LIPOPHOBIC EFFECTS ON PHOTOCHEMICAL AND PHOTOPHYSICAL BEHAVIOR OF MOLECULES WITH POLAR CHAINS IN NONPOLAR-SOLVENTS - EVIDENCE FOR INTERMOLECULAR AGGREGATION AND SELF-COILING

The emission spectra of naphthalene end-labeled poly (ethylene glycol) oligomers (N-Pn-N) in isopentane and hexane at different temperatures have been studied under stationary and nonstationary conditions. The photochemical dimerization of the end naphthalene groups of N-Pn-N has also been investigated. The results reveal that lipophobic interactions force single N-Pn-N chains to coil up at room temperature and pushthem together to form aggregates below-20 °C. The excimer emission dominates the fluorescence spectra of N-Pn-N at low temperature. The fluorescence depolarizationmeasurements demonstrate the existence of energy migration inside aggregates. The glassy solutions of N-Pn-N in isopentane (77 K) show strong delayed excimer fluorescence and phosphorescence of triplet excimer. The photoirradiation yields both intermolecular and intramolecular dimerization products of the naphthalene groups. This work provides the First example to apply lipophobic interaction to expediting the formation of macrocyclic entities, including a 42-membered-ring product. The conformational difference between singlet and tripletexcimers was confirmed by the experiments of formation and cleavage of the photodimers. © 1990, American Chemical Society. All rights reserved.