Tetra- and tri-thienyl ethenes: new fluorescent photochromic compounds

Abstract

In this work the photochemistry and photophysics of two new ethenic compounds, a tetra-thienylethene (tetrakis) and a tri-thienylethene (triakis), have been investigated by steady state and time-resolved UV-vis absorption and luminescence spectroscopies. The quantum yields of the UV-photoinduced ring-closure reaction (Φ_O→C) and of the visible-stimulated cycloreversion reaction (Φ_C→O) were determined as a function of the temperature in 3-methylpentane. The trends of Φ_O→C and Φ_C→O with temperature suggest that a barrier exists to the ring opening reaction which hampers the cycloreversion in a rigid matrix at 80 K. For the closure reaction, a negative effect of the temperature on Φ_O→C reflects an increased concentration of the more easily isomerizable antiparallel conformer with decreasing the temperature. Quantum yields and lifetimes of fluorescence, observed for tetrakis in both the colourless and coloured forms and for triakis only for the coloured form, were determined. The solvent and temperature effects on fluorescence were investigated. Nanosecond time-resolved experiments showed the formation of a triplet transient from tetrakis which could be a precursor of the main reaction photoproduct. For triakis no transient was detected.