Synthesis and characterization of new rhodamine dyes with large Stokes shift

Two new rhodamine dyes (Rh Q-H, Rh Q-Me) containing 1, 4-diethyl-1, 2, 3, 4-tetrahydroquinoxaline as an effective electron donor are designed and synthesized. The structures of the novel compounds are confirmed by 1H NMR, 13C NMR and ESI. Due to an excited-state intramolecular charge transfer (ICT), the new dyes exhibit longer absorption (>580 nm) and emission (>640 nm) compared with the model compounds, rhodamine 101 and rhodamine 6G. The new rhodamine dyes show large Stokes shift of 40–50 nm in commonly used solvents. Notably, when measured in a mixture of H2O/EtOH solution, significant stokes shift of 65–68 nm are achieved, which is among the largest Stokes shifts ever reported for rhodamine dyes.