Spectral properties of bichromophoric probes based on pyrene and benzothioxanthene in solution and in polymer matrices

Abstract

Abstract The new initiator for controlled radical polymerization labeled with highly fluorescent chromophores was prepared. Highly fluorescent chromophores, such as pyrene and benzo[k,l]thioxanthene (BTX), were used for fluorescent labeling. The chromophores were combined with nitroxide on 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO), which is one of the best systems for mediating the radical polymerization of olefins. The spectral properties of a labeled initiator were measured in solution and in polymer matrix. In the absorption spectra of the bichromophoric initiator there is a broad band with a maximum in the range 450–485 nm depending on the medium. This band corresponds to the BTX moiety, whereas the band with vibrational structure at 320–345 nm arises from the pyrene structural unit. The emission spectra of the probes, recorded at an excitation of 480 nm in solution and in the polymer matrices exhibit a broad band in the range 490–525 nm and it is red-shifted in polar media. Excitation of the labeled initiator in the region of the maximum absorption of pyrene results exclusively in emission due to BTX which is caused by efficient transfer of the excitation energy from pyrene to the benzothioxanthene chromophore. Quenching of the fluorescence of the donor chromophore pyrene moiety of the labeled initiator was observed in solution and in the polymer matrices, and this process is primarily intra-molecular. The quantum yield of florescence of the labeled initiator is very high in all the studied environments and is between 0.5 and 1 depending on the polarity of medium. Both chromophores exhibit fluorescence, but the lifetimes of the fluorescence are very different. The lifetime is 100–400 ns for pyrene and is 4–8 ns for benzothioxanthene. The intermolecular quenching rate constant of 3.6 × 10 10 dm 3 mol −1 s −1 was determined from the Stern–Volmer plot for the quenching of the decay of pyrene with the BTXD model dye. Based on the intermolecular quenching rate of the pyrene–BTXD, the rate of intramolecular energy transfer from pyrene to the BTX chromophore in this bichromophoric initiator was estimated to be greater than 10 11 s −1 .