A comprehensive investigation of the electronic spectral and photophysical properties of conjugated naphthalene–thiophene oligomers

Abstract

The photophysics and spectroscopic properties of six naphthalene–oligothiophene derivatives, three oligothiophenes (n = 2, 3 and 4) end-capped with naphthalene chromophores and three additional oligomers derived from the former with additional thiophene and bithiophene units in their terminal positions, were investigated in solution at 293 and 77 K. Detailed results are presented on absorption, emission and triplet–triplet absorption spectra together with all relevant quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation), excited state lifetimes, singlet energies and the overall set of deactivation rate constants: k_F, k_IC and k_ISC. From these, three important conclusions could be drawn: (1) the main deactivation channel for the molecules in solution is the radiationless S₁∼∼→ T₁ intersystem crossing; (2) the incremental addition of thiophene units leads to an increment of the fluorescence at the expense of a decrease of the S₁∼∼→ S₀ internal conversion yield and (3) from time-resolved fluorescence experiments, in the picosecond time domain, the decays were found to be monoexponential, excluding decay processes involving energy transfer or conformational relaxation between the chromophoric units. This is explored, by comparison with recent studies on conjugated thiophene and para-phenylene-vinylene (PPV) copolymers and oligomers, to establish that with polythiophenes the energy transfer process is responsible for the fast (few ps) component(s) whereas with PPV polymers a relaxation of the first singlet excited state ought to be considered. DFT theoretical calculations (B3LYP/3-21G* level) support the S₁ state of the compounds with a quinoidal-like structure (within the thiophene central moiety) with a degree of coupling with the terminal naphthalene rings. Singlet oxygen yields were determined and the triplet energy transfer to ³O₂ to produce ¹O₂ was found to be highly efficient with values of S_Δ (= ϕ_Δ/ϕ_T) varying from 0.61 to 0.75, thus revealing to be an important deactivation path for the triplet state of these compounds.