The absorption spectrum of 1 in PE at room temperature comprises a band with a maximum at 366 nm and a shoulder at 408 nm. When excited at the maximum of the absorption band, a group of bands in the range 400–450 nm and a wide band with a maximum at 503 nm were observed in the luminescence spectrum. The short-wavelength part of the spectrum was determined by the monomer luminescence of 1, the spectrum of which coincided with the luminescence spectrum of dilute solutions . The second band at 503 nm corresponds to the luminescence of excimers . The presence of two luminescence centers for 1 in PE is corroborated by the different structure of the excitation spectra at different values of λreg: at 416 nm, there is one band at 360 nm in the excitation spectrum; at 503 nm, the excitation spectrum has a maximum at 412 nm, the spectrum itself is much wider (270–450 nm) and represents a group of bands at 290, 360, and 412 nm. The wide excitation spectrum indicates the presence of several photoprocesses, which result in the excimer luminescence of 1 (Fig. 1a).
When comparing the absorption and excitation spectra of 1 in PE (Fig. 1 a), it was found that the maximum of the absorption spectrum (366 nm) corresponded to the excitation band of the monomer luminescence, whereas the 408 nm shoulder in the absorption spectrum corresponded to the most intense band in the excimer excitation spectrum (412 nm). The authors of  showed that, in crystals of 1, the structure of the stacks of molecules corresponded to J-aggregates consisting of dimers. The shoulder in the absorption spectrum of 1 is related to the absorption of J-aggregates and corresponds to the long-wave band of the excimer excitation spectrum. Therefore, at room temperature, the excimer luminescence of 1 in PE is realized by the interaction of two or more molecules that are not separated by hydrocarbon chains of PE. The excimer luminescence of 1 occurs both during the energy transfer from an excited molecule to an unexcited one (λex=360 nm) and during the excitation of excimer traps (dimers) in the structure of a J-aggregate (λex = 412 nm). The monomer luminescence of individual molecules of 1 is excited only by the light with λex = 360 nm.
For 2 and 3 in PE, a similar pattern was observed: a long-wave shoulder related to the absorption of J-aggregates was registered in the absorption spectrum (Fig. 1 b, c). This shoulder in the absorption spectrum corresponds to the intense long-wavelength band of J-aggregates in the excimer luminescence excitation spectrum (Fig. 1 b, c, spectrum 3). When luminescence is excited by the light with a wavelength corresponding to the maximum of the absorption spectrum (380 nm), monomer and excimer luminescence is observed. During the excitation in the absorption band of single molecules (380 nm), both monomer and excimer luminescence are excited. During the excitation in the band of J-aggregates (412 nm), only the luminescence of aggregates (excimers) is observed.
When the sample of 1 in PE is heated, before PE softens (T = 90°C), the film is discolored, and the luminescence color changes from yellow-green to blue (Fig. 2). The thermochromism was investigated under UV light (λ = 365 nm), i.e., in the excitation band of the monomer luminescence. Changes in the spectral properties of the sample under heating are shown in Figures 1 and 2. The long-wavelength shoulder disappears in the absorption spectrum, and the absorption spectrum of the heated film 1 coincides with the excitation spectrum of the monomer luminescence (Fig. 1 a). At the excitation at 360 nm in the luminescence spectrum of the heated film, the intensity of the monomer luminescence bands increases, whereas the intensity of the excimer luminescence decreases (Fig. 2 a). In the excimer luminescence excitation spectrum of 1 in PE, a significant increase in the short-wavelength component is observed during heating (Fig. 1S). The most significant changes were observed in the luminescence spectra recorded at λex = 360 nm (the maximum of the absorption spectrum of 1). Therefore, when 1 in PE is heated, and the matrix is softened, dissociation of J-aggregates is observed due to an increase in the solubility of 1 in PE, the intensity of excimer luminescence decreases, and the intensity of monomer luminescence increases.
Similar changes were also observed for the complexes 2 and 3 in PE (Fig. 2 b, c): when heated, the film was discolored and the luminescence color changed from yellow-green (2) and from aquamarine (3) to bright blue. Similarly to 1, the discoloration of complexes 2 and 3 in PE is related to the dissociation of J-aggregates under heating (a decrease in the intensity of the long-wave arm in the absorption spectrum (Fig.1 b, c)). The change in the luminescence color in 2 and 3 is also related to the dissociation of J-aggregates. Indeed, in the excimer excitation spectra of 2 and 3, the decrease of the intensity of the long-wave component related to the excitation of J-aggregates was observed (Fig. 2S, 3S). These processes are characterized by an increase of the intensity of the monomer luminescence bands and a simultaneous decrease of the intensity of the excimer luminescence bands (Fig. 2 b, c). When the film cools down, the color and luminescence color of the films restore to the original ones. The processes of aggregation and dissociation of molecules of 1–3 in PE are reversible and are reproduced in the repeated heating-cooling cycles.
