Long-lived room temperature phosphorescent system: Phenanthrene–β-cyclodextrin–tert-butylbenzene. Spectra and structure computer simulations

By V. G. Avakyan, S. V. Titov, V. B. Nazarov, M. V. Alfimov

The room temperature phosphorescence (RTP) of phenanthrene (PH) was studied before and after oxygen removal in a suspension formed by adding tert-butylbenzene (TBB) to an aqueous solution of PH and β-cyclodextrin (CD). The RTP decay curves indicate that PH molecules are located inside solid aggregates within two types of the self-assembled phosphorescent complexes. The structure of one of them, PH@2CD@2TBB (I), was proposed by the method of fluorescence spectroscopy and confirmed by the quantum-chemical (QC) calculation indicating the presence of the direct contact between PH and TBB inside the CD dimer in aggregate. The structure of the second, PH@3CD@2TBB (II), was calculated by the QC method. The molecular dynamics simulation (MD) was performed to elucidate the stability in an aqueous solution and in solid matrices of both I and II characterized by different degrees of PH packing density inside the aggregate. Due to the higher stability, I turned out to be the preferred structure as the basis of the phosphorescent center in a solid aggregate. To elucidate the mechanism of RTP quenching the O2 molecules was added to the solution and solid systems of both I and II. Oxygen mobility and penetration statistics to PH in these systems were calculated. Both the fluorescence data and computer simulations show that the longest-lived component of the RTP decay belongs to structure I, in which PH is sandwiched between two TBB molecules in the CD dimer, owing to which RTP is characterized by the minimum values of the quenching and nonradiative relaxation constants. PH molecules located in a less rigid environment like complex II, or as part of other less ordered structures, are characterized by shorter RTP lifetimes. The results demonstrate a high degree of protection of the substrate encapsulated in the CD dimer from oxygen.