Unique behavior of DMF as solvent in fluorescence quenching process

We did a comparative fluorescence quenching study with different derivatives of carbazole (CZ) as fluorophore and 1,4-dicyanobenzene (DCB) as quencher. It was found that the bimolecular fluorescence quenching rate constant (kq) values in N,N-dimethylformamide (DMF) were almost half of that reported for a much less polar solvent tetra-hydrofuran (THF), which is something exceptional. Moreover, this phenomenon was not system specific. Similar results were obtained for some other systems too. In this communication while trying to resolve this unique problem, we have found that micro heterogeneous nature of DMF, its high viscosity and certain impurities present in the commercial grade of this solvent are probably responsible for such anomalous result.