Uncovering elusive ultrafast charge transfer-driven structural changes in 4,4′-bis(9-carbazol-9-yl)-1,1′-biphenyl, a paradigmatic molecular triad

Organic semiconductors have attracted significant attention for their potential in flexible, lightweight, and tunable optoelectronic applications. Among them, 4,4'-bis(9-carbazol-9-yl)-1,1'-biphenyl (CBP) is one of the most widely employed host materials. However, despite its extensive applications, its intrinsic excited-state dynamics and subsequent structural evolution remain underexplored. In this study, we combined femtosecond transient absorption (fs-TA) spectroscopy and nonadiabatic molecular dynamics (NAMD) simulations to investigate the excited-state dynamics of CBP. A comprehensive analysis of fs-TA data obtained in solvents with varying polarity and viscosity, combined with NAMD simulations, reveals a sequential progression of excited-state structural rearrangements. Specifically, we find that intramolecular charge transfer from the carbazole to the biphenyl unit occurs on a sub-picosecond timescale, followed sequentially by biphenyl planarization accompanied by a slower torsional motion around the bond connecting the biphenyl and carbazole moieties. Notably, the vibrational energy redistribution between torsional vibrational modes is identified as a key factor governing the stepwise structural evolution. These findings provide new insight into the excited-state dynamics of CBP and offer design guidelines for the development of next-generation OLED host materials.