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

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 SI vertical excitation induces direct efficient (phi approximate to 0.9-1), and ultrafast (similar to 120 fs) photocylization from SI 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 SI state, then evolves toward the cyclized Si state and relaxes by ultrafast S-1-S-0 internal conversion. Alternatively, the possibility for a second conical intersection near hot SI 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.