Intramolecular energy transfer in a covalently linked copper(II) porphyrin-free base porphyrin dimer:: Novel spin polarization in the energy acceptor

The excitation wavelength dependence of time-resolved EPR is used to demonstrate the pathway of intramolecular energy transfer in a covalently linked copper(II)-free base porphyrin dimer. Spin polarized spectra are presented for selective excitation of both the copper(II) porphyrin donor (at 540 nm) and the free base porphyrin acceptor (at 640 nm) at 50 and 80 K. In all cases the observed spectra are assigned to the triplet state of the free base which is coupled weakly to the copper ground state doublet. The polarization pattern generated by selective excitation of the free base half is indicative of intersystem crossing (ISC), whereas excitation of the copper(II) half gives an eaa/eea polarization pattern. The latter is rationalized in terms of energy transfer via the lowest excited trip-quartet state of the copper(II) moiety, followed by selective depopulation from the spin states with doublet character in the weakly coupled free base triplet-copper doublet system. This leads to a spectrum which resembles that of the free base triplet state with overpopulation of the T+1 and T-1 sublevels. The spin-selective electronic relaxation is supported by the fact that the rise time of the polarization is consistent with the decay rate of the triplet signal generated via ISC following direct excitation of the free base. Superimposed on these triplet spectra is a narrow emissive feature at g = 2.02 and a very broad a/e pattern, both of which decay with the same rate. In addition, a short-lived absorptive feature at g = 2.00 is observed at temperatures below 50 K. From their g-values and temperature dependence these features are tentatively assigned to quartet and doubler states in conformations of the complex in which the coupling between the free base triplet and Cu(II) ground state is strong.