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Isomer formation in dimeric complexes of a chiral naphthalene derivative (2-naphthyl-1-ethanol) with nonchiral or chiral primary and secondary alcohols (n-propanol, 2-methyl-1-butanol, 2-butanol, 2-pentanol) has been studied by hole-burning spectroscopy. Besides the spectroscopic discrimination between the homochiral and heterochiral complexes, previously observed in the fluorescence excitation spectra, ground-state depletion experiments have shown that each diastereoisomer is cooled in the jet in several isomeric forms. To get information on the structures of the complexes and on the influence of the solvent conformations of these structures, semiempirical calculations that rely on the exchange perturbation method have been performed. It has been shown that the most stable complexes involve a H-bond between the chromophore acting as the donor and the solvent and that they involve anti and gauche conformations of the solvent. The binding energy of the complexes results from a subtle balance between electrostatic and dispersive forces: the complexes involving the gauche and anti conformers of the solvent differ from each other by the amount of dispersion energy relative to the total interaction energy. The increase in the dispersive forces calculated for the complexes with the anti conformers has been related to a larger red shift of the absorption spectrum and is suggested to play a role in the observed chiral discrimination.

The influence of methyl and methoxy substitution in the para position of the phenolic OH functional group on the intramolecular proton-transfer properties of electronically excited salicylic acid (ESIPT) has been investigated both in solution and in the isolated gas-phase conditions provided by supersonic cooling. The dual fluorescence observed for 5-methylsalicylic acid (5-MeSA) in alkane solutions has been attributed for its blue part to the excited tautomer resulting from the intramolecular proton-transfer process and for its UV component to the dimer. A single fluorescence emission peaking at 400 nm is observed in alkane solutions of 5-methoxysalicylic acid (5-MeOSA). In the presence of proton acceptors such as diethyl ether, the 5-MeSA solution emits only in the blue region while 5-MeOSA exhibits two fluorescence bands at 400 and 475 nm. This behavior shows that the ESIPT process is promoted by complexation with proton-accepting molecules. In the supersonic expansion, the excitation and dispersed emission spectra of 5-MeSA are very similar to those previously observed for unsubstituted salicylic acid and show that the ESIPT mechanism takes place without barrier, in agreement with the model of a distorted potential surface in the excited state. In contrast, the 5-MeOSA excitation and dispersed fluorescence spectra present a mirror-image relationship that indicates that the molecule keeps a similar geometry in the ground and excited state. In this case the ESIPT reaction is prevented. Complexation with diethyl ether and acetone does not give rise to a dual fluorescence as in solutions but results in a broad emission extending toward the visible. This result may be explained by a modification of the excited potential energy surface along the tautomerization coordinate without introducing an energy barrier in the proton-transfer reaction.

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van der Waals complexation in a supersonic expansion combined with laser-induced fluorescence techniques have been used to discriminate between isolated diastereoisomer pairs of weakly bound species. By using 2-naphthyl-1-ethanol (2-NetOH) as the chiral chromophore, we have investigated the effect of complexation with different aliphatic alcohols on the microscopic shifts of the S0−S1 transition and on the fluorescence decay times of the chromophore. The fluorescence excitation spectra of the complexes of 2-NetOH with nonchiral primary alcohols have been first examined, and the binding energy of the complex of 2-NetOH with methanol has been determined to be on the order of 1000 cm-1. In the case of complexation with chiral solvents such as 2-methyl-1-butanol or secondary alcohols, the homochiral and heterochiral pairs give rise to specific spectral shifts and patterns which allow them to be clearly distinguished. The fluorescence lifetimes following excitation of alcoholic complexes in every case are longer than those of the uncomplexed chromophore and also depend on the particular diastereoisomer excited. The chiral recognition evidenced on the spectral properties and on the dynamical relaxation processes of isolated enantiomeric pairs shows the nonequivalence of their interaction energy in both the ground and excited states. The nature of the stereochemically dependent interactions can be tentatively described on the grounds of a hydrogen bonded intermolecular structure involving a folded geometry of the alkyl chain with respect to the naphthalene nucleus.

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