Time-Domain Visualization of Electron-Phonon Coupling in Nanographenes

Coherent molecular vibrations determine many molecular properties like intersystem crossing or intramolecular charge transfer, holding potential for developing systems with vibrationally controlled electronic dynamics and reactivity. Research efforts have been focused mainly on localized vibrational modes, leaving collective vibrational modes widely unexplored despite their prominent role in driving molecular dynamics. Besides, the lower intensity associated to collective vibrational modes and their low frequency makes their study a demanding task. In this sense, nanographenes are promising materials that can be synthesized with tailored shapes and sizes-including edge substituents-, offering a great platform for studying collective vibrational modes. Here, femtosecond impulsive vibrational spectroscopy, Raman spectroscopy, and density functional theory calculations are combined to investigate for the first time low-frequency vibrational motions in two dibenzo[hi,st]ovalene (DBOV) nanographenes. The systematic study of mesityl-substituted DBOV (DBOV-Mes) and its chloro-functionalized derivative (Cl-DBOV-Mes) demonstrates that collective vibrational modes supported by DBOV derivatives can be altered with edge substitution, while optoelectronic properties are preserved. The multidisciplinary approach followed in this work sets the stage for studies on collective vibrational modes in nanographenes and other pi-conjugated systems.