Meso-Zn(II)porphyrins of Tailored Functional Groups for Intensifying the Photoacoustic Signal
Development of efficient molecular photoacoustic contrast agent plays significant role in the next generation biomedical imaging techniques. This article demonstrates a design criterion for small molecules to exhibit large photoacoustic effect by density functional theory (DFT) and its experimental validation. The method relies on controlling the effect of resonating structures on the vibrational energies of small molecules such that rapid thermalization pathways exist near higher-order unoccupied molecular orbitals. A series of Zn(II)porphyrin derivatives (Por-Cn-RAm, where n=12 or 8 and m=1–4) is designed as a model system by DFT and demonstrated a systematic variation in the absorption coefficients of C-H vibrational modes occurring at high-frequency spectral region. A systematic decrease in absorption coefficients was observed; and therefore, a similar variation in the photoacoustic signals is predicted. To validate the theoretical results, four Zn(II)porphyrin based molecules showing systematic variation in absorption coefficients, viz. Por-C12-RA1, Por-C12-RA2, Por-C12-RA3, and Por-C8-RA4 are synthesized in good yields (40–70%) and their optoelectronic properties are systematically studied as well as the effect of resonating structures in these molecules in determining the vibrational energies are discussed. Theoretical predictions are validated by photoacoustic coefficients measurements and photoacoustic tomography. The photoacoustic coefficients and tomographic intensities decreased in the order Por-C12-RA1>Por-C12-RA2>Por-C12-RA3>Por-C8-RA4, as predicted by DFT. Large photoacoustic coefficients are observed for the Por-C12-RA1, which is superior to that of the existing small molecules. Besides offering a superior molecule for photoacoustic tomography, the present criterion adopted here would enable to design simpler molecules with superior photoacoustic and other nonlinear optical properties.