Impact of Diamino and Imidazole Functionalization on the Photophysical Properties and Electronic and Structural Dynamics of the Pyrimidine Nucleobase

The photostability of the DNA and RNA bases to ultraviolet radiation is essential to life, as evidenced by their ability to dissipate excess electronic energy via ultrafast internal conversion. Understanding how the functionalization of pyrimidine and purine affects electronic and structural relaxation pathways is crucial for insights into their selection as life's building blocks. It is also relevant for developing fluorescent biomarkers and photosensitizers for therapeutic applications. This study investigates how the functionalization of the C5=C6 bond in pyrimidine with heavier amino groups instead of hydrogen atoms to form 4,5-diaminopyrimidine, or with an imidazole ring to form purine, affects the photophysical properties and electronic and structural relaxation pathways observed in pyrimidine. The excited state dynamics of 4,5-diaminopyrimidine, pyrimidine, and purine are disclosed by using steady-state and time-resolved spectroscopy, supported by quantum chemical calculations. It is shown that the lowest-energy absorption band of 4,5-diaminopyrimidine is significantly red-shifted compared to that of purine and pyrimidine in both aqueous solution and acetonitrile, while the fluorescence quantum yield also increases. In acetonitrile, the initial 1ππ* state population in 4,5-diaminopyrimidine decays radiatively and nonradiatively to the ground state and can also intersystem cross to populate a long-lived triplet state. In contrast, intersystem crossing is suppressed in aqueous solution, leading to relaxation of the 1ππ* state population through fluorescence emission and internal conversion to the ground state. Our results demonstrated that both the strategic functionalization of pyrimidine and the solvent properties play important roles in tuning the optical properties and the electronic and structural relaxation pathways of the pyrimidine derivatives.