## 3 Result and Discussion
### UV-Vis Absorption Properties  
After synthesis, the dyes were first evaluated by UV-Vis absorption spectroscopy to analyze the effects each structural modification had on the dye optical energy gaps (Figure 2, Table 1). All four dyes showed a single, broad charge transfer absorption band as the lowest energy transition in solution. Absorption maxima values were observed in the following order **AP6** < **AP8** < **AP9**\(\approx\)**AP12** in dichloromethane (479, 531, 590, and 588 nm, respectively). While the absorption maxima of **AP9** and **AP12** were similar in energy, the absorption onset values varied significantly due to a broader absorption from **AP9** in solution. Because DSC devices typically produce electricity until a dye onset, these values are more representative of the energies relevant to DSC devices. The dye \(\lambda_{\rm onset}\) values were estimated from both solution measurements in dichloromethane (DCM) and on TiO\({}_{2}\) films. The comparison with TiO\({}_{2}\) films is critical for dual anchor dyes, because the carboxylic acid is part of the charge transfer system, and the carboxylic acid conformation relative to the dye \(\pi\)-system can be dramatically affected by surface binding orientations [26]. **AP6** showed an absorption onset of 560 nm in DCM and only a slight red-shift on TiO\({}_{2}\) to 585 nm. The removal of the \(\pi\)-spacer phenyl groups between the quinoxaline and the carboxylic acids red-shifted the solution onset by 80 nm to give an onset of 640 nm for **AP8** in DCM. This red-shift was rationalized by the phenyl groups having significant steric influence on the dye conformation, which prohibited the carboxylic acids of **AP6** from being completely planar with the dye \(\pi\)-system as is needed for maximal influence on the charge transfer band. The carboxylic acid groups of **AP8** can access a completely planar orientation leading to a red-shift in absorption despite **AP8** having fewer \(\pi\)-electrons. Interestingly, **AP8** showed a 70 nm \(\lambda_{\rm onset}\) blue-shift on TiO\({}_{2}\). We attributed this to a significantly different film conformation being accessed relative to that observed in the DCM solution, which was potentially due to both anchors binding the TiO\({}_{2}\) surface leading to a dye conformation with a reduced \(\pi\)-system overlap between the carboxylic acids and the quinoxaline group. This highlights the need for careful assessment of the dyes on TiO\({}_{2}\) films rather than just from the solution when estimating dye energetics, as the change in the dye optical energy gap was 240 mV for the case of **AP8**. This was a substantial change, especially at longer wavelengths as there is little room in this spectral region for non-ideal energetics. **AP9** showed a dramatic shift in the absorption curve onset in solution to 770 nm when compared with **AP8**, where the only difference in the two dyes was a thiophene spacer between the quinoxaline and TPA groups. This could be due to both extending the conjugation of the dye \(\pi\)-system and to reducing the steric interactions between the quinoxaline \(\pi\)-bridge and donor region by the introduction of a 5-member ring at the quinoxaline bridge. The anchor group spacing is identical to that of **AP8**, and a significant shift in the absorption onset would again be expected on TiO\({}_{2}\). Indeed, a 110 nm blue-shift was observed for **AP9** on TiO\({}_{2}\). **AP12** compares most directly to **AP6**, and **AP12** had a solution absorption onset of 720 nm. This was a 160-nm red-shift in the absorption onset relative to **AP6**, where **AP12** differed by only the linking of the phenyl \(\pi\)-spacers between the quinoxaline and carboxylic acid to force planarity of this group and the addition of two nitrogen atoms to reduce sterics at the TiO\({}_{2}\) surface. Both changes likely influenced the absorption onset.