Theoretical and Experimental Investigations of the Spectroscopic and Photophysical Properties of Fluorene-Phenylene and Fluorene-Thiophene Derivatives: Precursors of Light-Emitting Polymers

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Abstract

The ground and excited states of covalently linked fluorene-based dimers were investigated by theoretical methods and by UV−vis and fluorescence spectroscopies. The optimized structures and the characterization of frontier molecular orbitals were obtained by HF/6-31G* ab initio calculations. All derivatives are nonplanar in their ground electronic states. The extent of nonplanarity depends on the nature of the aromatic ring bonded to the fluorene unit. All frontier orbitals involved both subunits of the dyads. The HOMO of each compound possesses an antibonding character between subunits, while the LUMO shows bonding character. The nature and the energy of the first 10 singlet−singlet electronic transitions have been obtained by ZINDO/S semiempirical calculations performed on the HF/6-31G* optimized geometries. All electronic transitions are of the ππ* type and involve both subunits of the molecules. For each derivative, excitation to the S1 state corresponds mainly to the promotion of an electron from the HOMO to the LUMO, and the S1 ← S0 electronic transition is strongly favored and polarized along the long axis of the molecular frame. The energy of the first electronic transition of all derivatives follows the HOMO−LUMO energy gap computed from HF/6-31G* ab initio calculations. The absorption and fluorescence spectra of the fluorene derivatives have been recorded in cyclohexane. The first absorption band of each derivative can be assigned to the S1 ← S0 electronic transition computed from ZINDO/S calculations. The overall shape of the absorption and fluorescence spectra suggests a smaller distribution of conformers in the S1 state than in the ground state. The fluorescence quantum yield and lifetime in cyclohexane have been obtained. In these systems, the photophysical properties are mainly governed by nonradiative processes.

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