Orthogonal effect of pyrene–porphyrin conjugates on the detection of volatile organic compounds under UV and visible light illumination through surface photovoltage†

In this work, we have developed two modular compounds featuring pyrene at the meso position of the freebase porphyrin (H₂PyP) and its complex with Zn (ZnPyP). Both compounds exhibited a unique energy transfer process due to the orthogonal pyrene units, demonstrating that appreciable electronic interactions existed between the peripheral units and the porphyrin π-system. These compounds were found to behave as strong donor materials in solid-state thin films. Detailed photophysical properties and excited-state interactions in the gas phase were modulated through surface photovoltage measurements using the scanning Kelvin probe (SKP) technique. These interactions were explored towards the detection of different volatile organic compounds (VOCs) (ethanol, acetone, 1-hexanol, triethylamine, nonanal, and acetonitrile) under dark, UV and visible light illuminations. H₂PyP and ZnPyP showed n-type behaviour with high selectivity towards 1-hexanol under UV light illumination, while under visible light illumination, ZnPyP exhibited n-type behaviour and H₂PyP showed p-type behaviour. The response and recovery studies demonstrated that H₂PyP and ZnPyP showed unprecedented selectivity towards 1-hexanol by altering their p- and n-type behaviour. H₂PyP exhibited a high photovoltage response of 93% for an exposure of 17 s with a recovery rate of 23% in 5 s, while ZnPyP showed 97% in 2 s with a recovery rate of 55% in 116 s under UV light. The unique response of H₂PyP and ZnPyP to 1-hexanol could be attributed to donor–donor interactions and intermolecular hydrogen bonding at the central core, as well as the variations in the energy transfer process. Furthermore, density functional theory studies revealed that the binding interactions of H₂PyP and ZnPyP with VOCs showed a greater affinity for alcohol vapours compared to other compounds.