Benzoate-modified rhodamine dyes: Large change in fluorescence properties due to photoinduced electron transfer

Abstract

Six rhodamine dyes for which the benzoate is modified with NH2 or COOH were studied by time-correlated single photon counting and steady state fluorescence technique. The absorption and fluorescence spectra, the fluorescence quantum yield as well as fluorescence lifetime values were measured in typical solvents. The data showed that molecular rigidity, the substituent type and position on the benzoate, as well as solvent have remarkable effects on fluorescence properties. NH2 on the benzoate showed very different effect from that of COOH. NH2 modified rhodamines exhibit much smaller fluorescence quantum yields and much shorter lifetimes, due to the very fast intramolecular photoinduced electron transfer (PET). On the substituent position effect, 5′-COOH on the benzoate leads to larger band maximum, lower fluorescence quantum yield and shorter fluorescence lifetime than that of 6′-COOH. For NH2 substitution, however, it is 6-NH2 that shows larger band maximum, lower fluorescence quantum yield and shorter fluorescence lifetime than that of 5′-COOH. Although the rate constant of nonradiation process for the rhodamines is varied in several orders with structure and solvent, the rate constant of radiation process, however, shows a constant value (0.18×109s−1). Quantum chemical calculations on B3LYP/6-31+G(d) level was also carried out to obtain molecular structures and free energy change to confirm the presence of PET.