Sumanta Bhattacharya

Photophysical and theoretical investigations on fullerene/phthalocyanine supramolecular complexes.

The present paper reports the photophysical aspects of a very interesting and unique host-guest interaction between fullerene and phthalocyanines, viz., free base phthalocyanine (H2-Pc) and zinc-phthalocyanine (Zn-Pc), in toluene medium. Ground state electronic interaction between these two supramolecules has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Vertical ionization potentials of the phthalocyanines have been determined utilizing CT transition energy. Magnitude of degrees of CT reveals that, in the ground state, 2-4% CT takes place. Binding constants (K) for the fullerene/phthalocyanine complexes were determined from the fluorescence quenching experiment. Large K values in the ranges approximately 4.7 x 10(4) to 7.3 x 10(4) and 2.3 x 10(4) to 2.5 x 10(4) dm(3) x mol(-1) were obtained for the 1:1 fullerene complexes of Zn and H 2-Pc, respectively. Values of K suggest that both H 2- and Zn-Pc could not serve as an efficient discriminators between C60 and C70. Theoretical calculations as well as (13)C NMR studies establish that the orientation of C 70 toward phthalocyanine is favored in end-on orientation, which proves that interaction between fullerenes and phthalocyanines were governed by the electrostatic mechanism rather than dispersive forces associated with pi-pi interaction.

Tunable solvatochromic response of newly synthesized antioxidative naphthalimide derivatives: intramolecular charge transfer associated with hydrogen bonding effect.

The solvatochromic behavior of two newly synthesized naphthalimide derivatives (I and II) which have potential antioxidative activities in anticarcinogenic drug development treatment, has been monitored in protic and aprotic solvents of different polarity applying steady-state and time-resolved fluorescence techniques. The compounds exhibit unique photophysical response in different solvent environments. The spectral trends do not appear to originate only from changes in the solvent polarity but also indicate that hydrogen bonding interactions and intramolecular charge transfer (ICT) influence the energy of electronic excitation of the compounds. Incorporation of an amino group at C(4) position of the naphthalimide ring in II makes it behave differently from I in terms of spectral characterization and fluorescence efficacy of the systems. The nonradiative relaxation process of the compounds is governed by medium polarity. The ground state geometry, lowest energy transition, and the UV-vis absorption energy of the compounds were studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) at the B3LYP/6-31G* level, which showed that the calculated outcomes were in good agreement with experimental data.

Room temperature solution studies of complexation between o-chloranil and a series of anilines by spectrophotometric method.

Electron donor-acceptor (EDA) complex formation between o-chloranil and a series of anilines has been studied in CCl4 medium. In all the cases, EDA complexes are formed instantaneously on mixing the donor and acceptor solutions. N,N-dimethylaniline and N,N-dimethyl-p-toluidine form stable EDA complexes with o-chloranil while the other complexes decay slowly into secondary products. The kinetics of all these reactions has been studied by UV-VIS absorption spectrophotometric method and the rate constants of the reactions and formation constants of the EDA complexes have been determined. The charge transfer (CT) transition energies of the complexes are found to change systematically with change in the number and position of the methyl groups in the donor molecules (methylanilines). From an analysis of this variation, the electron affinity of o-chloranil has been found to be 2.54 eV. A perturbational inductive effect Hückel parameter hMe has been found from this trend and the value obtained (-0.27) is very close to that (-0.3) obtained by Lepley (J. Am. Chem. Soc., 86 (1964) 2545) from a study of tetracyano ethylene (TCNE)-methylbenzene complexes.

Study of the formation equilibria of electron donor-acceptor complexes between [60]fullerene and methylbenzenes by absorption spectrometric method.

The electron donor-acceptor (EDA) interaction between [60]fullerene and three methylbenzenes, viz., durene, pentamethylbenzene and hexamethylbenzene has been studied in carbon tetrachloride medium at a number of temperatures. It has been found that these methylbenzenes form stable 1:1 EDA complexes with [60]fullerene. Charge transfer (CT) absorption bands of the complexes in the 410-460 nm region are more intense than the usual 420-700 nm absorption band of C60. The CT transition energies (hvCT) of the complexes change systematically with change in the number and position of the methyl groups in the donor molecules (methylbenzenes) and also with the donor ionisation potentials. From an analysis of this variation the electron affinity of C60 has been found to be 2.30 eV and also an inductive effect Hückel parameter of the methyl group has been determined. Formation constants (K) have been determined at three different temperatures from which the enthalpies and entropies of formation of the complexes have been determined.

Spectroscopic and theoretical investigations on effective and selective interaction of fullerenes C60 and C70 with a derivatized Zn-phthalocyanine: stabilization of charge-recombined state by side-on approach of C70.

The photophysical aspects of noncovalently linked fullerenes C(60) and C(70) with a designed metallophthalocyanine, namely, zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (1) have been investigated employing various spectroscopic tools such as UV-vis absorption spectrophotometry, steady state and time-resolved fluorescence, along with solution state IR measurements in toluene medium. The ground state interaction between fullerenes and 1 is first evidenced from UV-vis measurements. Binding constants (K) for the complexes of C(60) and C(70) with 1 are determined to be 13,235 and 27,670 dm(3) x mol(-1), respectively. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C(60) and C(70). Emission studies reveal K values of 8945 and 19,175 dm(3) x mol(-1) for C(60)/1 and C(70)/1 complexes, respectively. The magnitude of K suggests that 1 preferentially binds C(70) in comparison to C(60) as average selectivity in K of C(70) over C(60) is enumerated to be 2.0. Time resolved emission measurements establish that C(70)/1 complex is stabilized much more in comparison to C(60)/1 complex in terms of charge recombination. Liquid IR studies provide very good support in favor of ground state complexation between fullerenes and 1. Molecular mechanics calculations at force field model and semiempirical calculations employing third parametric method substantiate the strong binding between C(70) and 1, and at the same time, determine the orientation of bound guest (here C(70)) within the cavity of 1.

