Long-living structures of photochromic salicylaldehyde azine: polarity and viscosity effects from nanoseconds to hours

Abstract

In this study, we report on the effects of solvent viscosity and polarity on the photochromic salicylaldehyde azine (SAA) molecule by examining the steady-state and UV-visible absorption results in the time scale from nanoseconds to hours, in solution and in a polymer film. For the neutral structure, the viscosity strongly affects the lifetime of the photochromic (trans-keto) tautomer by suppressing the second order quenching process, and thus increasing the photochrome lifetimes in highly viscous solvents to 500 μs in polar triacetine, and to 65 μs in non-polar squalane. Trapping SAA in a non-polar polymer film (polyethylene) results in further elongation of the photochromic lifetime (700 μs) by one order of magnitude (with respect to that in squalane), due to the retardation of the intramolecular back-isomerization. Another species, living significantly longer and absorbing more in the UV comparing to the photochrome, was identified as the syn-enol tautomer. The lifetime of this tautomer, created in a competitive mechanism to the photochrome creation, is much longer in non-polar solvents (hundreds of minutes) than in polar ones (tens of minutes), opposite to the trend observed for the photochrome. For the SAA anion, the transient living on the ns-μs time scale can be exclusively assigned to the triplet state, which is not observed for the neutral form at room temperature.

Photochromic cycle of 2′-hydroxyacetophenone azine studied by absorption and emission spectroscopy in different solvents

“…22 Similarly as for SAA and other Schiff bases, the best fit for APA/HEX is obtained for the mixed first and second order kinetics model. 22,57,70 The fitted second order rate k 2 = 1.5 × 10 10 s −1 M −1 is similar to the averaged value found for SAA/HEX (1 × 10 10 s −1 M −1 ). 22,70 This process describes the intermolecular quenching process involving most probably two trans-keto tautomers and the proton exchange between them.…”

“…22,57,70 The fitted second order rate k 2 = 1.5 × 10 10 s −1 M −1 is similar to the averaged value found for SAA/HEX (1 × 10 10 s −1 M −1 ). 22,70 This process describes the intermolecular quenching process involving most probably two trans-keto tautomers and the proton exchange between them. 22,24 On the contrary, there is a large difference between the fitted first order rate, which is k 1 = 1 × 10 5 s −1 for APA/HEX, ten times higher than that found previously for SAA/HEX (1 × 10 4 s −1 ).…”

“…22,24 On the contrary, there is a large difference between the fitted first order rate, which is k 1 = 1 × 10 5 s −1 for APA/HEX, ten times higher than that found previously for SAA/HEX (1 × 10 4 s −1 ). 22,70 The first order decay process is likely due to the back cis-trans isomerization around a double C=C bond in the ground state. Moreover, due to high k 1 rates for APA the effect of the second order process on the kinetics of diluted solutions ([APA] = 1 × 10 −5 M) is negligible, and they are well fitted by the single exponential decay model ( Figure 6(b)).…”

Transient states and the role of excited state self-quenching of indoline dyes in complete dye-sensitized solar cells

“…of number of electrons per particle, the photo behaviour observed in transient absorption is probably more relevant than that found in TCSPC experiments. However, under photostationary conditions the concentration of the transient species is proportional to its lifetime [50], so the concentration of excited dyes per particle is several orders of magnitude smaller than concentration of electrons. Therefore, from the point of view of excited state selfquenching, the conditions of TCSPC experiments are more relevant to 1 Sun.…”

Gels with sense: supramolecular materials that respond to heat, light and sound

“…Ziółek et al reported that the lifetime of the trans-keto isomer of the anil-like compound salicylaldehyde azine can be increased from 65 to 500 μs by the use of more viscous solvents, and may even reach 700 μs if the compound is entrapped in a polyethylene film. 310 Viscosity is thought to inhibit thermal relaxation by providing friction to oppose the trans-cis isomerisation, whereas a polymer matrix is likely to restrict the space around the molecule such that more energy is needed to induce a conformational change. These effects are clearly of particular importance in gels, where -in contrast to bulk solids -interactions with solvent play a significant role.…”