## 3 Results
### Optical Properties
#### 3.2.2 Fluorescence Spectra  
We determined the fluorescence spectra in dichloromethane and acetonitrile by irradiation at different wavelengths. At a concentration of 10\({}^{-6}\) M and under irradiation at \(\lambda_{2}\) or \(\lambda_{3}\) abs frequency maxima, as shown in Figure 4 (Figure S26 in Supporting Information, all measured fluorescence spectra in dichloromethane 10\({}^{-6}\) M), the spectra in dichloromethane of all compounds exhibited the main maximum at 435 nm and two minor maxima at 410 and 457 nm. At a higher concentration of 10\({}^{-5}\) M (see Figure S27 in Supporting Information for all spectra at this concentration), the emission spectra for compound **5 mT**, with the maximum at 457 nm, was more intense, and the maximum at 410 nm disappeared under irradiation at \(\lambda_{2}\). For compound **5 nT**, the maximum at 456 nm disappeared under irradiation at \(\lambda_{2}\) abs maximum. Compound **5 mT** displayed the highest fluorescence intensities as compared with **5 nT**, **5 mF**, **5 mP**, **5 nP**, and **5 bF** in dichloromethane at both analyzed concentrations (Figure 4 and Figures S26 and S27 in Supporting Information). In acetonitrile, all compounds but **5 mT** exhibited strongly decreased emission with maximum at 435 nm (see Figures S28-S33 in Supporting Information for all fluorescence spectra in CH\({}_{2}\)Cl\({}_{2}\) and CH\({}_{3}\)CN). In acetonitrile solutions of 10\({}^{-6}\) M and 10\({}^{-5}\) M, **5 mT** exhibited frequency maxima at around 460 and 485 nm, respectively, with a higher intensity in the most concentrated solution (Figure S28 of Supporting Information). We focused on studying the fluorescence modulation of compound **5 mT** by detecting spectra at different concentrations and in different solvents. We analyzed the acetonitrile and dichloromethane solutions at around 10\({}^{-3}\) M. In dichloromethane, no emission was  
Figure 4: More intense fluorescence spectra in dichloromethane, concentration 10\({}^{-6}\) M, of all compounds. Figure 3: UV spectra of compounds **5 mT** and **5 nT** in CH\({}_{3}\)CN with indication of the electronic transitions responsible for the absorption maxima. detected; in acetonitrile, the maximum redshifts at 495 nm had a noticeably increased intensity as compared with the diluted solutions (Figure S34). We measured the emission in THF, ethanol, and THF/H\({}_{2}\)O 1:1. In these solvents, the UV spectra displayed the same absorption frequencies as in dichloromethane and acetonitrile (Figure S35, see Figures S36-S38 for all fluorescence spectra). In THF solutions with concentrations of around \(10^{-6}\) M, we observed a very weak emission maxima at around 348 and 463 nm. At concentrations around \(10^{-5}\) M, depending on the excitation frequency, we observed a major maximum at 432 nm with two minor maxima at 410 and 457 nm, or an emission centered on a maximum at 460 nm. At concentrations of around \(10^{-3}\) M, no emission was detected. In ethanol solutions, the emission frequency maxima appeared similar to those in acetonitrile solutions. The difference was that the intensity maxima were highest in the \(10^{-5}\) M solutions. In solution THF/water 1:1, the emission intensity was very weak (maximum at 485 nm) in \(10^{-6}\) M solutions and was strongest in \(10^{-5}\) M solutions. We registered the highest intensity with redshift at 495 nm in the most concentrated solutions, the \(10^{-3}\) M solutions, with noticeably increased intensity. Figure 5a,b presents plots describing the frequency and intensity of emissions that we recorded for the solutions analyzed; Figure 5c presents the most intense registered fluorescence spectra at concentrations of \(10^{-6}\), \(10^{-5}\), and \(10^{-3}\) M.  
Under long-wave UV irradiation, the solutions displayed emissions varying from blue to cyan (Figure S39), in agreement with the detected emission spectra. Dichloromethane solutions exhibited blue emission that tended to disappear with increases in concentration.