Highly Fluorescent 2‐Aminopurine Derivatives: synthesis, photophysical characterization, and preliminary cytotoxicity evaluation

New fluorescent nucleobase analogues (FBAs) are emerging as extraordinarily useful tools for DNA labelling technologies. The highly fluorescent adenine analogue 2‑aminopurine (2AP) is still the most used within the few hundreds of newly FBAs synthesized, but its excitation in the UV region demands for high energy sources endangering living cells. New and highly fluorescent 2AP derivatives, 2-amino-6-cyanopurines, were obtained using novel, simpler, but very efficient synthesis method. All the new compounds exhibit advantageous photophysical properties over 2AP, showing absorption and emission bands ranging the visible region (blue-green region), high fluorescence quantum yields and Stokes shifts, especially in non-protic organic solvents. Density Functional Theory calculations (DFT) of electronic and vibrational structure were performed, allowing to predict absorption and emission spectra in accordance with experimental data. The extensive solvatochromism of compounds gives them important microenvironmental sensitivity to be used as superior fluorescent probes, comparing to 2AP. In addition, these newly synthesized 2-amino-6-cyanopurines exhibit little to no toxicity in assays using yeast cells, making them also valuable candidates for fluorescent studies in living cells.