Photoinduced electron transfer reactions in a porphyrin-viologen complex: Observation of S-2 to S-1 relaxation and electron transfer from the S-2 state
M. Andersson et al., Photoinduced electron transfer reactions in a porphyrin-viologen complex: Observation of S-2 to S-1 relaxation and electron transfer from the S-2 state, J PHYS CH B, 103(16), 1999, pp. 3258-3262
Zinc-meso-tetrasulfonatophenyl-porphyrin (ZnTPPS4-) spontaneously forms a c
omplex with methyl viologen (MV2+) in aqueous solutions. Both the free porp
hyrin and its complex with viologen were studied by pump-probe spectroscopy
. Excitation in the porphyrin Soret band with a short laser pulse (fwhm = 2
00 fs) populated the upper excited-state S-2, and the subsequent processes
were followed by the transient absorbance changes involved. For the free Zn
TPPS,(4-) the transient absorption spectrum of the upper excited-state S-2
was different from that of the lowest excited-state S-1. The S-2 to S-1 int
ernal conversion could therefore be followed, and a S-2 lifetime of 1.3 +/-
0.2 ps was determined. In the ZnTPPS4--MV2+ complex, excitation in the Sor
et band produced spectral changes attributed to the charge separated state
ZnTPPS3--MV+, with a rate limited by the laser pulse (rate constant >5 x 10
(12) s(-1)). This suggests that the photoinduced electron transfer from the
excited porphyrin to the methyl viologen occurred directly from the S-2 st
ate of the porphyrin, before internal conversion to S-1. The reverse electr
on transfer that followed regenerated the ground state reactants with a rat
e constant (1.3 +/- 0.2) x 10(12) s(-1).