Effect of Triplet T1-State Population on Fluorescence Polarization Kinetics of Organic Fluorophores

The fluorescence polarization kinetics of fluorescent organic molecules and molecules with two interconverting photoisomeric forms and fixed molecular skeletons were investigated by numerical methods based on the solution of population balance equations. The depolarizing effect of the lowest triplet T 1 state and the polarization time dependence for quasi-continuous-wave (cw) and pulsed photoexcitation were studied using model compounds. The fluorescence depolarization with cw excitation was characterized by times that were shorter than the T 1 -state lifetime; for the isomers, also by times that correlated with the photoisomerization times. Intense pulsed photoexcitation distorted significantly the initial oscillator orientation distribution in the singlet S 0 and S 1 states and could lead to sign reversal of the polarization.