Bridged Photochromic Diarylethenes Investigated by Ultrafast Absorption Spectroscopy: Evidence for Two Distinct Photocyclization Pathways

Abstract

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

Theoretical investigation on a series of novel S,S-dioxide diarylethenes with abnormal photochromic properties and design of new dyads

“…The main charge change is concentrated on the central ring (C6, C7, C11-C14). Then it passes through a conical intersection 14,18 and releases energy to reach the ground state of product (P), in the following way, For instance, the charges of atoms C in metapositions of benzenes show similar trend, either increase or decrease (Scheme 2b), which agrees with molecular orbital well (Figure 3 LUMO).…”

Picosecond cyclization reaction dynamics of furan-based diarylethene with thiosemicarbazone side-chain groups

“…Therefore, the cyclization reaction dynamics, which involves the process of π -π molecular orbitals (MO) redistribution within the photochromic core, may have quite different time scale. Moreover, such reaction may be sensitive to the pump wavelength [18] and solvent polarity [15,35].…”

“…Most of these requirements are fulfilled, if appropriate photochromic core and side-chain groups are combined (2,8]. Among the known photochromic molecules, DAEs display optically-induced reversible switching in solutions in the femtosecond regime (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Furthermore, while DAEs being attached to metallic electrodes at least ring-closing reaction can be achieved (19,20].…”

Efficient Self‐Contained Photoacid Generator System Based on Photochromic Terarylenes

“…Although this value is rather larger than most of practical PAGs, there should be room for substantial improvement considering the capability of the mother compound, which showed a photon‐quantitative reactivity (QY=0.98) . As much effort have been devoted to the improvement of ring‐cyclization sensitivity of diarylethene and its analogues, we were highly motivated to explore further photosensitive PAGs by developing molecular scaffolds inspired by previous developments in the field, which aim to maximizing the population of the photoreactive conformation in the ground state . Although the bisthiazolylbenzo[ b ]thiophene molecule showed much suppressed photoreaction quantum yield in methanol, the replacement of one of the thiazolyl moieties with a thiophene derivative, which formed an asymmetric terarylene, interestingly led to a large improvement of the QY in methanol .…”