Femto- to microsecond excited-state dynamics of aggregated and monomeric regiopure zinc and copper phthalocyanines

Understanding how aggregation state and metal identity affect excited-state dynamics of phthalocyanines (Pcs) is essential for rationalising their performance as photosensitisers in phototherapeutic applications. Here, we investigate the steady-state and time-resolved photophysics of a matched pair of regiopure Pcs with either diamagnetic Zn(II) (1) or paramagnetic Cu(II) (2) in solvents that promote either monomeric (DMSO) or mixed aggregate-monomer (H₂O and MeCN) speciation. Using time-resolved multiple probe (490-900 nm) transient-absorption spectroscopy continuously spanning femtoseconds to hundreds of microseconds, complemented by fluorescence lifetime and steady-state measurements, we correlate speciation with excited-state behaviour for 1 and 2. ZnPc 1 is fluorescent and long-lived in its monomeric form, exhibiting efficient intersystem crossing to an oxygen-sensitive triplet state with a lifetime of 196 µs. Aggregation leads to rapid non-radiative decay on a tens-of-picoseconds timescale, quenching fluorescence and suppressing triplet formation. In contrast, the CuPc analogue 2 undergoes ultrafast intersystem crossing (<2 ps) and displays room-temperature phosphorescence with a triplet lifetime of ∼20 ns that persists even under aggregating conditions. These results highlight potential switchable photodynamic and photothermal behaviour in structurally related Pcs based on aggregation state and the nature of the metal centre acting as kinetic controls of excited-state pathways.