The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an e...

Unusual Photophysical Properties of Coumarin-151

. The triplet-triplet absorption spectra of psoralen, xanthotoxin, angelicin and bergapten have been determined using the technique of pulse radiolysis and laser flash photolysis in benzene and water. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 353 nm excitation. These yields vary considerably in passing from benzene to water as solvent. The highest yields were obtained for psoralen and angelicin in water, being 0.45 and 0.33, respectively. Reaction rate constants of the psoralen and angelicin triplets with various pyrimidine and purine nucleic acid bases and amino acids have been measured. The high reactivities support the suggestion that the photosensitising properties of furocou-marins are mediated by the corresponding furocoumarin triplet states. The similarly high S → T quantum yields, and triplet reactivities with pyrimidines of psoralen and angelicin, also support the suggestion that the observed differences in photosensitising properties of these two psoralens may be due to their differing geometries, which allow psoralen itself to intercalate DNA more easily than angelicin. The efficient reactions also found for psoralen and angelicin triplets with amino acids suggest that such triplets in vivo may also react with these components. Significant damage may thus be caused to protein as well as nucleic acid components of the cell.

Triplet excited state of furocoumarins: reaction with nucleic acid bases and amino acids

Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δν=νabs−νfl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(e−1)/(2e+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH2 group in a pyramidal configuration. In this structure, since the 7-NH2 group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH2 group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH2 group is highly restricted and thus there is no fast nonradiative deexcitation process for the excited dye.

Photophysical properties of coumarin-120: Unusual behavior in nonpolar solvents

Electrons are produced in several organic liquids by the two‐photon laser photolysis of dilute solutions of anthracene and pyrene in these liquids. The rate constants for reaction of the electron with biphenyl, carbon tetrachloride and oxygen in alkanes have been measured, and are found to be much larger than the values quoted in polar solvents. The rate constants for reaction of the electron with biphenyl increase with the square root of the electron mobility while the corresponding reaction with O2 is independent of the electron mobility. The mobility of the electron in hydrocarbon mixtures follows the exponential of the mole fraction of the hydrocarbon with the higher mobility. Complex behavior is observed in alcohol‐hydrocarbon mixtures, which may be explained in terms of aggregates of alcohol in these mixtures. The data are discussed in terms of the present theories of the state of electrons in liquids.

Dynamics of Electrons in Nonpolar Liquids

Extremely intense and sharp Sn←S1 absorption of cis‐ and trans‐stilbene was observed with a picosecond flash technique. The decay of the transient absorption of trans‐stilbene was exponential with one component (110±10 ps), being same as the fluorescence decay at room temperature, and thus assigned to the Sn←S1 absorption. The molar coefficient of the absorption was 7×105 M−1 cm−1 with the full width at half maximum of 1×103 cm−1. The band width had a strong temperature dependence and became only 500 cm−1 at 77 K. A theoretical calculation, using a semiempirical Hamiltonian of PPP type, was made to analyze the nature of this observed intense absorption, and was found that the character of the transition was of the charge‐resonance among the ethylenic and two phenyl moieties in stilbene.

J.K. Thomas

Papers

The pulsed laser photolysis of benzene in cyclohexane

Trapping of electrons by hydrocarbons and polar solutes

One electron oxidation and p-type delayed fluorescence of an aromatic amine studied by pulse radiolysis