Michael J. Shephard

Long-lived photoinduced charge separation in a bridged C60-porphyrin dyad

Abstract Long-range photoinduced energy and electron transfer have been investigated in a novel dyad containing a zinc tetraarylporphyrin (PZn) donor and a C60 acceptor separated by a saturated norbornylogous bridge nine sigma bonds in length (PZn-9σ-C60). In non-polar solvents singlet-singlet energy transfer from PZn to C60 is observed but in benzonitrile efficient (> 90%) charge separation occurs with a rate constant of 1 × 1010 sec−1 to yield PZn.+-9σ-C60.−. The charge separated state exhibits a remarkably long lifetime for a dyad of 420 ns. This behaviour is discussed with reference to the energetics, dynamics and orbital symmetry properties of the states involved.

A New Class of Giant Tetrads for Studying Aspects of Long-Range Intramolecular Electron Transfer Processes: Synthesis and Computational Studies

Theroleofsolvent-mediatedelectrontransfer can be studied in multichromophoric systems that possess “U-shaped” cavities (see scheme). One approach to building these geometries is through the use of Diels–Alder chemistry on norbornylogous scaffolds. 1H NMR and computational AM 1 and HF/3–21G studies indicate that the major isomer formed within this sequence has a “U-shape”. This conclusion is consistent with recently reported photophysical studies on the U-shaped system.

Synthesis and photophysical studies of a porphyrin–viologen dyad covalently linked by a flexible seven-atom chain

A dyad with terminal porphyrin (P) and viologen (MV 2C ) chromophores covalently linked by a flexible seven-atom hexanoate chain has been synthesized. Ab initio and semi-empirical solvent continuum calculations suggest that the preference for ‘extended’ or ‘folded’ conformations of the dyad depend strongly on solvent. Fluorescence lifetime and transient absorption studies in acetonitrile indicate that approximately 80% of the dyad population are in a suitable conformation to undergo photoinduced electron transfer (PET) to produce P C ‐MV C with a rate of 510 8 s 1 . A long-lived charge-separated state lifetime of 170 ns is observed. It is proposed that electrostatic repulsion between the positively charged end groups resulting from PET increases the average donor‐acceptor separation thus retarding the charge recombination process.

The porphyrin-C60 non-bonded interaction: an ab initio MO and DFT study

The non-bonded interactions between a porphyrin molecule and a C60 molecule, in the gas-phase, has been systematically investigated using various theoretical models. These are: (1) wavefunction-based methods, Hartree-Fock SCF (HF), second-order Moller-Plesset (MP2) theory, and the localized MP2 (LMP2) theory using the diatomics in molecules (DIM-LMP2) and triatomics in molecules (TRIM-LMP2) methods; (2) density functional theory (DFT), using non-local (BLYP, PW91), hybrid (B3LYP), and local (SVWN) functionals. Of the HF and DFT methods examined, corrected for BSSE using the counterpoise (CP) method, only the SVWN method predicts a close separation (2.5 A) between the porphyrin and the C60 molecules, in line with close contacts observed in crystal structures of cocrystallates of porphyrins and fullerenes (2.7-3.0 A). The MP2 and LMP2 methods also predict a close contact between the two molecules although the MP2 and TRIM-LMP2 methods overestimate the interaction giving a separation < 2.5 A while the DIM-LMP2 method gives a satisfactory separation of 2.9 A. The SVWN and DIM-LMP2 methods also predict a reasonable complexation energy of ca. −13 kcal/mol (SVWN and DIM-LMP2 CP-uncorrected) and −7.9 kcal/mol (SVWN CP-corrected), whereas the MP2 and TRIM-LMP2 methods probably strongly overestimate the complexation energy. The remaining methods underestimate the complexation energy. The CP-uncorrected DIM-LMP2/6-31G(d) method gave the best estimate of the porphyrin-C60 separation (2.9 A) with a complexation energy of −13.3 kcal/mol; however, the more cost-effective SVWN functional gives satisfactory values for these quantities with the SVWN/6-311+G(d) level providing the best estimate for the complexation energy (−16.5 kcal/mol). The porphyrin-C60 interaction was investigated in the giant triad 1 in which a zinc porphyrin, a dimethoxynaphthalene and a C60 fullerene are separated by two norbornylogous bridge sections of six and five bond lengths, respectively. All methods predict the existence of a compact form of the molecule in which the porphyrin and C60 moieties are only 2.9-4.3 A apart (contact distance). This finding is consistent with certain photophysical properties of 1.