Fluorescence Characteristics of Naphthalene Dendrimers at Low Concentration in Aqueous Solution

Abstract

Water-soluble naphthalene dendrimers WN1, WN2, and WN3, together with the lipophilic compounds N1, N2, and N3 were prepared and their photochemical properties were examined. Whereas lipophilic dendrimers N1, N2, and N3 gave monomer emission peaking at 330Í340 nm, water-soluble dendrimers WN1, WN2, and WN3 gave not only the monomer emission at 330Í340 nm but also excimer emission peaking at 400 nm in aqueous solution under considerably diluted conditions. The relative intensity of the excimer fluorescence at longer wavelengths compared to the monomer emission depended on the dendrimer generation and the concentration of dendrimer and added salt (KCl). In particular the excimer emission of the second generation dendrimer is considerably higher than those of the first and the third generation dendrimers. Based on these experimental results, the aggregation and the excimer formation of water-soluble naphthalene dendrimers in aqueous solution depending on the generation and the conditions were discussed.Dendrimers are expected to have unusual chemical and/or physical properties and many studies concerning dendrimer effects have been reported. 1Í5 We have prepared several kinds of dendrimers exhibiting photochemical and photophysical properties, which could be explained by usual analytical processes. Among these, stilbene dendrimers having large substituents of benzyl ether dendrons or polyphenylene dendrons exhibited either fluorescence or photochemical isomerization and almost no effect of the large dendron substituents was observed. 6Í8 However, the introduction of hydrophilic groups such as carboxylate anion at the periphery of the benzene ring of the benzyl ether stilbene dendrimers enabled these compounds to be soluble in water and to exhibit unusual ultrafast photoisomerization of the C=C double bond (in the range of tens of nanoseconds) and rather slow molecular structural change (ca. ms) due to the reorientation of surrounding dendron groups after the geometric isomerization of the C=C double bond of the stilbene core. 9,10 Thus, photochemical properties of water-soluble photoresponsive dendrimers seem to be interesting.We have already prepared water-soluble dendrimers of polyaromatic hydrocarbon such as pyrene and observed fluorescence emission due to the excimer or aggregated form of the core pyrene group even at relatively low concentration of ca. 10 Õ4 M. 11 Thus, the water-soluble fluorescent molecules may give aggregates showing characteristic fluorescence at longer wavelength in contrast to the monomer emission. Because of the shorter singlet lifetime and smaller aromatic skeleton of naphthalene compared to pyrene, it is not easy to observe the fluorescence emission of naphthalene in excimer or aggregated form in solution at low concentrations. In contrast to this, naphthalene excimer was observed by fluorescence emission in solid state, 12 polymers, 13 photodimers, 14 and dendrimers having several naphthalene chromophores at the periphery. 15 However, naphthalene excimer formed by intermolecular inter...

Self-assembly of a poly(glutamate) dendrimer: solvent-dependent expression of molecular chirality and a [2 + 2] photocrosslinking reaction

“…[1][2][3] In many cases, a functional molecule is placed at the central core with bulky dendrons used for shielding the core site. [4][5][6] Coneshaped dendrimers, where the bulky dendron(s) attach to one side of the core molecules, tend to aggregate efficiently [7][8][9][10][11][12] and therefore have received much attention as a new class of building blocks of molecular architecture. In this paper we report on the self-assembly of a stilbene-cored dendrimer in which bulky poly (glutamate) dendrons are attached at the 4,4′-positions of stilbene (Fig.…”