Unraveling the Photophysics of zinc porphyrin oligomers

Photophysics of zinc (II)-porphyrin oligomers PZ_n(ethyne) (n = 2–5) was investigated computationally using TDDFT and CC2 methods. The excitation energies of the S₁ state of porphyrin oligomers decrease from 16,400 cm⁻¹ (n = 2) to 12,500 cm⁻¹ (n = 5), while oscillator strength (S₀ → S₁) and radiative rate constant increase with n. The low fluorescence quantum yield for PZ_2(ethyne) (1 %) is attributed to fast intersystem crossing (S₁ → T₂), driven by small energy gap (ΔE≈0.02 eV) and large spin-orbit coupling matrix element (5.4 cm⁻¹). The energy gap ($\Delta E_{S_1-T_2}$) is influenced by meso-substituents, which can cause the quantum yield to vary by orders of magnitude.