Transient EPR Studies of Ion-Paired Metalloporphyrin Heterodimers

Abstract

Spin-polarized transient EPR spectra measured at 9 GHz (X-band) and 24 GHz (K-band) are observed for the metalloporphyrin heterodimers formed by MTTAP (metal/free base meso-tetrakis[4-trimethylanilinium]porphyrin, M = Zn, Mg or H2) and CuTSPP (copper meso-tetrakis[p-sulfonatophenyl]porphyrin). In the temperature range 5−120 K, the EPR transients obtained following pulsed light excitation at 532 nm show a strong temperature dependence. The dimers exhibit two distinct types of EPR signals which may be separated on the basis of their different decay kinetics. The spectrum of one of the signals can be assigned to a triplet state of the dimers, whereas the other spin-polarized spectrum clearly shows involvement of the Cu(II) metal ion. This spectrum is tentatively assigned to the doublet ground state of the CuTSPP moiety in the dimers. It is proposed that the observed spin polarization is generated by a radical triplet pair mechanism (RTPM). In ZnTTAP−CuTSPP and H2TTAP−CuTSPP both spectra are observed, whereas only the latter spectrum is obtained in MgTTAP−CuTSPP. In ZnTTAP−CuTSPP, the triplet-state spectrum shows only small but significant differences from that of the isolated ZnTTAP monomer, indicating that the triplet state is localized on the ZnTTAP component of the dimer. The corresponding spectrum in the free-base dimer is considerably narrower than that of the monomer H2TTAP which suggests that, in this case, the triplet state is partly delocalized over both halves of the dimer.

Light‐induced electron spin polarization in the ground state of water‐soluble copper porphyrins

“…[17][18][19] ESP in electrostatically bound porphyrin heterodimers containing paramagnetic copper has also been reported. 20 In all of these linked systems, the RTPM and ESPT mechanisms do not account for the observed spin-polarized EPR spectra of the excited and ground states. Several mechanisms have been invoked to explain the observed ESP but, as in solution, they all involve spin-selective relaxation and doublet-quartet mixing.…”

Electron Spin Polarizations of Phthalocyaninatosilicon Covalently Linked to One TEMPO Radical in the Excited Quartet and Doublet Ground States

“…Recently new kinds of CIDEPs, the radical triplet pair mechanism (RTPM) − and the electron spin polarization transfer (ESPT) mechanism, have been reported, focusing on the interactions between photoexcited triplet species and stable radicals. To obtain novel information, some excited triplet molecules bonded to a stable radical were investigated. , Corvaja et al studied C 60 covalently linked to a TEMPO radical (C 60 -TEMPO) in toluene solution .…”

Intramolecular Energy Transfer in a Covalently Linked Copper(II) Porphyrin−Free Base Porphyrin Dimer: Novel Spin Polarization in the Energy Acceptor

“…A great many kinds of porphyrin dimers and oligomers, which can be exploited for this purpose, have been synthesized and studied extensively over the past decade . In these covalently and noncovalently linked porphyrin systems, the interactions between the choromophores and intramolecular processes such as energy and electron transfer have been investigated − and discussed in terms of through-space and through-bond mechanisms. − In chemically bridged porphyrin dimers, the interaction between the two halves varies with the spacer unit, which not only can control their mutual distance and orientation but also can specify the nature of the intervening chemical bonds. − In particular, hybrid porphyrin dimers having two different central metal ions serve as excellent donor−acceptor systems, since the two halves have different excitation energies and redox potentials. − Moreover, the central metal ion of the macrocycle can sometimes change the excited-state dynamics drastically. Thus, by varying the two metals, a variety of photophysical processes can be studied in a given complex.…”

“…It is well-known that zinc(II) porphyrin−free base porphyrin dimers undergo singlet−singlet energy transfer and that the rate depends on the distance, orientation, and linkage between the two halves. − In contrast to the large number of studies on the diamagnetic zinc(II)−free base dimers, relatively little attention has been paid to dimers involving paramagnetic metal ions. − This is primarily because most paramagnetic metalloporphyrins have (d,d*) or charge transfer excited states below their (π,π*) excited states, and the presence of these low-lying states involving the metal excitation accelerates deactivation immediately after energy transfer, or directly prohibits energy transfer itself.…”