One should mention that the evolution of the absorption spectra and stationary luminescence spectra of complexes 1–3 under heating is identical (Fig. 1, 2). However, unlike the stationary luminescence spectra, the evolution of the time-resolved luminescence spectra of complexes 1–3 is different (Fig. 3, 4).
The luminescence attenuation kinetics was recorded for each sample at two registration wavelengths (λreg = 440 and 500 nm) (Fig. 3), as well as the time-resolved luminescence spectrum (Fig. 4). For 1, the kinetic curves recorded at varied λreg differ significantly (Fig. 3 a): at λreg= 440 nm, τ = 6.9 ns, three-exponential kinetics (20.5 ns (26.06 %), 2.7 ns (42.00 %), 1.2 ns (31.94 %); and at λreg= 500 nm, monoexponential τ = 19.0 ns. Both excimer of 20.5 ns and monomer luminescence of 2.7 and 1.2 ns contribute to the kinetics at 440 nm. In the time-resolved luminescence spectrum, as opposed to the stationary one, an intense band of the monomer luminescence and a low-intensity band of the excimer luminescence are registered at room temperature (Fig. 3 a), whereas the excimer band appears 1.2 ns after the system is excited. In the case of 1, the traditional scheme for the excimer formation is observed:
A + hν → A*
A + A* → AA*
In the heated sample of 1, the lifetime at λreg=440 nm reduced from 6.9 to 4.9 ns by lowering the contribution of a long-lived component. In the time-resolved luminescence spectrum of the heated film of 1, the intensity of luminescence of the excimers decreases significantly and the excimer band is not recorded (Fig. 4 a).
For 2, in contrast to 1, both kinetic curves (Fig. 3 b) are proximate: at room temperature at λreg= 440 and 500 nm, the average lifetime equals to 3.8 ns. In both cases, a three-exponential process is registered: the component with τ = 16.0 ns corresponds to the luminescence of excimers; the one with τ = 2.9 and 1.3 ns corresponds to the monomer luminescence. The difference between the luminescence attenuation curves at λreg= 440 and 500 nm is determined by the redistribution of the contribution of the components. In the time-resolved spectrum of 2 in PE at room temperature, only excimer luminescence is observed, the intensity of the monomer luminescence is too weak to be registered (Fig. 4 b). For the heated film, on the contrary, only the monomer luminescence is registered in the time-resolved spectrum (Fig. 4 b). For the heated film 2, a component corresponding to the luminescence of excimers with τ = 21.0 ns is observed only at λreg = 500 nm (Table ESI).
For 2, the tendency towards the excimer formation is lower than for 1. This can be related to the structure of a molecule. In the complex 1, a molecule is planar, while a molecule of 2 has two mobile substituents in phenyl rings – methoxy groups, which can cause steric hindrances when overlapping neighboring molecules. Analysis of the crystal structure of 1  and 2  showed that, in crystals of 1, there existed a more complete overlap of the excimer-forming molecules than in 2 (Fig. 4S). A molecule of 3 contains an even more bulky substituent – a bromine atom, which is supposed to create even greater steric hindrances in the formation of excimers. Indeed, unlike the cases of 1 and 2, the lifetime of 3 in the excited state does not depend on the luminescence wavelength and equals to 1.5 ns, the value of τ does not change when heated (Fig.3 c). In addition, the luminescence band of 3 in PE is narrow, the half-width is only 57 nm (Fig. 2 c, 4 c), which is not typical for the excimer luminescence. Taking into account that there is a long-wavelength shoulder related to the absorption of J-aggregates in the absorption spectrum of 3 in PE, and the excitation spectrum also has a narrow long-wave excitation band of J-aggregates, which is mirror-symmetric to the luminescence spectrum, then the long-wave luminescence of 3 can be attributed to the emission of J-aggregates. When 3 is heated, J-aggregates dissociate (decrease in size), and bands related to the emission of monomers appear in the stationary and time-resolved spectra, simultaneously with the luminescence of the J-aggregates (Fig. 4 c).