Using Three-Pulse Femtosecond Spectroscopy to Probe Ultrafast Triplet Energy Transfer in Zinc meso-Tetraarylporphyrin-Perylene-3,4-dicarboximide Dyads

Linear arrays of zinc meso-tetraarylporphyrin (ZnP), perylene-3,4- dicarboximide (PMI), and either naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI) were synthesized and studied by ultrafast transient absorption spectroscopy. PMI was covalently linked in one of two orientations relative to ZnP. In one set of molecules, the 9 position of the perylene core is connected to the para position of a meso-phenyl in ZnP to give ZnP-PMI-N-X, where X = NI or PI is attached to the imide nitrogen atom of PMI. In the second set of compounds, the imide nitrogen atom of PMI is connected to the meso-phenyl in ZnP to give ZnP-N-PMI-X, where X = PI or H. Selective excitation of ZnP using 420 nm, 110 fs laser pulses in each molecule in toluene produces ¹*ZnP, which intersystem crosses (ISC) to ³*ZnP with τ = 2.3 ns. For ZnP-PMI-N-X, triplet energy transfer (TET) from ³*ZnP to PMI is much faster than ISC, so that ³*ZnP is not observed by one-pump-one-probe transient absorption spectroscopy. Following its formation, the lowest excited triplet state of ³*PMI was excited with a 575 nm, 110 fs laser pulse to produce an upper excited triplet state, ³**PMI. In ZnP-PMI-N-X, subpicosecond TET from ³**PMI re-populates ³*ZnP, which subsequently undergoes TET back to PMI with a rate of (7 ps) ^-1. The same experiment carried out on ZnP-N-PMI-X reveals that the TET process ³*ZnP-N-PMI-X → ZnP- ³*N-PMI-X occurs with a rate of (55 ns) ^-1. The nearly 8000-fold larger TET rate from ³*ZnP to PMI in ZnP-PMI-N-X relative to that in ZnP-N-PMI-X is a consequence of the larger π-orbital coefficients at the 9 position in both the HOMO and LUMO of PMI relative to that on its imide nitrogen atom. This basic asymmetry allows optimization of energy and electron and/or hole transfer rates in large assemblies containing PMI for use in organic molecular electronics.