Spectrochemical investigations in dendritic media: evaluation of nitromethane as a selective fluorescence quenching agent in aqueous carboxylate-terminated polyamido amine (PAMAM) dendrimers

Alternant and nonalternant polycyclic aromatic hydrocarbons (PAHs) are empl oyed to compare carboxylate-terminated polyamido amine (PAMAM) dendrimers t o typical anionic micelles. Nitromethane is a known, selective, quenching a gent of alternant PAHs. However, recent studies by Acree and co-workers hav e found that nitromethane will also quench the fluorescence emission intens ity of nonalternant PAHs in the presence of anionic surfactants above the c ritical micelle concentration. The quenching of alternant and nonalternant PAHs by nitromethane is used to compare dendritic 'unimolecular micelles' t o traditional micelles. Experimental results indicate that the PAHs' associ ation with these dendrimers does not appear to occur in the 'palisade' regi on, as seen in traditional micelles, but rather deeper within the dendrimer structure. Solvent polarity probe studies and quenching in the absence of nitromethane also support this conclusion. Due to the fact that the PAHs do not reside near the negatively charged surface groups, the nitromethane se lective quenching rule is obeyed in carboxylate-terminated PAMAM dendrimers (10 mM in surface groups). (C) 1999 Elsevier Science B.V. All rights reser ved.