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

Authors：Zhang, Wei; Kong, Jie; Hu, Dehua; Tao, Min; Niu, Xinmiao; Vdovic, Silvije; Aumiler, Damir; Ma, Yuguang^*; Xia, Andong^*

Source：Journal of Physical Chemistry C, 2020, 124(10): 5574-5582.

DOI：10.1021/acs.jpcc.0c00003

• ACTIVATED DELAYED FLUORESCENCE
• INTRAMOLECULAR PROTON-TRANSFER
• TIME-RESOLVED FLUORESCENCE
• LIGHT-EMITTING-DIODES
• HIGHLY EFFICIENT
• PHOTOPHYSICAL PROPERTIES
• NARROW EMISSION
• DERIVATIVES
• RELAXATION
• DESIGN

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.

• Institution
  Chinese Academy of Science; Graduate University of Chinese Academy of Sciences