Light-induced electron transfer as compared with energy transfer in molecular thin-film systems

Abstract

Quenching of fluoranthene (FA) singlets by tetrabromo-o-benzoquinone (TBBQ) and NNN′N′- tetramethyl-p-phenylenediamine (TMPD) was studied both in xylene solutions and in spin-cast polystyrene (PS) films. Emphasis was placed on time-resolved fluorescence transients resulting from pulsed excitation. Linear Stern–Volmer plots were obtained for quenching in solution and gave diffusion-controlled rate constants, of 1.45 × 10¹⁰ and 1.53 × 10¹⁰ dm³ mol^–1s^–1 for TBBQ and TMPD, respectively. TBBQ was found to quench FA singlets in PS over the studied concentration range 12 <[TBBQ]/mmol dm^–3 <48, but in its presence FA singlets decayed non-exponentially. The results were interpreted quantitatively in terms of pure Förster's transfer from FA to TBBQ without diffusion of excitons. The critical transfer radius R₀ was experimentally determined to be 24.3 Å, which is in good agreement with the theoretical value of 23 Å calculated from spectral data. Quenching of FA singlets in PS films was found to be independent of FA concentration over a 300–1200 mmol dm^–3 FA concentration range for a constant TBBQ concentration of 24.0 mmol dm^–3. TMPD was only slightly effective as a quencher of FA singlets in PS because it apparently behaves strictly as a contact quencher based on reversible charge transfer. The implications of these results for the design of systems intended to exploit light-induced electron transfer are discussed.