Internal conversion in poly(1-vinylnaphthalene)—IV. Photochemical processes in polymeric systems

Using Excimeric Fluorescence to Study How the Cooling Rate Determines the Behavior of Naphthalenes in Freeze-Concentrated Solutions: Vitrification and Crystallization

“…84,85 In this context, for example, the adsorption of Np on the surface of silica gel produces a second excimer emission at 375 nm; 86 a second excimer is also observed in polyvinylnaphthalene glass. 74,87 Np-doped amorphous silica glasses show the second excimer peaking at around 380 nm at 0.5 mol/L. 77,88 Thus, the similarity between the Np emission spectra in the FCS and the amorphous silica glasses is indicative of the FCS being glassy, with a concentration of at least 0.5 M. As shown below, the thermal dependence of the fast-frozen samples' luminescence confirms this hypothesis even further.…”

“…73 A part of the spectra of the pure MeNp appear like monomer emission (around 325 nm), and there is also a shoulder at around 430 nm, indicating other possible excimer structures. 74 This signal is higher in energy than the phosphorescence signal of the MeNp (471 nm) or the Np (469 nm). 47,75 These spectral features probably do not originate from the impurity, as they are absent from the spectra of the (frozen) solutions.…”

“… The amount of reabsorbed light directly corresponds to the thickness of the crystals, as is observed in Figure S1 and S2 and well documented in the literature. , The pure MeNp is a liquid at room temperature and shows mainly excimer-like emission in the fluorescence spectrum (Figure ). A part of the spectra of the pure MeNp appear like monomer emission (around 325 nm), and there is also a shoulder at around 430 nm, indicating other possible excimer structures . This signal is higher in energy than the phosphorescence signal of the MeNp (471 nm) or the Np (469 nm). , These spectral features probably do not originate from the impurity, as they are absent from the spectra of the (frozen) solutions.…”

“…This property stands in contrast to solutions of nonpolar solvents at high concentrations or an ethanol solution at a low temperature, which allows for excimers with a well-developed π–π overlap; the scenario then results in a distinct excimer emission having large bathochromic shifts around 385–410 nm. , A similar excimer emission peaking at around 400 nm is observed in layers of amorphous Np deposited on either gold or ice at temperatures below 77 K. Excimers are known as stabilized dimers of one excited- and one ground-state molecule, with dissociative ground-state interactions. Conversely, the presence of a second excimer, stabilized in both the ground and excited states, is typical of constrained media with restricted molecular mobility. , In this context, for example, the adsorption of Np on the surface of silica gel produces a second excimer emission at 375 nm; a second excimer is also observed in polyvinylnaphthalene glass. , Np-doped amorphous silica glasses show the second excimer peaking at around 380 nm at 0.5 mol/L. , Thus, the similarity between the Np emission spectra in the FCS and the amorphous silica glasses is indicative of the FCS being glassy, with a concentration of at least 0.5 M. As shown below, the thermal dependence of the fast-frozen samples’ luminescence confirms this hypothesis even further.…”

“…This signal is higher in energy than the phosphorescence signal of the MeNp (471 nm) or the Np (469 nm). , These spectral features probably do not originate from the impurity, as they are absent from the spectra of the (frozen) solutions. The particular excimer structures were proposed by Gupta et al , based on observation of two distinct lifetimes in polymeric matrices. , The presence of the methyl group on the aromatic ring reduces the symmetry of the molecule, allowing multiple mutual orientations in the dimeric structures, which could explain the dual-fluorescence bands. The X-ray investigation of the liquid MeNp does not exclude such a possibility .…”