Solvatochromism, aggregation and photochemical properties of Fullerenes, C<SUB>60</SUB> and C<SUB>70</SUB>, in solution

Fullerenes, C60 and C70, display interesting physicochemical properties in solutions, especially due to their unique chemical structures and their good electron accepting abilities. Solubility of fullerenes in different organic solvents and their unusual solvatochromic behavior, the ability of the fullerenes to form aggregates in solutions, and their electron transfer and charge transfer interactions with variety of electron donors, are the subjects of extensive research activities for more than one decade. Many research groups including ours have contributed substantially in the understanding of the solvatochromism, aggregation behavior, and the photoinduced electron transfer and charge transfer chemistry of fullerenes, in condensed phase. Present article is aimed to summarize the important results reported on the above aspects of fullerenes, subsequent to the earlier report from our group. (D.K. Palit and J.P. Mittal, Full. Sci. &amp; Tech. 3, 1995, 643-659)

Pulsed laser excitation of phosphate stabilised silver nanoparticles

Laser flash photolysis studies were carried out on two types of silver nanoparticles prepared by &#947;-radiolysis of Ag+ solutions in the presence of polyphosphate as the stabiliser. Type I silver nanoparticles displayed a surface plasmon band at 390 nm. Type II silver nanoparticles showed a 390 nm surface plasmon band with a shoulder at 550 nm. On photoexcitation in the surface plasmon band region, using 35 picosecond laser pulses at 355 nm and 532 nm, the type II solutions showed transient bleaching and absorption signals in the 450-900 nm region, which did not decay appreciably up to 5 nanoseconds. These transient changes were found to get annealed in the intervalt where 5ns &lt; t &lt; 100 ns. Extended photolysis of the nanoparticle solutions with repetitive laser pulses resulted in a decrease in the values of the average particle size which were measured by employing the dynamic light scattering technique

Pulse radiolytic studies on cis-dichlorobis-(2,2'-bipyridine)cobalt(III) and cis-dichlorobis-(1,10-phenanthroline)cobalt(III) complexes

690-694The reactions of hydrated electron (e⁻q) with Co(III) polypyridyl complexes of the type [Co(NN)2Cl2]Cl where NN = 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen) have been studied by pulse radiolysis. The rate constants for the reactions at 300 K have been evaluated to be (7.6±0.2)x10¹⁰, and (6.9±0.2)x10¹⁰ dm³ mol⁻¹ s⁻¹, respectively. Time resolved transient absorption spectra show two broad peaks at 360 and 610 nm for the bpy complex and a single broad peak at 420 nm for the phen complex at 1 μs. Comparison with reported transient spectra of the anion radicals of ligands indicates that the electron is located on the complex as a whole. The anion radicals of both the complexes initially produced, decay in the time scale of ~80 μs. Steady state absorption spectra on irradiation point out to breakdown of the phen complex, and the bpy-Co(III) complex is reduced to Co(II) complex. Conductance of the solution substantially increases on irradiation for both the complexes and can be attributed to aquation/de-ligation of the phen complex. The phen complex anion radical undergoes aquation/de-ligation by intramolecular electron transfer leading to dissociation of the complex. For bpy complex the conductance increases due to the release of chloride ions and reduction to Co(II) complex species is observed

Photoinduced charge transfer interaction between fullerene[60] and diphenylpolyenes in solution: evidence for photocycloaddition reaction

The charge transfer interaction between C<SUB>60</SUB> and diphenylpolyenes, namely 1,4-diphenylbutadiene and 1,6-diphenylhexatriene, has been studied in benzene and benzonitrile solutions using transient absorption techniques with pico to microsecond time resolution. Polyenes form weak ground state charge transfer complexes with C<SUB>60</SUB> in benzonitrile, but not in benzene. In benzonitrile the charge transfer complex undergoes charge separation on photoexcitation to form a contact ion-pair, which undergoes a fast charge recombination process. Diphenylpolyenes also interact with the excited triplet state of C<SUB>60</SUB>. The interaction of the polyenes with the excited states of C<SUB>60</SUB> gives rise to a cycloaddition product

Aggregation of C<SUB>70</SUB> in solvent mixtures

The unusual solvatochromism of C70 is investigated in a variety of solvent mixtures by optical absorption and fluorescence techniques. Distinct reversible color change from pink to purple is seen in the solvent mixtures studied. Such changes are seen also for C60 solutions in some solvent mixtures. Formation of clusters is found to be responsible for the observed optical changes. Light scattering studies are carried out to confirm the presence of clusters which show that the particle size varies from ~100 to ~1000 nm depending on the concentration of the fullerene. It is found that the solubility of the fullerene in the solubilizing solvent and that in the solvent mixtures are the major factors governing the aggregation behavior of the fullerene. Polarity of the solvent plays a minor role in the formation of aggregates

Charge recombination reactions in photoexcited fullerene C<SUB>60</SUB>-amine complexes studied by picosecond pump probe spectroscopy

