EXTREME VISCOSITY DEPENDENCE FOR THE MAGNETIC-FIELD-SENSITIVE RECOMBINATION RATES OF CHAIN-LINKED TRIPLET RADICAL-ION PAIRS

Two chain-linked triplet radical ion pairs (RIPs), 3[P.+-(CH2)n-Vi.+] (P.+ = Zn porphyrin and Vi.+ = viologen radical cations; n = 6 and 10) have been studied by transient optical absorption and external magnetic field (B less-than-or-equal-to 0.21 T) effects on the RIP recombination rates, k(r)(B) in methanol-glycerol mixtures of widely varying viscosities (0.55-123 cP). Extreme (with a maximum in k(r)(B = 0)) and monotonous (with a decrease of k(r)(B = 0)) viscosity dependences have been found for 3[P.+-(CH2)6-Vi.+] and 3[P.+-(CH2)10-Vi.+], respectively. The increase of k(r)(B = 0) correlates with the narrowing of the corresponding magnetic-field-dependent recombination rate spectra, whereas the decrease of k(r)(B = 0) correlates with the broadening of those.