The photophysics of a number salicylic acid derivatives (SADs) in aqueous solutions was investigated in a wide range of pH by time-correlated single photon counting (λ_ex = 350 nm, τ_resp = 300 ps) and fluorescence up-conversion (λ_ex = 266 nm, τ_resp = 300 fs) techniques. The acid-base equilibrium constants in the ground (pK_a) and the excited states (pK_a*), the fluorescence quantum yields as well as the lifetimes of anionic, neutral, and cationic forms of SADs were determined. Evidence of ultrafast excited-state intramolecular proton transfer (ESIPT) leading to the formation of the proton-transferred excited state of SADs was obtained from the fluorescence up-conversion measurement. The nature of the ESIPT process is discussed.

The kinetics of the dual fluorescence of several derivatives of dimethylaminobenzonitrile (DMABN) has been compared using fs-fluorescence upconversion experiments. Variation of the size and twist angle of the donor (dialkylamino group) suggest a large amplitude solvent-viscosity controlled diffusional twisting motion towards larger twist angles as the rate limiting step. Large rate differences were observed for an ester group as acceptor. Temperature dependent studies indicate that these differences are not connected with different activation barriers but with changes in the Arrhenius preexponential factor. It is argued that conical intersections along the reaction path can bring about these entropy changes.

The influence of solute−solvent interactions on the vibrational energy relaxation dynamics of perylene and substituted perylenes in the first singlet excited-state upon excitation with moderate (<0.4 eV) excess energy has been investigated by monitoring the early narrowing of their fluorescence spectrum. This narrowing was found to occur on timescales ranging from a few hundreds of femtoseconds to a few picoseconds. Other processes, such as a partial decay of the fluorescence anisotropy and the damping of a low-frequency oscillation due to the propagation of a vibrational wavepacket, were found to take place on a very similar time scale. No significant relationship between the strength of nonspecific solute−solvent interactions and the vibrational energy relaxation dynamics of the solutes could be evidenced. On the other hand, in alcohols the spectral narrowing is faster with a solute having H-bonding sites, indicating that this specific interaction tends to favor vibrational energy relaxation. No relationship between the dynamics of spectral narrowing and macroscopic solvent properties, such as the thermal diffusivity, could be found. On the other hand, a correlation between this narrowing dynamics and the number of low-frequency modes of the solvent molecules was evidenced. All these observations cannot be discussed with a model where vibrational energy relaxation occurs via two consecutive and dynamically well-separated steps, namely ultrafast intramolecular vibrational redistribution followed by slower vibrational cooling. On the contrary, the results indicate that both intra- and intermolecular vibrational energy redistribution processes are closely entangled and occur, at least partially, on similar timescales.

A series of 6-styryl-2,4-diphenylpyrylium salts exhibiting dual fluorescence has been investigated by fluorescence up-conversion in conjunction with quantum chemical calculations. The short-wavelength emission is due to an excited state localized on the pyrylium fragment and the long-wavelength emission arises from a charge-transfer state delocalized over the whole molecule. The two fluorescing states do not exhibit a precursor−successor relationship. The rise time of the short-wavelength fluorescence is smaller than 200 fs, and that of the long-wavelength emission depends on the electron-donating property of the styryl group substituent. The rise is almost prompt with the weaker donors but is slower, wavelength and viscosity dependent with the strongest electron-donating group. A model involving a S₂/S₁ conical intersection is proposed to account for these observations.

Electronic energy transfer (EET) rate constants between a naphthalene donor and anthracene acceptor in [ZnL^4a](ClO₄)₂ and [ZnL^4b](ClO₄)₂ were determined by time-resolved fluorescence where L^4a and L^4b are the trans and cis isomers of 6-((anthracen-9-yl-methyl)amino)-6,13-dimethyl-13-((naphthalen-1-yl-methyl)amino)-1,4,8,11-tetraazacyclotetradecane, respectively. These isomers differ in the relative disposition of the appended chromophores with respect to the macrocyclic plane. The trans isomer has an energy transfer rate constant (k_EET) of 8.7 × 10⁸ s^-1, whereas that of the cis isomer is significantly faster (2.3 × 10⁹ s^-1). Molecular modeling was used to determine the likely distribution of conformations in CH₃CN solution for these complexes in an attempt to identify any distance or orientation dependency that may account for the differing rate constants observed. The calculated conformational distributions together with analysis by ¹H NMR for the [ZnL^4a]²⁺ trans complex in the common trans-III N-based isomer gave a calculated Förster rate constant close to that observed experimentally. For the [ZnL^4b]²⁺ cis complex, the experimentally determined rate constant may be attributed to a combination of trans-III and trans-I N-based isomeric forms of the complex in solution.

Ultrafast time-resolved fluorescence measurements have been carried out to investigate vibrational relaxation dynamics of perylene derivatives in solution. The early results obtained with cyanoperylene in acetonitrile are presented and discussed.