Photoexcitation of complexes between fullerene C60 and organic amines in benzene solutions is known to result in charge separation (CS) and subsequent charge recombination (CR) reactions, which lead to varying yields of fullerene triplet formation. Picosecond flash photolysis studies are carried out on C60-diphenylamine (DPA), C60-triethylamine (TEA), C60-diazabicyclooctane (DABCO), and C60-triphenylamine (TPA) systems to find out mechanistic details of the triplet formation on CR by inducing heavy atom and polarity effects by using suitable solvents. It is found that in the case of C60-DPA, C60-TEA, and C60-DABCO systems proton transfer from the amine cation to the C60 anion in the ion pair state dominates, leading to poor triplet yields, which improve in heavy atom containing solvents.In TPA, proton transfer is not possible and hence fullerene triplet yields are high. Increase of solvent polarity for this system results in decreased C60 triplet yields with a consequent increase in the ion dissociation yield. A suitable reaction scheme is proposed to explain the results obtained

Formation of radical adducts of C<SUB>60</SUB> with alkyl and halo-alkyl radicals. Transient absorption and emission characteristics of the adducts

C60 is known to undergo addition reactions with several radicals. Laser flash photolysis and pulse radiolysis studies have been carried out on C60 solutions containing various halocarbons. Transient absorption peaks observed in pulse radiolysis experiments at 460 and 640 nm are attributed to different radical adducts between C60 and halocarbon radicals. No evidence is obtained for the radical cation C60&#8226;+. On a longer timescale (ca. 30 &#181;s), a unique strong emission is observed with a maximum at 745 nm which is attributed to the excited state of C60 radical adducts generated owing to absorption of the strong probe light. Rate constants for the formation and decay of the radical adducts have been estimated and compared with some reported values

Aggregation of fullerene, C<SUB>60</SUB>, in benzonitrile

C60 solutions in benzonitrile have been found to show concentration dependent optical absorption behaviour. At lower concentrations (&lt;100 &#181; M) the UV-vis absorption characteristics of C60 in benzonitrile are exactly similar to those in benzene and decalin. At higher concentrations (&gt;100 &#181; M), however, the C60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also visually opaque. From picosecond laser flash photolysis experiments it is seen that the triplet quantum yield of C60 in benzonitrile at higher concentrations (~400 &#181; M) is much less than unity and increases with the dilution while in decalin and benzene it is always close to unity and independent of the C60 concentration. Dynamic light scattering experiments indicate the presence of particles of mean size of about 250 nm in C60 solutions in benzonitrile with concentration &gt;100 &#181; M, while in &lt;100 &#181; M solutions no such particles have been observed. Such particles are also not observed for C60 solutions in benzene and decalin, even up to ~500 &#181; M. Scanning electron microscopy also shows particles of size ~250 nm. It is inferred that C60 forms aggregates in benzonitrile at concentrations &gt;100 &#181; M and that the aggregated and the monomeric form of C60 are in equilibrium

Dynamics of OH formation in the dissociation of acrylic acid in its (n,&#960;<SUP>&#8727;</SUP>) and (&#960;,&#960;<SUP>&#8727;</SUP>) transitions excited at 248 and 193 nm

The (n,&#960; &#8727; ) and (&#960;,&#960; &#8727; ) transitions in acrylic acid (H2CCHCOOH) are excited by KrF (248 nm) and ArF (193 nm) laser pulses, respectively, and the dynamics of its photodissociation to give OH fragments is studied using laser induced fluorescence technique. At both the photolysis wavelengths, the OH fragments produced are vibrationally cold, but have different rotational state distributions. To get an insight into the potential energy surface involved in the dissociation process, spin-orbit and &#8743;-doublets ratios are also measured. Average relative translational energy partitioned into the photofragments is determined using linewidth of the Doppler profiles to be 13.2&#177; 3.1 and 10.2&#177; 2.8kcal/mol at 193 and 248 nm excitations, respectively. High percentage of translational energy released into the photofragments suggests the presence of an exit barrier for the dissociation. On 248 nm excitation, the OH radicals are formed instantaneously during the laser pulse, while on 193 nm excitation, a risetime of &#8764;2 &#181;s is seen. Another difference between the photodissociation at 193 nm and 248 nm is the observation of an intense fluorescence in UV-visible region at the former, and no fluorescence at the later wavelength. Our experimental results are compared with those obtained by recent ab initio calculations by Fang and Liu. It is concluded that when (&#960;,&#960; &#8727; ) transition of acrylic acid is excited at 193 nm, the initially prepared S2 state undergoes nonradiative transitions to S1 and T2 states, and from where the molecule subsequently dissociates, while excitation to (n,&#960;&#8727; ) transition at 248 nm leads to dissociation solely from the initially prepared S1 state

Change in reaction mechanism with driving force in photoinduced dissociative electron transfer (PDET) reaction - a subpicosecond transient absorption study

Electron-transfer reactions that are dissociative in nature can proceed via two mechanisms, stepwise and concerted. In this work, the mechanism of photoinduced dissociative electron transfer between carbon tetrachloride and four phenothiazine derivatives has been investigated by subpicosecond transient absorption study. Clear evidence was obtained for the stepwise mechanism for the reaction between phenothiazine and carbon tetrachloride, whereas for the other three derivatives concerted reaction is seen to be operative. The mechanism of the dissociative electron-transfer reaction has been correlated with the driving force of the reactions studied. A crossover of reaction mechanism from concerted to stepwise was observed with the increase in the reducing power of the phenothiazine donor, i.e., with the increase in the driving force of the reaction. It was also shown that changes in solvent polarity can trigger the crossover of the reaction mechanism. The change in the reaction mechanism with the driving force of the reaction has been explained by potential energy surfaces involved in the two different mechanisms