Excited-state photophysics of donor-appended cobalt(II) porphyrins from picosecond transient absorption spectroscopy

Picosecond absorption spectra, obtained with 4-ps pump and probe pulses, are reported for the Co(II) complex of octaethylporphyrin (OEP) and of octaalkylporphyrins with single meso substituents: phenyl, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), dimethylaniline-N,N,N′,N′-tetramethyl-p-phenylenediamine (DMA-TMPD), or a phenyl-TMPD. The donor substituents were attached in order to investigate the possibility of transferring charge to the putative Co^(I)P⁺ species transiently formed in the excited-state deactivation of cobalt-(II) porphyrins, thereby increasing the lifetime of a Co(I) phototransient. The substituents shifted the porphyrin ground-state absorption bands only slightly, and the redox potentials of the constituent parts of the constructs were essentially unaffected by the substitution. For all of the cobalt(II) porphyrins, photoexcitation produced transient bleaching of the ground-state Soret band and an induced absorption at ca. 410 nm. Ground-state recovery was complete within 50 ps in all cases. The lifetime of the phototransient was determined to be 12 ± 4 ps for OEP, as well as all the meso-substituted octaalkylporphyrins. For the TMPD- and DMA-TMPD-appended porphyrins, the transient absorption spectrum in the red region showed a single band at ca. 577 nm, identical to that observed for OEP. The absence of a band at 606 nm excludes significant involvement of oxidized TMPD; from the known absorptivity of TMPD cation, the extent of electron transfer was estimated to be less than 10%. Yet the 12-ps lifetimes should have been sufficient to support electron transfer, judging from rates and driving forces in analogous systems. It is therefore concluded that the observed phototransient is not the Co^(I)P⁺ charge-transfer (CT) state, as had previously been supposed, but rather a dd excited state. Deactivation of CT states via lower-lying dd states is likely to be a general phenomenon for first transition row metalloporphyrins. Stabilization of transiently reduced or oxidized metal centers via electron transfer from or to porphyrin peripheral substituents may, however, prove fruitful for second or third transition row metalloporphyrins, in which the available dd states may be raised above accessible CT states.