# Sunlight Bleaching of Subporphyrazine Dye Films
## 3 Results and Discussion
### Absorption Spectra  
As seen from Figure 1, SubPzS\({}_{3}\)H\({}_{0}\) is a peripherally modified porphyrin dye without terminal hydrogen atoms. It can be treated as a subphthalocyanine derivative where all three benzene rings are replaced by the strongly electron-accepting thiadiazole rings [15]. This leads to a downward shift of the frontier molecular orbitals and to some widening of the optical bandgap [14; 15; 18]. The electronic absorption spectra of SubPzS\({}_{3}\)H\({}_{0}\) consisted of two broad, intensive bands, Q- at 538 nm and B- (Soret) at 308 nm [16; 18], and one less intensive band at \(\sim\)390 nm (measured in DCM)--Figure 4. The latter band is absent in subphthalocyanines because it relates to the charge transfer (CT) from the \(\pi\)-system of subporphayazine chromophore to annulated 1,2,5-thiadiazole rings [18]. The most intensive Q-band located in the center of the visible range is assigned to the lowest-energy \(\pi\)-\(\pi^{*}\) transition (2a\({}_{2}\)\(\rightarrow\)1e*), i.e., corresponds to the HOMO-LUMO gap frequently used in the band diagrams of optoelectronic devices. The other intensive band, Soret-band, is composed of two transitions (2a\({}_{1}\)\(\rightarrow\)1e* and 1a\({}_{1}\)\(\rightarrow\)1e*) from HOMO-1 to the closely lying  
Figure 3: AFM image of the surface of pristine SubPzS\({}_{3}\)H\({}_{0}\) film (V-metallized) in a scan window of 2.25 \(\times\) 2.25 \(\upmu\)m\({}^{2}\) (**a**), and SEM image of this surface in a window of 25 \(\times\) 25 \(\upmu\)m\({}^{2}\) (**b**, nonmetallized). The lower part of the AFM image displays the edge of a large surface object that is seen in SEM—inset. excited states B1 and B2 in the near-ultraviolet range [18]. In most devices, the photons reach or leave the photoactive molecular layer after passing through a substrate coated with a transparent electrode, glass/ITO for example. The substrate itself does not transmit light in the UV range; thus, the absorption spectra of films deposited on it become practically interesting at wavelengths longer than 320-340 nm (Figure S4). During aggregation, the bands broadened and shifted bathochromically compared to the spectrum of a diluted solution due to the intermolecular interactions [24; 25]--Figure 4a. Judging from the low splitting and shift energies, the interaction between SubPzS\({}_{3}\)H\({}_{0}\) molecules in a film is even weaker than it is in subphthalocyanines [4], which correlates with the X-ray data showing the absence of molecular ordering. Probably, this is associated with the lack of hydrogen bonding in SubPzS\({}_{3}\)H\({}_{0}\), which can play an important role in a solid-state arrangement of subphthalocyanine molecules [26]. Figure 4b,c show the deconvolution results for the diffuse Q-band. The insertion of highly overlapping Gaussian peaks to deconvolve the experimental spectral profiles of phthalocyanines can give a virtually unlimited number of potentially successful fits and  
Figure 4: (a) Absorption spectra of SubPzS\({}_{3}\)H\({}_{0}\) dissolved in DCM and thin sublimed film on glass; deconvolution results for Q-band in solution (b) and film (c). is usually greeted with skepticism [27]. That is why here we do not strive here to achieve an ideal deconvolution procedure, but are just seeking to find a minimum admissible intraband composition and then trace its possible transformations in samples subjected to irradiation. As seen in Figure 4b, the Q-band for the SubPzS\({}_{3}\)H\({}_{0}\) molecule in the dissolved state can be modelled by four Gaussian subpeaks, the main one being centered at 539 nm. Other subpeaks correspond to the vibronic satellites or may partly be due to the mixing between the singlet Q-state and the lowest triplet state [25]. The detailed identification of the vibronic structure, which is clearly seen in the experimental Q-band of SubPc/SubPz, based on theoretical spectra is still difficult [18]. In the film, the main subpeak in the Q-band shifted by 15 nm, and subpeaks at 476, 498 and 519 nm shifted by 10 nm (Figure 4c).