# Sunlight Bleaching of Subporphyrazine Dye Films
## 3 Results and Discussion
### Photobleaching
#### 3.4.2 On Transparent Conductive Oxides (Glass/ITO, PET/ITO, Glass/FTO)  
Generally speaking, the ITO (In\({}_{2}\)O\({}_{3}\):SnO\({}_{2}\) = 90:10 wt%) coating on glass can be non-inert with respect to molecular film grown over it, albeit this circumstance is very seldom taken into account. There are two ways in which the substrate material can influence the overlying molecules. One is the so-called orientation effect that manifests itself in a specific arrangement of molecules in the first (and subsequent) layers of the growing film due to interaction with a substrate. The features of bulk molecular stacking determine optical and other characteristics, including the color of dyes [31]. Since our films were relatively thick and disordered (see Section 3.2), this effect is unlikely. The other method involves the doping of the molecular layer with a very little amount of admixtures migrating from the substrate. Despite the inertness of the ITO coating and the gentleness of the thermal evaporation technique, such a conjecture has been proved valid by applying precise analytical techniques [29; 30; 31]. Unlike bare glass substrate, ITO coating contains oxidized indium and tin species, and their presence in a layer due to upward diffusion could trigger the photolysis of dye molecules. First of all, the spectra of the pristine SubPc/SubPz films deposited on bare glass and on ITO do not completely coincide--Figure 5. In the SubPzS\({}_{3}\)H\({}_{0}\) films deposited on ITO, the red edge of the Soret-band broadens and the CT-band transforms into a shoulder of the Soret-band by moving to the left and decreasing in intensity (details in Figure S11). A similar, but less pronounced trend was demonstrated by SubPzS\({}_{2}\)H\({}_{4}\). SubPcS\({}_{0}\)H\({}_{12}\) is not sensitive to the substrate material. At this moment, we have no explanation for such spectral profiles. Secondly, the photobleaching also depends on whether the film was deposited on bare glass or on ITO--Table 3. Each compound has its own peculiarities of interaction with light/air/substrate--Figure 5. The most dramatic destruction occured with SubPzS\({}_{3}\)H\({}_{0}\) on ITO. After irradiation in laboratory or synthetic air, the SubPzS\({}_{3}\)H\({}_{0}\) films on ITO lost their original color and became almost transparent. For the other two compounds, the ITO underlayer had a rather stabilizing effect. In argon, this stabilizing effect ensures the same degree of photobleaching for all compounds. Despite such a strong photobleaching effect of SubPzS\({}_{3}\)H\({}_{0}\) films on ITO in air, their absorption spectrum, again, did not change fundamentally. This indicates that surviving dye molecules retained the original structure of their chromophores, and the end-products of destruction were mostly transparent or highly volatile. Nonetheless, a closer look reveals some distinct spectral features. First, there appeared a new component at the red side of the diffuse Q-band (580-600 nm) in the spectra of irradiated SubPzS\({}_{3}\)H\({}_{0}\) films--Figure S12. It could be an artifact of the fitting procedure, but, as we will show further, this is unlikely to be the case. Secondly, we noticed that the relative intensity of all subbands composing the Q-band increased in the films after irradiation (Table S1). As said above, the nearest-neighbor exciton coupling in the condensed state of zinc phthalocyanine was analyzed in Ref. [25], and the authors arrived at an important conclusion that an increase in the length of molecular aggregates leads to a decrease in the intensity of high-energy subpeaks in the vibrationally resolved Q-band spectrum. Conversely, growing blue subpeaks indicate shortening of the aggregation chain. Applying this model to our system and assuming that short-range molecular aggregates actually exist in an intact film, we do see that photobleaching destroys the ordering of molecules of any compound deposited on a glass/ITO substrate.