The Unusual Fluorescence Strengthened by Steric Hindrance

In four dyads of fluorescein and carbazole, photoinduced intramolecular electron transfer occurred in polar solvents and was negligible in nonpolar solvents. However, unprecedented fluorescence strengthening in the dyad linked with a semi-rigid bridge (-CH2-Ph-CH2-) at the carboxyl of fluorescein (as 3.2 and 10 times as the model dibutyl derivative of fluorescein in toluene and cyclohexane, respectively) was observed. Experimental data showed it could result from steric hindrance.