Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.

The possibility of overcoming the Shockley–Queisser limit of organic solar cell (OSC) efficiency by multiexciton generation through singlet exciton fission has recently attracted significant research interest. Herein we show that 9,10-bis­(phenylethynyl)­anthracene (BPEA), an ethynyl derivative of anthracene and a widely used fluorescence molecular probe, exhibits an efficient singlet exciton fission process in the solid state. Steady state and time-resolved emission experiments carried out on a nanoaggregate and thin film of BPEA reveals an orders-of-magnitude reduction in emission yields and the singlet lifetime as compared to the near-unity emission yield and long emission lifetime in solution. Femtosecond and nanosecond resolved transient absorption studies unraveled exciton–exciton annihilation (at high excitation fluence) and singlet exciton fission to be the dominant relaxation processes with a fission yield of about 74 ± 6%. A high singlet fission yield with a long triplet lifetime (of about 30 μs) of BPEA in thin films and aggregates makes this material an interesting candidate for further study in OSC applications.

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2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine} (ZnPC) exists as monomeric species in DMSO and is reasonably strong fluorescent. But ZnPC forms H-aggregates in water and hexafluoroisopropanol, which are strong hydrogen bonding solvents. Nanoaggregates are nearly nonemissive. Transient absorption spectroscopic technique has been used to investigate the excited state relaxation processes in both monomeric and aggregated forms of ZnPC. The lifetime of the S 1 state of the monomoric form in DMSO is long (τ = 3.4 ns) but the excited states of ZnPC nanoaggregates show much faster ground state recovery (within 100 ps). The longest lifetime component, τ 3, which is independent of excitation density, has been assigned to the unimolecular decay of the S 1 -exciton in the absence of annihilation reaction, while τ 1 and τ 2 are the lifetimes obtained by the two-component fit of the nonexponential decay arising due to the time-dependent decay rates of the S 1 -excitons because of diffusive migration controlled exciton -exciton annihilation reaction. Rates of the annihilation reaction (2.0 × 10 −6 cm 3 s −1 ) and exciton migration (4.3 × 10 −5 m 2 /s) as well as diffusion length (about 85 nm) of the S 1 -exciton created in the ZnPC nanoaggregates in HFIP have been determined.

Wavenumber (cm -1 ) Absorption spectrum Excitation Spectrum (λ em = 425 nm) 400 380 360 340 320 Wavelength (nm)