I am doing fluorescence studies of two molecules (one containing dibenzothiophene-S,S-dioxide and other benzothiadiazole) in THF/ water mixture. Fluorescence intensity decreases with increasing water content in both cases. But fluorescence decay time increases in case of the first molecule having dibenzothiophene-S,S-dioxide while decreases in other case. It appears to me that oxygen atoms of dibenzothiophene-S,S-dioxide might be playing some role in this.