Search

A new push-pull stilbene was synthesized, and its photophysical properties investigated by spectroscopic and theoretical methods. Solvatochromic shifts in the fluorescence were in accordance with Onsager's continuum model, whereas shifts in the absorption spectra only correlated with the polarizability of the solvents. Excited state dipole moments were determined and compared to semi-empirically calculated values. With increasing solvent polarity the fluorescence quantum yield increases steeply as the solvent dielectric permittivity exceeds 8. It is concluded that a strongly luminescent and highly dipolar π, π ∗ state becomes the lowest excited state in polar solvents, and radiative deactivation therefore competes effectively with non-radiative pathways. Preventing internal rotation of molecular subunits is thus no prerequisite for obtaining highly fluorescent push-pull stilbenes.

Exciplex formation between excited triethylamine and ethanol in hydrocarbon solution is demonstrated by steady-state and time-resolved fluorescence spectroscopy. At least two types of excited complexes can be resolved. The 1:1 complex shows only small spectral shift versus the monomer and a binding energy of 12 kJ/mol is obtained.

The influence of various solvents (water, methanol, diethyl ether, n -hexane, cyclohexane, as well as of mixtures) and of the excitation energy (λ exc ) on the fluorescence quantum yield ( Q F ) of phenol has been investigated in O 2 - free solutions. At excitation energies above 5.1 eV (λ exc ⩽ 240 nm) a strong decrease of Q F was observed in apolar solvents and in water. Addition of polar solvents reduces the effect. Various possibilities are discussed in order to explain this behaviour, especially the influence of H-bonds between the phenol molecules and the solvent on the occurrence of radiationless transitions.