Influence of the energy of charge transfer on non-covalent interactions between fullerenes and a designed bisporphyrin

Abstract

The present paper reports the spectroscopic and theoretical investigations on the formation of supramolecular complexes of a designed bisporphyrin (1) with C60 and C70 in toluene. Absorption spectrophotometric studies establish appreciable amount of ground state electronic interaction between fullerenes and 1. The interaction is facilitated through charge transfer (CT) transition as evidenced from well defined CT absorption bands in the visible region of the electronic spectra. In our present case, the CT interaction may be claimed as one of the rare findings, especially on account of interaction between fullerenes and bisporphyrin in a non-polar solvent. Other than fullerenes C60 and C70, various other electron acceptors, viz., 2,3-dichloro-5,6-dicyano-p-benzoquinone, tetracyanoethylene, o-chloranil and p-chloranil form CT complexes with 1. Utilizing the CT transition energies for various electron donor–acceptor complexes of 1, vertical ionization potential (IDv) of 1 is determined to be 6.37eV in solution. Estimation of degrees of CT, oscillator and transition dipole strengths evoke that the fullerene–1 non-covalent complexes are of neutral character in ground state. Higher magnitude of electronic coupling elements for the C70–1 complex compared to C60–1 complex indicates strong binding between C70 and 1. Steady state fluorescence studies elicit efficient quenching of the fluorescence of 1 in presence of fullerenes. Both UV–Vis and steady state fluorescence measurements reveal large value of binding constant (K) for C70–1 system (∼6.94×104dm3mol−1) than that of C60–1 system (K∼2.1×104dm3mol−1). Time resolved emission studies establish charge-separated state for the fullerene–1 systems. Transient absorption measurements in the visible region establish the formation of 1+ and fullerene− in toluene medium. Molecular mechanics calculations employing force field method in vacuo evoke the single projection structures of the fullerene–1 complexes and interpret the stability difference between C60 and C70 complexes of 1 in terms of heat of formation values of the respective complexes.