New insights into the photoswitching mechanisms of normal dithienylethenes

Abstract

The photoswitching and competitive processes of the referent photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) and a novel bridged analog DTE-m5 have been investigated by state-of-the-art TD-DFT calculations and ultrafast spectroscopy supported by advanced chemometric data treatments. Focusing on DTE, the overall deactivation pathway of both antiparallel (AP) and parallel (P) conformers of the open form (OF) (1 : 1 in solution) has been resolved and rationalized starting from the Franck-Condon (FC) region to the ground state recovery. For the photo-excited P conformer, after ultrafast relaxation (∼200 fs) towards the S relaxed state, an expected ISC occurred (55 ps) to produce a triplet state, P, the latter relaxing within 2.5 μs. Concerning the AP conformer, the photocyclization reaction is reported to proceed immediately (100 fs) starting from the FC region while the relaxed singlet state is populated in parallel. For the first time, we discovered that the latter state evolves through an unexpected ISC process (1 ps) giving rise to a second triplet state,AP. For DTE-m5, by slightly constraining the molecule with the bridge, this triplet becomes reactive and participates in the formation of 10% of closed form (CF) probably through an adiabatic mechanism. Concerning the photoreversion, in accordance with the literature, we report on a two-step process, a 190 fs vibrational relaxation followed by a 6 ps ring-opening reaction. For the overall species at the singlet or triplet manifold, the use of advanced MCR-ALS allows us to obtain specific spectral signatures. This study is therefore a new step within the comprehension of DTE photochemistry.

Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes

“…The initial formation of a broad positive transient band at around λ max =680 nm could be observed as well as a negative absorption that can be ascribed to the bleaching of the stationary band of the open isomer at around 390 nm (Figure , top left). Both bands decay with a time constant of τ =2.9 μs that is typical for the triplet excited state of the parallel conformer (see also Figure S4 in the Supporting Information) . In accordance with the stationary experiments, after 50 μs the initial spectrum is not fully recovered, but a new band appears at around 580 nm corresponding to the formation of the closed isomer 1 c .…”

Cyclization Dynamics and Competitive Processes of Photochromic Perfluorocyclopentene Dithienylethylene in Solution

“…It immediately rationalizes the quantum yields of the cyclization for DMTPF (0.22) being more than 2 times lower compared to DTE (0.46) paralleling the decrease of AP population from 48 to 20 %. Another important findings concerns the triplet quantum yields measured by the method of energy transfer with betacarotene we already applied to DTE [32] indicated in Table 1. For DMTPF this value is as high as 0.75 while for DTE, it decreases down to 0.4.…”

“…Figure 2 shows time-resolved transient absorption spectra obtained after the excitation of the DMTPF OF (for DTE, see Figure 6 of Ref. [32]) in chloroform solution with a femtosecond 320 nm laser pulse, recorded in the 380-700 nm spectral range within four temporal windows (À 1-0.25, 0.28-0.5, 0.52-1 and 2-500 ps). Global fitting of transient signal with four-exponential functions, convoluted with a Gaussian profile simulating the pump-probe correlation function are shown in supporting information (see Figure S.3).…”

“…[22,23] Indeed, the rationalization of complex excited state mechanisms are highly desired to still improve photochromic performances and so considerable investigations have been devoted worldwide to explore the excited state topologies of DTEs (or DAEs) via advanced optical spectroscopies [24][25][26][27] and/or computational chemistry. [28][29][30] In this perspective, our group has actively investigated the spectrokinetic features on the series of bridged DAEs, [31] bridged DTEs [29,32,33] or DTEs dedicated to supramolecular chemistry [34] using ultrafast spectroscopies combined with (TD)-DFT calculations. Our leitmotiv was the attempt to detangle the photochemistry of the antiparallel (AP) conformers from the photophysics of parallel (P) conformers by comparing bridged/ constrained versus free analog molecules (the initial purpose of the former being the blockade of the molecule into a photoactive conformation).…”

“…Due to its commercial availability, normal dithienylethene 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl) perfluorocyclopentene (denoted as DTE, hereafter) has become a reference [26,[35][36][37][38] of photoswitch, that is the reason why we reinvestigated its photochemistry together with a series of polyether bridged analogs. The main outcomes [32] were: (i) for the P* conformer, an Intersystem crossing (ISC) occurred (40-150 ps) to produce a first triplet state ( 3 P) after ultrafast relaxation toward S 1 relaxed state. Later on the 3 P state relaxed within few tens of μs to the ground state; (ii) for the AP* conformer, the photocyclization reaction occur immediately after light absorption (100-200 fs) starting from the Franck-Condon (FC) region while the relaxed singlet state is populated in parallel way and evolves through an ISC reaction (1-40 ps) and generate a second triplet state, 3 AP with a lifetime depending on the structural constraints; (iii) interconversion between the two triplet states has been experimentally identified, e. g. 3 AP $ 3 P only for some polyether bridged DTE (DTE-O 6 in ref [33]); (iv) an unidentified species (denoted as "X" hereafter) have been reported with variable statistical weight (0-50 %) along with the two triplet states within μs regime.…”