increase of solvent polarity. The stimulated emission is mainly from the LEdominated lowest excited state in low-polar TOL, whereas CT dominates the final relaxation process in high-polar ACN because of strong solvation. Furthermore, the excited states being dominated by LE and CT simultaneously in medium-polar tetrahydrofuran is observed, while the quick equilibrium LE. CT is established just after a femtosecond pulse excitation, indicating the typical HLCT character. The excited state deactivation process of TPA-PPI-OH is faster than that of TPA-PPI, which is attributed to the higher proportion of the LE component and the additional vibrational decay paths induced by the H-bond in TPA-PPI-OH. The results herein offer a guidance to understand the solvent-modulated excited state deactivation mechanism of HLCT molecules.