Photophysical Properties of Fluorescent 2‐(Phenylamino)‐1,10‐phenanthroline Derivatives

The present study details the experimental and theoretical characterization of the photophysical properties of 14 examples of 2‐(phenylamino)‐1,10‐phenanthrolines (1). The absorption spectra of 1 are substituent‐dependent but in a general manner present absorption bands at wavelengths of ~230; ~300; ~335 and a shoulder at ~380 nm. Electron‐donating groups (EDG) and electron‐withdrawing groups (EWG), respectively, result in bathochromic and hypsochromic shifts. Compounds 1 are highly luminescent, in contrast to phenanthroline, and emit in the region between 350 and 500 nm with substituent‐dependent λmax emission. The emission spectra show a redshift for EDG (4‐OMe 62 nm; 4‐Me 19 nm) and a blueshift for EWG (4‐CN 41 nm; 4‐CF3 38 nm) relative to the emission of the unsubstituted parent compound 1a. Plotting the λmaxEM against Hammett σ+ constants gave an excellent linear correlation demonstrating the electron‐deficient nature of the excited state and how the substituents (de)stabilize S1. Theoretical calculations revealed a HOMO‐LUMO π‐π* electronic transition to S1 which in combination with difference (S1–S0) in electron density maps revealed charge‐transfer character. Strongly electron‐withdrawing substituents switch off the charge transfer to give rise to a local excitation. The photophysical properties of luminescent 2‐(phenylamino)‐1,10‐phenanthroline derivatives were experimentally and theoretically investigated.