Triplet-Singlet Transitions in Organic Molecules. Lifetime Measurements of the Triplet State

Abstract

The emission lifetimes of the metastable triplet states (phosphorescent states) of a large variety of organic molecules have been measured. The lifetimes are in the range from 10−4 to about 10 seconds. It is shown that the transition probabilities corresponding to the shorter lifetimes are of the same magnitude as found in the light atoms of which the molecule is composed. The longer lifetimes, on the order of seconds, are found only among the aromatic compounds. A consideration of the perturbing singlet states in aromatic compounds shows that the matrix elements for the intercombination must be very much smaller than those responsible for the intercombination in the free carbon atoms. Direct evidence that the long-lived states of the aromatic compounds are triplet states is obtained by showing that as the atomic number of chemically similar substituents is increased (e.g. substitute Br for Cl), the transition probability increases approximately in proportion to the increase in the square of the spin-orbit interaction energy of the substituent atom. The luminescences are usually observable only when the substance is dissolved in a rigid solvent, and the effect of such a solvent on the lifetimes is discussed. Experimental data relating singlet-triplet absorption strength to triplet state lifetime is presented.

External heavy-atom effect on fluorescence kinetics

“…Heavy atoms in the fluorophore or in close contact to it increase the rate of intersystem crossing (ISC) by strengthening spin-orbit coupling, 2,3 like in the electronic transitions in heavy atoms (e.g. Hg).…”

“…Firstly, the heavy atom can be part of the chromophore under study; this is referred to as the internal heavy-atom effect. 2,4 If however the heavy atom is external to the chromophore, either intramolecularly (quenching moiety 11,14 ) or intermolecularly, as the quencher (e.g. I -, Xe) or part of the quencher [e.g.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Heavy atom quenching of fluorescence may occur in two different ways. Firstly, the heavy atom can be part of the chromophore under study; this is referred to as the internal heavy-atom effect.…”

Electronic spectroscopy of the β-carboline derivatives nitronorharmanes, nitroharmanes, nitroharmines and chloroharmines in homogeneous media and in solid matrix

“…[10][11][12][13][14][15] The spin-orbital coupling mechanism of the heavy atom enhances the rate of intersystem crossing to the triplet state. 12,15,16 Bromine and iodine atoms, as heavy atoms, have been preferentially used in order to study both intramolecular and intermolecular HAE. 3,4,[10][11][12][13][14][15][16] Furthermore, carbonyl and nitro groups attached to aromatic compounds also show a spin -orbital coupling mechanism that reduces significantly the fluorescence quantum yield of the aromatic compound and enhances the intersystem crossing process.…”

“…12,15,16 Bromine and iodine atoms, as heavy atoms, have been preferentially used in order to study both intramolecular and intermolecular HAE. 3,4,[10][11][12][13][14][15][16] Furthermore, carbonyl and nitro groups attached to aromatic compounds also show a spin -orbital coupling mechanism that reduces significantly the fluorescence quantum yield of the aromatic compound and enhances the intersystem crossing process. 3,4 In the present work the effect of the nitro group as substituent on the fluorescence (φ f , τ f , τ 0 f , k 0 f ) and phosphorescence (φ p , τ p , k 0 p ) parameters of several β-carboline derivatives is investigated: i) in acetonitrile and in methanol at 298 K; ii) in the same solvents in the presence of added mineral acids at 298 K; iii) in a mixture of isopropanol -ethyl ether (1:1; v:v) at 77 K and iv) in a mixture of isopropanol -ethyl ether (1:1) with added mineral acids at 77 K by using time correlated single photon counting technique and phosphorescence lifetime spectroscopy.…”