Thermodynamic and kinetic aspects of the intermolecular exciplex formation between 2-methylnaphthalene and aliphatic amines

Proton-coupled charge-transfer reactions and photoacidity of N,N-dimethyl-3-arylpropan-1-ammonium chloride salts

Excited-state, intermolecular proton-transfers of aromatics tethered to ammonium groups are solvent mediated and coupled to either the formation of an exciplex or a solvent-separated ion pair.

Electronic Energy-Transfer Rate Constants for Geometrical Isomers of a Bichromophoric Macrocyclic Complex

The rate of electronic energy transfer (EET) between a naphthalene donor and an anthracene acceptor in [ZnL(3)](ClO(4))(2) and [ZnL(4)](ClO(4))(2) was determined by time-resolved fluorescence measurements, where L(3) and L(4) are the geometrical isomers of 6-[(anthracen-9-ylmethyl)amino]-trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-13-amine (L(2)), substituted with either a naphthalen-1-ylmethyl or naphthalen-2-ylmethyl donor, respectively. The energy-transfer rate constant, k(EET), was determined to be (0.92 +/- 0.02) x 10(9) s(-1) for the naphthalen-1-ylmethyl-substituted isomer, while that for the naphthalen-2-ylmethyl-substituted isomer is somewhat faster, with k(EET) = (1.31 +/- 0.01) x 10(9) s(-1). The solid-state structure of [ZnL(3)Cl]ClO(4) has been determined, and using molecular modeling calculations, the likely distributions of solution conformations in CH(3)CN have been evaluated for both complexes. The calculated conformational distributions in the common trans-III N-based isomeric form gave Förster EET rate constants that account for the differences observed and are in excellent agreement with the experimental values. It is shown that the full range of conformers must be considered to accurately reproduce the observed EET kinetics.

Wavelength-Dependent Stereodifferentiation in the Fluorescence Quenching of Asymmetric Naphthalene-Based Dyads by Amines

In the present contribution, wavelength has been used as a tunable parameter to achieve selective control of the photophysics of two novel asymmetric bichromophoric dyads composed of naphthalene units, i.e., 6-methoxynaphthalene (NPX) and 1-methylnaphthalene (NAP) derivatives, with different electronic properties, connected by an amide spacer [(S,S) and (S,R)-NPX-NAP]. As model systems, relevant monochromophoric compounds (NPX-M and NAP-M) have also been investigated. While upon excitation at 325 nm the light energy remained in the NPX moiety, at 290 nm an efficient singlet-singlet energy transfer (phi(SSET) of about 97%) from the NAP unit to the NPX chromophore dominated. A remarkable stereodifferentiation was observed in the excited-state quenching by triethylamine via exciplex formation. The results demonstrate that it is possible to control configuration-dependent interactions in the excited state by wavelength tuning. This can be rationalized through intramolecular interactions of pi systems leading to modulation of the redox properties.

Intramolecular electron transfer in diastereomeric naphthalene–amine dyads: a fluorescence and laser flash photolysis study

Two dyads containing a naphthalene-like chromophore linked to a pyrrolidine-derived moiety, namely (S,S)- and (R,S)-NPX-PYR, have been synthesised by esterification of (S)- or (R)-naproxen (NPX) with (S)-N-methyl-2-pyrrolidinemethanol (PYR) and submitted to photophysical studies (steady-state and time-resolved fluorescence, as well as laser flash photolysis). The emission spectra of the dyads in acetonitrile were characterised by a typical band centred at 350 nm, identical to that of the reference compound (S)-NPX. However the intensities were clearly different, revealing a significant intramolecular quenching in the dyads, as well as a remarkable stereodifferentiation (factor of 1.6). Accordingly, the fluorescence lifetimes of the two dyads were different from each other and markedly shorter than that of (S)-NPX. The quenching mechanism is intramolecular electron transfer, that is thermodynamically favoured. Exciplex formation, that is nearly thermoneutral, does not compete efficiently. The electron transfer rate constants for (S,S)- and (R,S)-(NPX-PYR) were 1.8 x 10(8) and 2.8 x 10(8) s(-1), respectively. By contrast, no significant intramolecular quenching was observed for the excited triplet states (lambda(max)= 440 nm), generated by laser flash photolysis; this is in agreement with the fact that intramolecular electron transfer is thermodynamically disfavoured, due to the lower energy of excited triplets.