7539c is not very small, the decay of the IP can be reproduced approximately by a single exponential. In this case, the rate of C R deactivation of CIP and its solvation to form LIP are rather slow and may be close to the rate of CR deactivation and dissociation of LIP, which seems to make difficult the detection of the double-exponential decay of IP.(d) The C R rate constant of CIP (k,) is larger than that of LIP (k,) in the case of TCNB-toluene and -xylene complexes in acetonitrile solution where both kinds of CR rate constants have been obtained, in agreement with the general tendency observed thus far in the case of various donor-acceptor systems.I*I5 The energy gap dependence of the CR rate constant of CIP formed by excitation of TCNB complexes in acetonitrile solution including a wide range of aromatic hydrocarbon donors of various strength has been confirmed to agree qualitatively with the previous result that the C R rate increases exponentially with decrease of the energyIn this way, we have examined a wide range of aromatic hydrocarbon donors by changing their oxidation potentials, molecular structures including their size, etc., in the case of TCNB complexes in relation to their photoinduced CS and ionic dissociation mechanisms, and have established the existence of several different cases concerning the behaviors of IP by referring also to our previous results.615In the above discussions, we have assumed the existence of SSIP in addition to CIP in the course of ionic dissociation. The results in case a seem to indicate strongly the existence of such an intermediate state of ionic dissociation. In case c, however, the existence of such an intermediate state in the course of ionic dissociation is not so evident as in case a. Moreover, some model calculations21*22 on ion pair-solvent dipole systems indicate the existence of LIPS with more intervening solvents between ions in the pair than SSIP, that is, the existence of the multiple kinds of IPS in the course of the dissociation. Actually, we have obtained some results that indicated the existence of multiple kinds of IPS in the course of CR and d i s s o c i a t i~n ,~~ which will be discussed elsewhere. N.M. acknowledges the support by a Grant-in-Aid (No. 62065006) from the Japanese Ministry of Education, Science and Culture. Acknowledgment. ~ (21) Levesque, D.; Weis, J. J.; Patey, G. N. J. Chem. Phys. 1980, 72, 1887. (22) Salem, L. Electrons in Chemical Reactions: First Principles; Wi-(23) Mataga, N.; Kanda, Y.; Asahi, T.; Hirata, Y.; Okada, T., manuscript (24) Gould, I. R.; Moser, J. E.; Ege, D.; Farid, S. J. Am. Chem. Soc. 1988.Photophysical properties of cis-stilbene and several homologues are investigated in supersonic expansions as free molecules and in large Ar clusters. Assignment of sharp vibrational structure in the spectra of 1,2-diphenylcyclobutene to long progressions in phenyl twisting and bending vibrations shows that in the SI state cis-stilbene phenyl twist angles are more planar and the phenyl-ethylene bend angles are more bent than i...

2-(2′-Hydroxy-5′-methylphenyl)benzotriazole, 1, Tinuvin P, and o-hydroxybenzophenone, 2, are thought to achieve exceptional photostability through highly reversible deactivation associated with their intramolecular hydrogen bonds. Excimer laser excitation (10 mJ, 20 ns at 308 nm) of these molecules in argon-bubbled hexane solution at room temperature affords no discernible transient signals (absorption or emission) between 320 and 800 nm. In DMSO solution under the same conditions, however, strong transient absorptions in the visible (λ max 410 and 425 nm for 1 and 2, respectively) are observed. The transients responsible for the absorptions are quenched by oxygen and acid, and the spectra match well with the ground state difference spectra generated from the corresponding phenolate ions. When a bulky group is incorporated ortho to the hydroxyl function such as in 2-(2′-hydroxy-3′-cumyl-5′-(1,1,3,3-tetramethylbutyl)phenyl)benzotriazole, 3, Tinuvin 928, no appreciable transient absorption is observed even in DMSO solution. These results are consistent with the disruption of the intramolecular hydrogen bond of 1 and 2 (but not 3) as the result of intermolecular hydrogen bond formation with DMSO. The "dramatic" ortho effect implies that the bulky group adjacent to the hydroxyl function sterically shields the intramolecular hydrogen bond from disruption by polar basic environments.

No abstract

No abstract

No abstract

No abstract

Magnetic field effects (MFEs) on the intramolecular exciplex fluorescence of a chain-linked phenanthrene (Phen)/N,N-dimethylaniline (DMA) system, Phen–(CH2)n–O–(CH2)2–DMA, have been studied in the magnetic fields (≤0.62 T) as functions of (1) solvent polarity (ε = 7.6—36.7), (2) chain length (n = 4—12), and (3) temperature (223—333 K). The MFEs on the exciplex fluorescence are attributable to the singlet–triplet intersystem crossing in the intramolecular radical ion pair which is in dynamic equilibrium with the exciplex. (1) Solvent polarity increases significantly the MFEs on the exciplex fluorescence of Phen–(CH2)10–O–(CH2)2–DMA. This is attributable to the stability of the intramolecular radical ion pair relative to the exciplex. (2) A remarkable influence of chain length on MFEs in the exciplex fluorescence in N,N-dimethylformamide (DMF) was observed. The results were discussed in connection with the edge-to-edge distances <r> of two radicals obtained from molecular dynamics calculation for model compounds. (3) The temperature-induced high-field shift of the exchange energies |2J| for Phen–(CH2)4–O–(CH2)2–DMA in DMF was discussed in connection with <r>.

Kinetic stabilization and reactivity of π single-bonded species have been investigated in detail by generating a series of singlet 2,2-dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls (DRs). The lifetime at 293 K in benzene was found to increase when the carbon chain length of the alkoxy groups was increased; 292 ns (DRb; OR = OR' = OCH3) <880 ns (DRc; OR = OR' = OC2H5) <1899 ns (DRd; OR = OR' = OC3H7) ≈2292 ns (DRe; OR = OR' = OC6H13) ≈2146 ns (DRf; OR = OR' = OC10H21). DRh (OR = OC3H7, OR' = OCH3; 935 ns) with the mixed-acetal moiety is a longer-lived species than another diastereomer DRg (OR = OCH3, OR' = OC3H7; 516 ns). Activation parameters determined for the first-order decay process reveal that the enthalpy factor plays a crucial role in determining the energy barrier of the ring-closing reaction, that is, from the π-bonding to the σ-bonding compounds. Computational studies using density functional theory provided more insight into the structures of the singlet species with π single-bonded character and the transition states for the ring-closing reaction, thereby clarifying the role of the alkoxy group on the lifetime and the stereoselectivity of the ring-closing reaction.