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 demons trate 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 ca ses the observed spectra are assigned to the triplet state of the free base which is coupled weakly to the copper ground state doublet. The polarizati on pattern generated by selective excitation of the free base half is indic ative of intersystem crossing (ISC), whereas excitation of the copper(II) h alf gives an eaa/eea polarization pattern. The latter is rationalized in te rms of energy transfer via the lowest excited trip-quartet state of the cop per(II) moiety, followed by selective depopulation from the spin states wit h doublet character in the weakly coupled free base triplet-copper doublet system. This leads to a spectrum which resembles that of the free base trip let state with overpopulation of the T+1 and T-1 sublevels. The spin-select ive electronic relaxation is supported by the fact that the rise time of th e polarization is consistent with the decay rate of the triplet signal gene rated 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 s hort-lived absorptive feature at g = 2.00 is observed at temperatures below 50 K. From their g-values and temperature dependence these features are te ntatively assigned to quartet and doubler states in conformations of the co mplex in which the coupling between the free base triplet and Cu(II) ground state is strong.