Phthalazine-trione as a blue-green light-emitting moiety: crystal structures, photoluminescence and theoretical calculations

Abstract

Phthalazine (2,3-diazanaphthalene) and phthalhydrazide (2,3-dihydro-1,4-phthalazinedione) compounds are heterocycles with several biological properties. Most recently, analogous phthalazine-triones also emerged as drug candidates. Herein, we introduced a phthalazine-trione moiety as a promising fluorophore in the blue and green spectral regions. This desired optical property was rationalized based on their crystal structure and time-dependent density functional theory (TD-DFT) calculations. Under ultraviolet (UV) excitation (ca. 360 nm), two light emission maxima at ca. 460 nm and 480 nm were observed for ten phthalazine-triones, regardless of the substitution pattern. This conservation in the light-emitting property was rationalized by our time-dependent DFT calculations for the three phthalazine-triones whose crystal geometries were also determined in this study (4-chlorophenyl, 3-methoxyphenyl and 4-nitrophenyl derivatives) and for those available in literature (4-fluorophenyl and 4-bromophenyl derivatives). Electronic transitions with the largest oscillator strengths were near 360 nm and were in excellent agreement with the experimental excitation energy (UV-visible and photoluminescence spectra). Lifetime values of selected samples could also be determined, which were between 1.59 ns and 3.21 ns. In all compounds, the Frontier molecular orbitals involved in these excitations were distributed over the phthalazine-trione moiety and were not localized on the changeable substituent.