The present study quantitatively analyses the gated photochromism and the acidity photomodulation properties of a diacid dithienylethene compound. Photoisomerisation between the open and closed isomers was investigated by UV/visible and (1)H NMR spectroscopy. It was found that the photocyclisation quantum yield of the diacid form was remarkably high (around 90%). Partial neutralisation of the open isomer revealed a gated photochromism as the photocyclisation quantum yield of the mono- and dianion were 50 and 67%, respectively. A considerable photomodulation of the acidity was observed: the closed isomer is more acid than the open one by more than one pK(a) unit. This effect has been shown to be exploitable for a reversible photo-acid generation. This is the first time that a complete quantitative investigation that allows for the determination of the main photochromic, spectral and thermodynamic parameters of a base-sensitive photochromic diarylethene has been carried out.

A thermally irreversible dithienylethene (DTE) photochrom can be turned into a thermally reversible one in presence of Cu(II) triflate. A ring opening (DTEC closed→DTEO open) occurs through the formation of a copper-containing fast transient intermediate. Stopped-flow experiments monitored at 410 and 780 nm have allowed to show that the stoichiometry of this intermediate is DTE/Cu=1:1. At longer monitoring times (i.e., several seconds after mixing), the intermediate undergoes a slow decay while the residual DTEC closed form opens. A joint detailed kinetic and electrochemical analysis has unveiled a proton catalysis scenario in which electron transfer between DTEC and Cu(II), ligand exchange, protonation-deprotonation equilibria of the cation radicals and ring opening are embedded into two main reaction cycles. At the beginning of the reaction, Cu(II) is reduced into Cu(I) and DTE is degraded without ring opening. Then, as the reaction progresses, the triflic acid released from the Cu(II) reduction switches-on a propagation cycle during which ring opens without any more Cu(II) consumption. Cyclic voltammetry, spectro-electrochemical measurements, delayed photocoloration experiments in presence of Cu(II) and acid-base additions have confirmed the main features of the proton catalysis.