Generation of Phosphorescent Triplet States via Photoinduced Electron Transfer: Energy and Electron Transfer Dynamics in Pt Porphyrin-Rhodamine B Dyads

Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multi-chromophoric systems in which emissive triplet states are generated via a pathway involving photoinduced electron transfer (ET), as opposed to local intrachromophoric processes. In model dyads, PtP-Ph-n-pRhB(+) (1-3, n = 1-3), comprising platinum-(II) meso-tetraarylporphyrin (PtP) and Rhodamine B piperazine derivative (pRhB(+)), linked by oligo-p-phenylene bridges (Ph-n), upon selective excitation of pRhB(+) at a frequency below that of the lowest allowed transition of PtP, room-temperature T-1 -> S-0 phosphorescence of PtP was observed. The pathway leading to the emissive PtP triplet state includes excitation of pRhB(+), ET with formation of the singlet radical pair, intersystem crossing within that pair, and subsequent radical recombination. Because of the close proximity of the triplet energy levels of PtP and pRhB(+), reversible triplet-triplet (TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time by the long decay of the pRhB(+) triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the distance attenuation factors beta between these two closely related processes.