Synthesis and near-infrared luminescence of a deuterated conjugated porphyrin dimer for probing the mechanism of non-radiative deactivation

Abstract

β,meso,β-Fused porphyrin oligomers have many attractive photophysical features such as strong absorption in the near-IR at wavelengths greater than 1000 nm, and high two-photon cross sections. However their ultrafast S₁–S₀ deactivation (k_d > 10¹¹ s⁻¹) limits potential applications. We have synthesised a deuterated fused porphyrin dimer to test whether deuteration influences the rate of non-radiative deactivation. An efficient synthetic strategy was developed, starting with deuteration of dipyrromethane. Deuteration of the zinc porphyrin dimer does not affect its fluorescence quantum yield in CD₂Cl₂ (Φ_fD/Φ_fH = 1.00 ± 0.05). This implies that the ultrafast non-radiative deactivation is not simply a consequence of the small S₁–S₀ energy gap. Comparison with other conjugated porphyrin oligomers confirms that the deactivation rate in the edge-fused oligomers is faster than would be expected from the energy gap law. This result indicates that it should be possible to create near-IR dyes with similar S₁–S₀ energy gaps to the β,meso,β-fused porphyrin oligomers but with slower rates of S₁–S₀ decay.