RESUMO
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.
RESUMO
Two types of unstable growth of a precipitation layer in gel are discussed. A cation and an anion that are reactive diffuse from opposite ends of the gel to its center. A white turbid zone forms due to their reactions. When the concentration ratios for both the ions are far from stoichiometry, the turbid zone expands toward the lower-concentration side. However, when the ratio is nearly stoichiometric, unstable growth occurs. In a glass tube, a protrusion of the precipitation region from the turbid zone grows, which forms a long needle-like shape. When a free surface is present on the gel, the precipitation region protrudes from the gel surface to form a rising structure. Mapping the growing structure on a concentration diagram and using scanning electron microscopy to examine contained particles suggest that the reaction is restricted to a narrow region and the reaction product migrates through a path formed in the protrusive structure to form a bulk solid at the edge.
