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.

Multiswitchable Acidichromic and Photochromic Bisdiarylethene. An Experimental and Theoretical Study

“…90 nm, implies a continuous modulation of color of the solutions from orange to red, violet, and blue-green The molar absorption coefficient of the color band is nearly independent of the pH/ H 0 . Its value (∼20 000 ± 2000 dm 3 mol –1 cm –1 ) is notably higher than those usually determined for other photochromic DAEs bearing a single isomerizable photochromic unit (∼10 000–15 000 dm 3 mol –1 cm –1 ). ,, …”

“…Previously reported equations (eqs –), suitable for the kinetic treatment of photoreversible systems, were applied. In eq , describing the color-forming kinetics, A C is the absorbance; ε C is the molar absorption coefficient of the closed (colored) form at the analysis wavelength; I 0 is the intensity of the irradiating light; A 0 and ε iso are the absorbance and the molar absorption coefficient, respectively, at the isosbestic wavelength where irradiation was carried out.…”

Light and pH tunable luminescence in a photochromic bisdiarylethene

“…For other diarylethenic compounds, 13,20 more marked solvatochromism had been found, associated to decrease of fluorescence quantum yield (about one order of magnitude from a hydrocarbon to an alcohol). Due to the very large Stokes shift (∼8000-10 000 cm −1 ), this behaviour, that was observed for various DAEs, 8,10,13,17,35 was attributed to occurrence of emission from an excited singlet state having a dominant CT character.…”

“…Singlet lifetimes of bisDAE are in the nanosecond domain as also found for other DAE analogs. 6,13,14,20,27 Since the rate constant for any photophysical or photochemical process can be obtained from the quantum yield for the process of interest and the lifetime of the excited state, the fluorescence rate constant was calculated from eqn (3):…”

“…Generally, the DAEs exhibit fluorescence from one of the isomeric forms, very often the colourless open form [5][6][7][8][9][10][11][12][13] but in some cases the coloured closed form is the fluorescent one, [14][15][16][17][18] even with significant quantum yield, 19 and in a few cases both forms emit. 20,21 Materials responding to different environmental stimuli by changing specific properties can act as sensors and probes. Therefore great efforts have been devoted to design molecular systems in which the photochromic behaviour is associated to an efficient luminescence emission.…”