The real-time intramolecular electronic excitation transfer dynamics of 9′,9-bifluorene and 2′,2-binaphthyl in solution

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Abstract
The dynamical behavior of intramolecular electronic excitation transfer in 9,9’‐bifluorene and 2,2’‐binaphthyl has been investigated in various solvents using a heterodyned and background‐subtracted femtosecond polarization technique. It has been found that the excitation transfer between fluorenyl moieties occurs on a time scale of around 300 fs in hexane, but evidently slows down to around 970 fs in CCl4, and there appears little observable variation in transfer dynamics when the solvent is changed through the hexane–decane–hexadecane series. It has also been found that excitation transfer between naphthyl moieties in CCl4 undergoes damped oscillations which have an apparent period of 1.2±0.1 ps and a damping time constant of 180±20 fs. While there is no clear sign of any oscillation in hexane, the transfer dynamics decays only slightly faster. All the phenomena observed can be neatly categorized, within the simple phenomenological Bloch theory, as either over‐ or underdamped motion relevant to the ratio of the exciton splitting to the pure dephasing rate. However, it is the thorough analysis of all the secular elements of the Redfield relaxation matrix that pinpoints the role of correlated fluctuations in the excitation transfer, and provides a quantitative relation of the pure dephasing between the excited local states to that between the excited and ground states. The equivalency of the pure dephasing rate between the two local states to the population transfer rate between the two delocalized states also prompts us to propose that a local libration of solvent CCl4 could play a key role in underdamping the excitation transfer coherence.