Influence of a diketopyrrolopyrrole spacer on the ultrafast nonlinear optical properties and excited state dynamics of dimeric zinc porphyrin molecules†

Zinc porphyrins (ZnP) are an important class of small molecule donors that have gained significant attention in the domain of organic photovoltaics. In this work, we investigate and correlate various photophysical and third-order nonlinear optical (NLO) properties of three different zinc porphyrin molecules: a monomer – ZnP1 (ZnP-TEH-DPP) with two diketopyrrolopyrrole (DPP) units bridged to zinc porphyrins, its corresponding porphyrin dimer ZnP2 (ZnP2-DPP) with an additional porphyrin unit at the centre, and another porphyrin dimer ZnP3 (HDDPP-EHTOM) having two zinc porphyrin donors linked with DPP and endcapped with 3-ethylrhodanine. To investigate the excited state dynamics of the systems, a femtosecond transient absorption spectroscopy (fs-TAS) experiment was employed in the visible range by exciting the molecules at 2.53 eV. The estimated lifetime values from the global analysis were in the range of 230–560 fs, 10–27 ps, and 200–350 ps, which is attributed to the internal conversion, vibrational relaxation, and relaxation from the lowest excited singlet state, respectively. Furthermore, the NLO features of ZnP1, ZnP2, and ZnP3 were investigated using a femtosecond Z-scan technique. All the three samples exhibited saturable absorption (SA) at lower intensities and SA followed by reverse saturable absorption (RSA) at higher input intensities. The second hyperpolarizability (γ) from closed aperture Z-scan measurements was estimated to be 5.72 × 10⁻³¹, 7.02 × 10⁻³¹, and 10.6 × 10⁻³¹ esu for ZnP1, ZnP2, and ZnP3, respectively, which were consistent with the theoretically obtained ones using the DFT calculations. Zinc porphyrin dimers (ZnP2 and ZnP3) having extended π-conjugated systems exhibit better NLO properties in comparison to the monomer ZnP1. The observed properties of these samples revealed that these porphyrins can be useful for organic photovoltaic as well as other NLO applications.