Avinash V. Sapre

Photophysical properties of the fullerenes, C60 and C70

Photophysical properties of the singlet and triplet states of the fullerenes, C60 and C70, have been investigated in toluene, benzene, hexane and ethanol solutions by pico- and nano-second laser flash photolysis techniques. Lifetimes, extinction coefficients, quantum efficiencies of the formation of the excited states and such properties have been determined. True tripleti?½triplet absorption spectra of C60 and C70 are reported. C60, unlike C70, is found to undergo photoionization in ethanol solution giving solvated electrons.

Effect of solvent polarity on the aggregation of C60

C60 forms aggregates in solution when the dielectric constant of the solvents exceeds some critical value viz. ⩾13. Further, for aggregation the C60 concentration has to exceed some critical value. The aggregation process is reversible and the aggregates exist in equilibrium with monomers. Picosecond laser flash photolysis experiments indicate that the excited (S1) aggregates undergo very fast relaxation without contributing towards the triplet formation.

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Abstract Effect of solvent polarity on the aggregation behaviour of C70 has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C60. It is seen that similar to C60, aggregation of C70 also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C70 aggregation is found to be in the range of ∼27–31, which is much higher than that required for C60 aggregation (∼12–14). The large difference in the critical solvent polarity required for C60 and C70 aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules.

Dynamics of charge transfer in the excited amine complexes of the fullerenes C60 and C70.: A picosecond laser flash photolysis study

In benzene solution, C60 and C70 interact weakly in the ground state with amines having favourable oxidation potentials. Picosecond time-resolved absorption measurements show that on photoexcilation, the weak complexes undergo charge separation to produce ion pairs which in turn undergo fast geminate recombination either to produce the triplet state of the fullerenes or give back the ground slate of the complex, depending on the oxidation potential of the amine. Free-ion yield is generally negligible.

Dual Solvatochromism of Neutral Red

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine‐based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low‐polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high‐polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life‐times of NR have been determined in different solvents and correlated with the solvatochromic shifts.

Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions

Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromatic amines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state and time-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of the coumarin dyes by the amine quenchers always shows a positive deviation from linear Stern–Volmer relationship, which arises due to the localized high quencher concentrations at the micellar Stern layer. In time-resolved fluorescence measurements, the analysis of the fluorescence decays following a micellar quenching kinetics model assuming a unified quenching constant (kq′) per quencher occupancy does not give satisfactory results, especially for the higher quencher concentrations used. The observed fluorescence decays are, however, seen to fit reasonably well following a bi-exponential analysis for all the quencher concentrations used. The average fluorescence lifetimes of the coumarin dyes in the micellar solution as estimat...

Rotational dynamics of pyrrolopyrrole derivatives in alcohols: Does solute-solvent hydrogen bonding really hinder molecular rotation?

Rotational reorientation times of two structurally similar nonpolar molecules, 2,5dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) have been measured in n-alcohols using steady-state fluorescence depolarization technique. While both DMDPP and DPP contain two C=O groups, the latter also has two NH groups. As these groups are known to form hydrogen bonds with alcohol solvents, the present work is aimed at finding out whether or not such solute–solvent hydrogen bonding is effecting the rotation of the probe molecules. The rotational dynamics of DMDPP is explained reasonably well by the Stokes–Einstein–Debye (SED) hydrodynamic theory with slip boundary condition. It is also found that the hydrogen bonding between the two C=O groups of the probe and the solvent molecules is not influencing the rotation of DMDPP. However, the reorientation times of DPP are found to be longer by a factor of 2.2 to 3.3 compared to that of DMDPP, and followed a supe...

Rotational dynamics of neutral red: Do ionic and neutral solutes experience the same friction?

The rotational dynamics of neutral and cationic forms of the phenazine dye neutral red has been studied in n-alcohols, amides, and aprotic solvents using picosecond time-resolved fluorescence depolarization spectroscopy. While both the neutral and cationic forms of neutral red experienced more or less the same friction in alcohols, the cationic form experienced 16%–26% more friction in amides and aprotic solvents exceptions being formamide and propylene carbonate (PC). The results were analyzed in terms of the Stokes–Einstein–Debye (SED) hydrodynamic theory and dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes. Both the Nee–Zwanzig and van der Zwan–Hynes dielectric friction theories overestimate the dielectric friction contribution for the neutral form of neutral red in alcohols. The rotational dynamics of neutral form of neutral red in N, N-dimethyl formamide (DMF), N, N-dimethyl acetamide (DMA), N, N-dimethyl propionamide (DMP), and dimethyl sulphoxide (DMSO) is adequately described by ...

Photophysical Properties of the Cationic Form of Neutral Red

Abstract— Photophysical properties of the cationic form of neutral red (NRH+), a phenazine‐based dye of biological importance, have been investigated in several protic and aprotic solvents using optical absorption, steady‐state and time‐resolved fluorescence and picosecond laser flash photolysis techniques. Absorption and fluorescence characteristics of the dye in protic solvents indicate the existence of intermolecular hydrogen bonding between the NRH+ and solvent molecules in the ground state as well as in the excited state. Measurements of the fluorescence lifetime in normal and heavy water also support the formation of intermolecular hydrogen bonding. Time‐resolved transient absorption spectra obtained in the picosecond laser flash photolysis experiments show only the absorption band due to the Sn← S1 absorption. The picosecond transient absorption results do not indicate any spectral shifts attributable to the hydrogen bond formation dynamics between the excited NRH+ and the protic solvent molecules. It is inferred that the hydrogen bonding dynamics are much faster than the time resolution of our picosecond setup (∼35 ps).

Rotational dynamics of pyrrolopyrrole derivatives in glycerol: A comparative study with alcohols

The rotational dynamics of two structurally similar nonpolar molecules, 2,5-dimethyl-1,4dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c] pyrrole (DPP) has been studied in glycerol in the temperature range of 300–380 K using both time-resolved and steady-state fluorescence depolarization techniques. While the reorientation times of both the probes are varying linearly as a function of viscosity over temperature, the rotational dynamics of DMDPP is described by the Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition, whereas the reorientation times of DPP are in between slip and stick limits and are about a factor of 1.5 longer than that of DMDPP. This is due to the hydrogen bonding between the two NH groups of the probe molecule and the oxygen atoms of the hydroxyl groups in glycerol. It has also been observed that the rotational dynamics of a nonpolar and noninteracting molecule like DMDPP is essentially the same, both in glycerol and in n-alcohols.