The molecular and excimer fluorescence bands recorded for concentrated solutions of pyrene, lY2-benzanthracene, 2-methylnaphthalene, 1-fluoronaphthalene and acenaphthene exhibit an " isosbestic " point over certain limited ranges of temperature ; from the temperature coefficient of the ratio of excimer to molecular fluorescence intensities at the isosbestic wavelength the enthalpy and entropy of photoassociation can be estimated. The heat of photoassociation is found to be significantly higher for pyrene than for the other compounds examined, and for these systems the repulsion energy between unexcited molecules in the excimer configuration contributes substantially to the red-shift of 5000-6000 cm-1 of the excimer band maximum relative to the O"-O' molecular fluorescence band. Entropies of photoassociation are in the region of -20 cal/mole deg. Except at low concentrations the photoassociation of identical uncharged planar aromatic molecules A, competes with fluorescence emission,'A*-+A + hvM, 1A*+3A, and intersystem crossing, of the lowest electronically-excited singlet state 1A* in a fluid .environment. Tlic evidence for process (1) is the emission of a broad structureless band,ly 2 red-shifted by some 5000-6000 cm-1 from the structured molecular fluorescence spectrum, which originates in the radiative relaxation of the excimer 1AZ to a dissociated ground state Process (4) competes in turn with the formation of a stable dimer A2, internal conversion and possibly intersystem crossing 'A;-+2A+ Itv,. (4) 1 * &-+A,, 2A or 3A,(?j, together with dissociation relaxation, at higher temperatures.3The molecular configuration of the photoassociated state 'A; is believed to be that in which the aromatic planes are perpendicular to the axis joining the molecular centres of gravity4; this is supported by the emission of characteristic excimer * presented at the 7th European Congress on Molecular Spectroscopy,

No abstract

VAg reactivity indices (ß) measured for naphthalene, anthracene, 1,2-benzanthracene, naphthacene, 1,2,5,6-dibenzanthracene, pentacene, and/or certain derivatives vary by six orders of magnitude in benzene at 25°. An orbital correlation through the transition state is presented for anthracene as a model acceptor in an attempt to identify those acceptor properties which contribute to the activation energy for O^Ag addition insofar as this largely controls acceptor reactivity. It is shown that the x-relocalization energy provides a satisfactory reactivity parameter for unsubstituted acceptors if this is estimated as the difference in x-electron energy of the acceptor and of those odd-alternant fragments which provide electron localization at the site of CVAg addition, with a value of -36 kcal for the bond integral. Electron donating substituents at the site of (VAg addition increase acceptor reactivity in the order H < C6H5 < CH30 ~CH3, each methyl substituent producing an increase in reactivity by a factor of *~13 for anthracene.

No abstract