Adsorption Interaction, Azo Bond, and Surface-Enhanced Raman Spectra of Azopurine on Silver Investigated by a DFT Study

Surface-enhanced Raman scattering (SERS) spectroscopy has been widely employed in the study of adenine and its derivatives due to its ability to provide fingerprint information on interfacial adsorption molecules. Adenine, as one of the most important biomolecules, dominates the SERS spectra of DNA and coenzymes. In our recent work, we demonstrated that adenine undergoes a photo-oxidative coupling process to yield (E)-1,2-di(7H-purin-6-yl) diazene (azopurine) through experiments and density functional theory (DFT) calculations. However, due to the structural sensitivity of SERS spectroscopy and the specificity of azo-containing functional groups, DFT spectroscopic simulations are highly dependent on the functionals and parameters used. In this work, we employed DFT calculations to conduct a theoretical analysis of the structures and spectra of azopurine and its complexes formed with charged silver clusters. We investigated the effects of azopurine conformational isomerism, interactions with metal ions, and the choice of theoretical methods on the spectra and established the correlation between the isomers and Raman spectra of azopurine molecules. Additionally, through the calculations of Raman spectra for a series of molecules containing N=N bonds, it has been found that the characteristic Raman spectral peaks of molecules with N=N bonds are intimately related to the theoretical methods, bonding properties, and accuracy of N=N bond length calculations. The solvation effect can also significantly influence the vibrational frequency shift and relative intensities of Raman spectral bands. Therefore, it is crucial to select the appropriate theoretical approaches and comprehensively consider the relevant computational conditions during theoretical calculations.