Physicochemical insights in supramolecular interaction of fullerenes C60 and C70 with a monoporphyrin in presence of silver nanoparticles.

The present article reports for the first time on supramolecular interaction between fullerenes (C60 and C70) and a designed monoporphyrin in solution, e.g., 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (1), in absence and presence of silver nanoparticles (AgNp) having varying diameter of range between 3 and 7 nm. Ground state electronic interaction between fullerenes and 1 has been evidenced from the observation of decrease in the intensity of the Soret absorption band of 1 after complexation with C60 and C70 in toluene. However, in presence of AgNp, extent of decrease in the intensity of Soret absorption band of 1 has been reduced following its complexation with fullerenes. Steady state fluorescence measurements establish quenching of fluorescence of 1 by fullerenes and the most interesting aspect of the present work is that quenching efficiencies of C60 and C70 are found to be less in presence of AgNp. Steady state fluorescence measurement reveals reduction in the binding constant (K) value for both C60-1 (KC60-1=2355 dm3 mol(-1) and C70-1 complex (KC70-1=11,980 dm3 mol(-1)) in presence of AgNp (KC60-1=340 and KC70-1=7380 dm3 mol(-1)). The new physical insight of the present studies is that 1 acts as excellent discriminator molecule for C70 in presence of AgNp as selectivity in binding is estimated to be ∼21.7 in presence of AgNp compared to the situation when fullerene-1 mixture does not contain any AgNp (i.e., selectivity in binding=∼5.0) in solution. Time-resolved fluorescence studies establish the role of static quenching mechanism behind fluorescence decay of 1 by fullerenes in absence and presence of AgNp. Magnitude of rate constant for charge separation and quantum yield of charge separation indicates that C70-1 complex exhibits highest value of such parameters in absence of AgNp compared to the situation when AgNp particles are present in the composite mixture of C70 and 1. Dynamic light scattering (DLS) measurement reveals while particle size of AgNp is estimated to be ∼4.8-5.0 nm in presence of 1, the size of the AgNp particles in 1 become larger in presence of C60 (∼13.0 nm) and C70 (∼37.0 nm) solution in toluene. Conductance measurement establishes that AgNp particles reduce the generation of electrical conductivity value for both C60-1 and C70-1 systems in toluene with respect to time; the rate of decrease of electrical conductivity become much slower in presence of C70-1 complex. Scanning electron microscopic experiment provides excellent support for DLS measurements regarding increase in the size of the nanoparticles in presence of C60 and C70. Transmission electron microscope clearly demonstrates that the electrostatic attraction between porphyrin-based supramolecules and silver nanoparticles is very much responsible behind the formation of larger aggregates. Semiempirical PM3 calculations in vacuo establish the single projection structures for the fullerene-1 complexes and well interpret the stability difference between C60- and C70-complexes of 1 in terms of heat of formation values of the respective complexes.