Excited-State Planarization as Free Barrierless Motion in a π-Conjugated Terpyridine

We present femtosecond time-resolved transient absorption data and results from nanosecond time-resolved emission studies for an extended terpyridine system (4′-(4-((2,5-bis(octyloxy)-4-styrylphenyl)ethynyl)phenyl)-2,2′:6′,2′′-terpyridine). By variation of selected solvent properties, that is, solvent viscosity, polarity, and temperature, we can dissect kinetic components, which are due to photoinduced molecular structure rearrangements of the terpyridine system. This real-time observation allows us to quantify the influence of the solvent viscosity on the structural rearrangements, which becomes slowed from 20 to 100 ps when changing the solvent from methanol to n-butanol. By adding temperature-dependent time-resolved emission experiments to the study, we show that the relaxed S1 state partially decays via a nonradiative channel which can be assigned to an intersystem crossing to a lower lying T1 state. The data presented in this paper directly visualize excited-state planarization of the terpyridine sphere in real time. Such motion of the terpyridine with respect to the adjacent conjugated rest leads to a highly conjugated chromophore and is causative for the observed photophysical features of the ligands and their transition metal complexes.