The photophysical properties of the N-H and N-methyl derivatives of 1,2-, 2,3-and 1,8-naphthalimides have been studied. The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 26, 2.8 D for 36 and <2 D for 46, 1 D z 3.33564 x C m, and 26, 36 and 46 are N-methyl-1,2naphthalimide, N-methyl-2,3-naphthalimide and N-methyl-1 ,&naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 46 increases dramatically by three orders of magnitude compared with that of 26 ; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, k,, increases. This difference is found to arise from the energy gap between the lowest '(n,n*) singlet excited state and the upper 3(n,n*) triplet state, which is of the order of 9 kcal mol-' for 26 and less than 2 kcal mol-' for 46 in acetonitrile solution. Protic solvents increase the energy difference between the n,n* and n,n* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 46 is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet-triplet annihilation process occurs with the N-methyl derivatives 36 and 46 which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

Absorption and fluorescence spectra, fluorescence decay times, and quantum yields of fluorescence and triplet state formation have been determined for N-phenyl and substituted N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides, using stationary irradiation and laser flash excitation methods. The effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied. A short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives, and/or a considerably red shifted long-wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties. A striking characteristic of the Nphenylnaphthalimides (in contrast to the N-alkyl derivatives) is the very efficient internal conversion which results in short fluorescence decay times and in low fluorescence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometrical relaxation of the Franck-Condon state yields two emitting excited states, the SW and LW states, which emit the short-wavelength and long-wavelength fluorescence, respectively. The geometry of the SW state is similar to that of the ground state, while twisting of the phenyl group toward a coplanar geometry is assumed to be required in the formation of the LW state. The extended conjugation comprising the phenyl and naphthalimide moieties, attributed to the coplanar geometry, together with the charge transfer character endows the LW excited state with an extra stability. Solvent cage and geometrical (twisting) relaxation induces efficient internal conversion by virtue of pseudoJahn-Teller coupling of the two low-lying excited states ("proximity effect") as well as by the decrease of the energy gap between the LW excited state and ground state ("energy gap law").

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946J. Am. Chem. SOC. diffusing carbonium ions by acetonitrile solvent followed by hydrolysis (the Ritter reaction starting from an unactivated C-H bond). The other minor byproducts including those derived from photooxidation of the tetra-n-butylammonium cations and the acetonitrile solvent as well as a small percentage of the epimerization product arise from freely diffusing intermediate radicals.Not shown in Scheme I11 are the processes involving reoxidation of the reduced [or photoreduced] polyoxotungstates. Reoxidation can be facilitated by (1) reaction with O2 and protons, evolved from the dehydrogenation reactions, to form H 2 0 , (2) reaction solely with protons to form H2, or (3) in some circumstances, by reaction with other oxidants. These processes, although peripheral to the novel chemistry in the complex redox processes which we have elaborated in this paper, are nevertheless significant for they limit the overall rate of catalytic turnover. 1992,114,946-953The substantial degree to which the first and second redox potentials, protonation states, and other characteristics of the polyoxometalates that affect the relative rates of the key processes in Scheme I11 and analogous processes in other reactions can be systematically altered through rational syntheses of polyoxometalates suggests potential applications of these complex inorganic molecules to transformations of organic materials in synthesis and various technologies.Acknowledgment. We thank the National Science Foundation (Grant CHE-9022317) for support.Registry No. cis-1, 1579-21-1; trans-1, 16021-08-2; 2, 13837-12-2; 3, 062, 110294-54-7; Q4W10032,68109-03-5; Na4Wl,OJ2, 126752-51-0; D2, 7782-39-0; cis-decalin, 493-01-6; trans-decalin, 493-02-7.1196-55-0; DTBO, 1876-22-8; Q3PW12040, 53749-37-4; (NH4) Abstract The luminescence properties of 2,3-naphthalimides have been studied using picosecond and nanosecond spectroscopies. In acetonitrile solution N-phenyl-2,3-naphthalimide (3) is found to emit dual fluorescence with emission maxima at 385 and 490 nm, respectively. The short-wavelength emission corresponds to the known fluorescence of the naphthalimides and is demonstrated for 3 to originate from a molecular conformation in which the phenyl substituent and the naphthalimide skeleton are orthogonal to each other. The long-wavelength emission is assumed to originate from a singlet excited state formed by a ca. 90° rotation of the phenyl group so that the two moieties are coplanar. Only a small dipole moment change is found between this excited state and the ground state. Only short-wavelength emission is observed with a lifetime in the nanosecond range as in the case of 1 and 2 when phenyl rotation is blocked with a bulky ortho tert-butyl group (compound 4). Increasing the viscosity of a glycerol/ethanol medium enhances both the efficiency and the lifetime of the short-wavelength emission of 3. It appears that at 77 K the emission originates directly from the Franck-Condon state. At room temperature, the other two emitting species are shown to ari...

The photoreduction of N-phenyl-1,8-naphthalimide and N-phenyl-2,3-naphthalimide by aliphatic amines has been studied by laser flash photolysis with transient absorption and transient conductivity methods in different solvents. Analysis of transient time profiles establish for most systems the occurrence of a fast primary and a slower secondary reduction process. Primary reduction is ascribed to the reaction between a triplet naphthalimide and an amine, while secondary reduction is assigned to the reaction of an amine-derived a-aminoalkyl radical with a ground-state naphthalimide molecule. In polar solvents, with aliphatic amines both primary and secondary reductions proceed by electron transfer. In solvents of intermediate polarity, hydrogen atom transfer (primary reduction) is succeeded by electron transfer (secondary reduction). Finally, in nonpolar solvents, only primary reduction by hydrogen atom transfer is found to occur. Rate constants are obtained for most of these processes by computer modeling of the transient time profiles. In polar solvents, reaction AH2+ + AH2 -A H + AH3+ (where AH2 and AH are the amine and a-aminoalkyl radical, respectively) is a key reaction in which the a-aminoalkyl radical is formed. Its rate constant is found to decrease by more than 4 orders of magnitudes when AH2 varies from tertiary, through secondary, to primary amine. This is explained by the significant change in the dissociation energy of the C-H bond in the a-position to the nitrogen. The electron transfer between the a-aminoalkyl radical and the naphthalimide molecule is found to occur in polar solvents with a rate close to the diffusion controlled limit, whatever the type of the aliphatic amines.

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