Charge separation processes in porphyrin-quinone compounds with several flexible bridges

2 Charge separation processes in porphyrin-quinone compounds with several flexible bridges 

The high conformational mobility of porphyrin-quinone compounds with flexible bonding makes it difficult to elucidate in sufficient detail the mechanism of electron transfer between porphyrin and quinone fragments. Far greater possibilities for determining the role of mutual orientation of P and Q and the distance between them are offered by P-Q compounds in which the P and Q fragments are linked by several bridges. A P-Q compound of this 

In case of co-facial quinone-capped porphyrins (P and Q are linked by four tetraamidophenoxy bridges and are located at a distance of 10 A from each other), the quantum yield of charge separation is much bigger and reaches 30% for short distances between P and Q [53, 54]. Luminescence quenching via electron transfer from P to Q is observed for both singlet- and triplet-excited states of the porphyrin fragment of P-Q. The appearance of the additional channel for luminescence decay via electron transfer manifests itself in the biphase character of P-Q luminescence decay kinetics. 

A decrease in the length of the bridge and an orientation of P and Q fragments in co-facial quinone-capped porphyrins which is favourable for electron tunneling result in a sharp rise of the efficiency of electron phototransfer from P to Q [54]. 

2 Charge Separation Processes in Porphyrin-Quinone Compounds with Several Flexible Bridges 

In Ref. 142, a detailed analysis of the forward and reverse electron transfer rates for capped P-L2-Q in a variety of solvents was given. The results show that forward electron transfer is in the normal region Er — AG° and charge recombination is in the Marcus inverted region, Er — AG°. 

Despite the presence of four bridging groups the quinone is not held rigidly above the plane of the porphyrin. Two channels of electron transfer from the singlet excited state of the porphyrin were found for this compound [145]. They were ascribed to slowly equilibrating introverted and extroverted conformers in which the estimated interplanar porphyrin.quinone separation is respectively, 6.5 and 8.5 A. The faster of the two rate constants is independent of the temperature over the range 80-300 K. 

---