A fluorescent guest encapsulated by a photoreactive azobenzene dendrimer

Abstract

We have studied the adduct formed by N,N′-2,7-didecyldiazapyrenium (P²⁺), as the hexafluorophosphate salt, with a third generation dendrimer (D) that comprises 14 tertiary amine units in the interior, and 16 naphthyl and 16 trans-azobenzene units in the periphery. A strong charge-transfer interaction between the electron-accepting diazapyrenium dication and electron-donating amine units of the dendrimer interior leads to a host–guest complex with 1 : 1 stoichiometry, namely D(16t)⊃P²⁺. The self-assembly process can be easily monitored by strong changes in the absorption and emission spectra: (i) new absorption bands (λ_max ≈ 445 and 565 nm) at lower energy compared to the isolated species arise, (ii) a strong quenching of the intense diazapyrenium fluorescence is observed, and (iii) a new broad emission band, typical of the charge-transfer complex, appears with a maximum at 660 nm at 298 K and 600 nm at 77 K. By global analysis of the absorption spectra, an association constant of 1.0 × 10⁶ M⁻¹ is obtained for D(16t)⊃P²⁺, compared to 3.0 × 10⁴ M⁻¹ in the case of the adduct of P²⁺ with triethylamine under the same experimental conditions. This result demonstrates that the presence of bulky substituents at the dendrimer periphery does not hinder the formation of a stable complex and suggests a cooperative effect due to the multiple amine groups of the dendrimer branching points. No energy transfer from the azobenzene or naphthyl chromophores to the charge-transfer complex has been evidenced. The self-assembly process can be reversed upon addition of an equimolar amount of trifluoromethanesulfonic acid. An adduct with 1 : 1 stoichiometry is formed also between P²⁺ and D(2t14c), a species obtained from D(16t) upon irradiation at 365 nm: the corresponding association constant is very similar to that of D(16t)⊃P²⁺, showing that the isomeric state of the peripheral azobenzene units does not affect the stability of the adduct.