Study of ground state EDA complex formation between [70]fullerene and a series of polynuclear aromatic hydrocarbons

[70]fullerene has been shown to form 1:1 EDA complex with anthracene, naphthalene, phenanthrene, pyrene and acenaphthene in CCl4 medium. Charge transfer (CT) bands have been detected in all the cases. Isosbestic points have been observed in the cases of phenanthrene and acenaphthene complexes. Ionisation potentials of the donors and CT transition energies have been found to correlate in accordance with Mulliken equation and from this correlation the electron affinity of C70 has been found to be 2.59 eV. Enthalpies and entropies of formation of the complexes have been estimated from the formation constants of the complexes determined spectrophotometrically at three different temperatures.

Photophysical and theoretical insights on fullerene/zincphthalocyanine supramolecular interaction in solution.

The present article reports photophysical studies on supramolecular interaction of a zinc phthalocyanine derivative, namely, zinc-2,9,16,23-tetra-tert-butyl phthalocyanine (1) with C(60) and C(70) in solvents having varying polarity, i.e., toluene and 1,2-dichlorobenzene (DCB). The interesting feature of the present work is the observation of charge transfer (CT) absorption bands of the fullerene/1 complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of 1, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of 1 on the UV-vis spectral characteristics of C(60) and C(70) have been explained using a theoretical model that takes into account the interaction between electronic subsystems of 1 with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of 1 in the presence of both C(60) and C(70). The average binding constants of the C(60) and C(70) complexes of 1 (estimated by UV-vis and steady state fluorescence measurements) are determined to be 18,330 dm(3)·mol(-1) (12,595 dm(3)·mol(-1)) and 19,160 dm(3)·mol(-1) (15,292 dm(3)·mol(-1)) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C(70)/1 species. The faster charge recombination of the fullerene/1 systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the ab initio method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and 1 during fullerene/1 interaction. A variable temperature (13)C NMR study nicely demonstrates that the end-on orientation of C(70) is very much responsible for the low selectivity in binding between C(60)/1 and C(70)/1 systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited 1 to C(60) and C(70) in the C(60)/1 and C(70)/1 complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from (T)C(60*) (and (T)C(70*)) to 1 occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of 1.

New Photophysical Insights in Noncovalent Interaction between Fulleropyrrolidine and a Series of Zincphthalocyanines

The present article reports, for the first time, the photophysical aspects of noncovalent interaction of a fullerene derivative, namely, C(60) pyrrolidine tris-acid ethyl ester (PyC(60)) with a series of zincphthalocyanines, for example, underivatized zincphthalocyanine (1), zinc-1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (2), and zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (3) in toluene. Ground state electronic interaction of PyC(60) with 1, 2 and 3 has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Utilizing the CT transition energy, vertical electron affinity (E(A)(v)) of PyC(60) is determined. Steady state fluorescence experiment enables us to determine the value of binding constant (K) in the magnitude of 2.60 × 10(4) dm(3)·mol(-1), 2.20 × 10(4) dm(3)·mol(-1), and 1.27 × 10(4) dm(3)·mol(-1) for the noncovalent complexes of PyC(60) with 1, 2, and 3, respectively. K values of PyC(60)-ZnPc complexes suggest that PyC(60) is incapable of discriminating between 1, 2, and 3 in solution. Lifetime experiment signifies the importance of static quenching phenomenon for our presently investigated supramolecules and it yields larger magnitude of charge separated rate constant for the PyC(60)-1 species in toluene. Photoinduced energy transfer between PyC(60) and ZnPc derivatives, namely, 1, 2, and 3, in toluene, has been evidenced with nanosecond laser photolysis method by observing the transient absorption bands in the visible region; transient absorption studies establish that energy transfer from (T)PyC(60)* to the ZnPc occurs predominantly, as confirmed by the consecutive appearance of the triplet states of PyC(60). Theoretical calculations at semiempirical level (PM3) evoke the single projection geometric structures for the PyC(60)-ZnPc systems in vacuo, which also proves that interaction between PyC(60) and ZnPc is governed by the electrostatic mechanism rather than dispersive forces associated with π-π interaction.

Absorption spectroscopic study of EDA complexes of [70] fullerene with a series of methyl benzenes

[70]Fullerene has been shown to form 1:1 molecular complexes with toluene, p-xylene, m-xylene, 1,2,4,5-tetramethyl benzene (durene) and pentamethyl benzene (PMB) in CCl4 medium by absorption spectroscopic method. Isosbestic points have been detected in case of complexes with PMB and durene. Charge transfer absorption band could not be detected but the intensity of the broad absorption band of C70 in CCl4 decreases systematically with increase in the concentration of the added methylbenzenes. From this trend the formation constants (Kc) of the complexes have been determined at three different wavelengths. The constancy of Kc with respect to change in the wavelength of measurement supports the view that complex of a single stoichiometry (1:1) is formed in each case.

Spectrophotometric study of molecular complex formation between o-chloranil and a series of methylated pyridines

o-Chloranil has been shown to form 1:1 molecular complexes with pyridine and 2-, 3- and 4-picolines in CCl4 medium. Isosbestic points have been found but charge-transfer bands could not be detected. The formation constants of the complexes exhibit a very good linear free energy relationship from which the Hammett p parameter for the complexation reaction is found to be -3.67.