Solvent polarity and temperature effect on the photophysical properties of two 1,2-benzopyrone dyes, namely, coumarin-152 (C152) and coumarin-481 (C481), have been investigated using steady-state and time-resolved fluorescence measurements. In nonpolar (NP) solvents (e.g., hexane, cyclohexane, methylcyclohexane, 2-methylpentane, and 3-methylpentane), the Stokes' shifts (Δν̄) and fluorescence lifetimes (τf) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Δν̄ correlates linearly with the solvent polarity function, Δf = {(ε − 1)/(2ε + 1) − (n 2 − 1)/(2n 2 + 1)}, and the slopes of these plots indicate that the fluorescent states for both the dyes in these solvents are of intramolecular charge transfer (ICT) character. The unusually lower Δν̄ and τf values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields (Φf) and τf values of the two dyes also show an unusual reduction in higher polarity solvents (Δf > ∼0.2). Furthermore, unlike in all other solvents, in solvents with Δf > ∼0.2, the Φf and τf values are strongly temperature-dependent. These results indicated the involvement of a new activation-controlled nonradiative deexcitation channel in higher polarity solvents, assigned to the involvement of the nonfluorescent twisted intramolecular charge transfer (TICT) state. Though the activation barrier (ΔE a) for such processes usually decreases with solvent polarity, for the present systems the ΔE a is seen to increase with Δf. This unusual behavior has been rationalized assuming that the ΔE a arises because of the potential energy (PE) crossing of the TICT and ground states rather than that of the ICT and TICT states as is usually the case for most TICT molecules. On increasing Δf, since the highly polar TICT state not only gets better stabilization but also its PE surface becomes more steeper than the ICT state, the ΔE a effectively increases with solvent polarity.

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 (Δν̄=ν̄abs−ν̄fl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(ε−1)/(2ε+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.

Electron transfer (ET) from aniline, N-alkylanilines, N,N-dialkylanilines, and o-phenylenediamine to a number of excited (S 1 ) 4-CF 3 -coumarin dyes having differently substituted 7-amino groups have been investigated in acetonitrile solutions using steady-state (SS) and time-resolved (TR) fluorescence quenching measurements. Direct evidence for the ET reactions in the present systems have been obtained by characterizing the amine cation radicals using picosecond transient absorption measurements in the visible region. The experimentally determined bimolecular quenching constants (k q ) are seen to correlate nicely with the free energy changes (∆G°) for the ET reactions, within the framework of the Marcus ET theory. The total reorganization energy (λ) estimated from such correlation indicates that the solvent reorganization (λ s ) plays the major role in governing the ET dynamics in the present systems. A comparison of the present results under diffusive conditions with those reported earlier under nondiffusive conditions indicates that the ET dynamics in the latter cases are much faster than in the former. The results are discussed considering a structural difference between the encounter complexes formed under diffusive and nondiffusive conditions.

Photophysical properties of 7-NHEt-4-CF 3 -1,2-benzopyrone dye (coumarin-500, C500), have been investigated in different solvents and solvent mixtures using optical absorption and fluorescence measurements. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 2-methylpentane, 3-methylpentane, and Decalin, the dye displays unusual properties in comparison to those in other solvents. Thus, in the former solvents, the Stokes' shifts ∆ν j and the fluorescence lifetimes (τ f ) for C500 are found to be unusually low. Though in these solvents the fluorescence quantum yields Φ f are seen to be only marginally higher, the radiative decay rate constants k f are found to be unusually higher than those in the other solvents. From comparing the results of C500 with those reported earlier for its lower analogue, C151 (coumarin-151, 7-NH 2 -4-CF 3 -1,2-benzopyrone; Nad, S.; Pal, H. J. Phys. Chem. A. 2001, 105, 1097, it is inferred that just like C151, in nonpolar solvents the dye C500 also exists in nonpolar structures, whereas in all other solvents the dye exists in a polar intramolecular charge transfer (ICT) structure. Unlike C151, the C500, however, does not show any activationcontrolled nonradiative deexcitation channel for the S 1 state in nonpolar solvents, which arises due to the flip-flop motion of the 7-amino group. Due to the heavier mass, the flip-flop motion of the 7-NHEt group in C500 is supposed to be much slower than that of the 7-NH 2 group in C151. Thus, such a motion cannot introduce any deexcitation channel, which can compete with the intrinsic fluorescence k f and the internal conversion k IC rates in the excited C500 dye. Thus, unlike C151, there is no activation-controlled nonradiative deexcitation channel for C500 in nonpolar solvents though both the dyes appear to exist in similar structural forms in these solvents, which are different than those existing in other solvents of lower to higher polarities.

Electron transfer (ET) interactions of a series of 7-aminocoumarin dyes with aliphatic amine donors have been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching, picosecond laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. For different coumarin–amine pairs, the estimated quenching constants (kq) from SS and TR fluorescence measurements are found to be similar within the experimental error. That ET from amine donors to excited (S1) coumarin dyes takes place has been established from the LFP and the PR results. For different coumarin–amine pairs, the kq values are seen to correlate well with the free energy changes (ΔG0) for the ET reactions following Marcus’ outer-sphere ET theory. The total reorganization energy (λ) estimated from this correlation is seen to be just similar to the solvent reorganization energy (λs). The leveled-off kq value under diffusion-controlled condition (kqDC) appears to be much lower (∼2.5 times) for the present systems compared to the corresponding value obtained for the ET reactions in coumarin–aromatic amine systems. The large difference in the kqDC values with aliphatic and aromatic amines as the electron donors has been rationalized on the basis of the shapes of the highest occupied molecular orbitals (HOMO) of the amine donors. For aliphatic amines, since their HOMOs are largely localized on the amino group, a large orientation factor is involved in the encounter complexes for the ET reaction to take place. With π like HOMOs, such orientational restriction is just nominal with the aromatic amines as the electron donors. Present ET results under diffusive conditions have also been discussed with a comparison to the ET rates observed under nondiffusive conditions, where the amines are directly used as the solvent donors.