Dieter Gloyna

Picosescond fluorescence lifetime standards for frequency- and time-domain fluorescence.

We characterized a series of dimethylamino-stilbene derivatives as standards for time-domain and frequency-domain lifetime measurements. The substances have reasonable quantum yields, are soluble in solvents available with a high purity, and do not show significant sensitivity to oxygen quenching. All the fluorophores displayed single exponential intensity decays, as characterized by frequency-domain measurements to 10 GHz. The decay times vary from 880 to 57 ps, depending on structure, solvent, and temperature, which is a useful range for modern picosecond time-domain or gigahertz frequency-domain instruments. These fluorophores may be used either to test an instrument or as reference compounds to eliminate color effects. We also characterized two-fluorophore mixtures, with the decay times spaced twofold (150 and 300 ps), with varying proportions. These mixtures are useful for testing the resolution of other time- and frequency-domain instrumentation. The excitation wavelength ranges from 260 to 430 nm, and the emission from 350 to 550 nm. The decay times are independent of the excitation and emission wavelengths.

Effect of substituents on the T1 energy of trans-stilbenes

Abstract Oxygen-induced singlet—triplet absorption and electrochemiluminescence quenching experiments with substituted stilbenes indicate a small influence of monosubstitution or donor—acceptor disubstitution on the triplet energy. This implies a decrease of the S 1 T 1 energy difference particularly in the case of donor—acceptor substituted stilbenes.

Einfluß der Polarität und Viskosität des Lösungsmittels auf die Deaktivierung des S1-Zustandes von Donator-Akzeptor-substituiertentrans-Stilbenen

The dependence of the fluorescence constantskfnand the adiabatic twisting constantsktpof the S1-State upon the solvent polarity, the capacity of hydrogen-bonding, as well as the solvent viscosity, were investigated for donor-acceptor substitutedtrans-stilbenes [4-dimethylamino-4′-cyano-stilbene (1 g), 4-dimethylamino-4′-diphenylphosphinyl-stilbene (1 b), 4-dimethylamino-4′-brom-stilbene (1 h) and 4-dimethylamino-4′-methoxistilbene (1 k)] by using the multiparameter model. An increase of the solvent polarity results in the decrease of the fluorescence ratekfnfor all these compounds. The specific interaction between the strongly polar stilbenes such as1 g and1 b and a solvent leads to additional decrease inkfnbut in the case of a relatively weak polar compound1 k to its growth. The linear combination of universal and specific solvent-solute interactions yields distinct relations forktp. Both interactions have an influence onktpfor1 g,1 b, and1 k to an equal degree. The twisting observed for1 g and1 b due to the interaction with solvent is hindered, however it is being supported for1 k. The influence of the investigated small range of viscosity onktpat simultaneous variable solvent polarity (pure solvent) is only visible for the weak polar1 k. In the other cases the effect of solvent-polarity is predominant.The viscosity dependence ofktpwas also experimentally proved for1 g,1 b and1 k by applying the obtained different dimethylformamid-acetonitril mixtures. According previous results the radiationless deactivation process is caused by twisting around ethyl-bonding, thereforektpevidently decreases with an increase of viscosity.

Photophysics of derivatives of trans-stilbenes in poly(vinyl alcohol) films

Absorption and fluorescence spectra of eight trans-stilbene derivatives have been measured in poly(vinyl alcohol)(PVA) films. Fluorescence quantum yields, ϕF, and mean fluorescence lifetimes, τF, have also been determined for these substances. Fluorescence quantum yields investigated in PVA films were found to be markedly higher than those measured in liquid solutions, this effect being even more pronounced with the mean fluorescence lifetimes. Radiative and radiationless deactivation constants kF and kd, respectively, were determined based on the measurements of ϕF and τF. The effect of donor–acceptor properties of substitutes δp upon the deactivation constants was examined. The energies of the 0–0 transitions between electronic states S0 and S1 were also affected by the donor–acceptor properties of the substituent, remaining in correlation with the Hammett constants δp.

Substituentenabhängigkeit bei der direktentrans →cis-Photoisomerisierung von donator-substituierten 4′-Diphenylphosphinyl-trans-stilbenen

From the fluorescence quantum yield ϕf, the fluorescence decay time τfw and the quantum yield ϕtc of thetrans →cis photoisomerization results the fraction α of perpendicular singlet (perp-S1) decaying to ground statetrans-stilbenes1. The α values of the donor substituted 4′-diphenylphosphinyl-trans-stilbenes1 in the solvents toluene andn-propanol are dependent on the donor substituent in1 and the solvent. It is concluded for1 that increasing polarity of the first excited singlet state (tr-S1) promotes thetrans →cis photoisomerization.