Ultrafast excited-state dynamics of ferrocene-bridge-acceptor system

• Published in: Chemical physics 2010-06, Vol.372 (1), p.17-21
• Main Authors: Mohammed, Omar F., Sarhan, Abdelwareth A.O.
• Format: Article
• Language: English
• Subjects: Charge transfer; Time-resolved fluorescence; Ultrafast internal conversion
• ISSN: 0301-0104
• DOI: 10.1016/j.chemphys.2010.04.014

The photoexcitation of the Fc-ph-DCV at 390 nm generates the S2 state. The S 2 → S 1 internal conversion (IC) occurs with a time constant of ∼100 fs and generates the S 1 state having a CT character. The IC process is faster than the solvation processes and is independent of solvent polarity. On the other hand, the lifetime of S 1 state shows a substantial solvent dependence, changing from 50–60 ps in ACN to ∼20 ps in CHX, reflecting that the S 1 state is more stabilized in more polar solvent. The up-conversion data indicate the presence of ultrafast solvation and/or vibrational relaxation in the S 1 state. The present study clarified the ultrafast dynamics of the CT process following electronic relaxation of a newly synthesized donor-π-acceptor system, Fc-ph-DCV. We employed both femtosecond fluorescence up-conversion and transient absorption techniques with ∼150 fs time resolution to study the excited-state deactivation process of an intra-molecular charge transfer model compound, 4-(ferrocen-1-yl)benzylidene-malononitrile (Fc-ph-DCV), which consists of ferrocene (Fc) unit as an electron donor, dicyanovinly (DCV) as an electron acceptor and phenyl (ph) ring as the central bridge. The results showed that after photoexcitation into the higher excited S 2 state, ultrafast internal conversion into S 1 takes place. The rate of S 2 → S 1 internal conversion is markedly faster (with a typical time of 120 fs ± 20 fs) than the diffusive solvation process. On the other hand, the lifetime of the relaxed S 1 state was strongly dependent on the solvent polarity, changing from 40 to 50 ps in acetonitrile to ∼20 ps in cyclohexane. Time-resolved fluorescence data also showed subpicosecond transient component that is attributable to the spectral relaxation caused by solvation and/or vibrational relaxation in the S 1 state.