Tandem Synthesis and Photophysical Behavior of Pyrazolo[3,4‐ h ][1,6]naphthyridines: A Theoretical–Experimental Approach

Abstract

A microwave‐assisted tandem synthesis of pyrazolo[3,4‐ h ][1,6]naphthyridines bearing substituents with diverse stereoelectronic character (i.e., alkyl, aryl, CN, and MeO) is reported. Products were obtained in good yields by reacting 5‐aroyl‐pyrazolo[3,4‐ b ]pyridines with malononitrile in methanol, where the expected Knoevenagel reaction does not proceed. Instead, a solvent molecule drives a tandem reaction that affords three tricyclic N ‐heteroaromatic compounds. Their molecular architecture prompted us to explore their photophysics, including solvatofluorochromism and aggregation properties. The 4‐methoxyphenyl‐substituted derivative exhibited the best behavior ( ɸ F up to 62%, with up to 80% water tolerance) due to intramolecular charge transfer within the π ‐extended tricyclic core; this process is notably attenuated or enhanced by the aryl substituent. Density functional theory calculations were used to interpret the experimental results, establish a mechanism for product formation, and correlate the structural and spectroscopic properties of the three synthesized tricyclic compounds.