## 3 Results
### Optical Properties of the Dyes  
All optical properties of the dyes and precursors discussed herein were collected in DCM solutions and are collated in Table 1. In addition, absorption and fluorescence spectra are included in the supporting information (Figure S2). All the dyes (and aldehyde precursors) exhibit weak to moderate fluorescence behavior in solution, consistent with previous observations [54]. Upon conversion of the aldehyde precursors (**3a-e**) into the dye derivatives (**2a-e**), there is a significant broadening and bathochromic shift in the UV-Vis absorption spectra for all dyes. Since the Knoevenagel condensation does little to perturb the highest occupied molecular orbital (HOMO) energies of the dye electrochemically (_vide infra_, Figure S1), it is reasonable to conclude that this bathochromic shift is a result of a decrease in the lowest unoccupied molecular orbital (LUMO) energy in the dye family. This can be attributed to extended conjugation. Figure 2a contrasts the experimental absorption behavior of the symmetrical dyes **2a**, **2c**, and **2e**, and Figure 2b depicts the calculated frontier molecular orbitals involved in dye absorption. At first glance, there is little difference between the observed absorption of dye **2a** and **2c**, but significant differences (blue shifted, with a low energy tail) are observed for dye **2e**. To better understand the nature of these optical behaviors in Family 2, time-dependent density functional theory (TDDFT) calculations were performed (summarized in Figure 2b) to determine the dominant optical transitions. Ground-state geometries were optimized with the B3 LYP hybrid functional. All optimizations were calculated without any symmetry constraints using the 6-311G(d,p) basis set with the Gaussian 16 (rev C.01) software package [57]. Predicted absorption spectra (by TDDFT) had a much broader spectral envelope that would be well suited for panchromatic absorption, but that was not observed experimentally. In each of the dyes studied, the HOMO-LUMO optical transitions are not significant contributors to the absorption spectra because of insufficient wavefunction overlap between these orbitals and a resulting weak transition oscillator strength. The \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Molecule**} & \multicolumn{3}{c}{**UV-Vis \(\lambda_{\text{max}}\) nm**} & \multicolumn{3}{c}{**Fluorescence**} & \multirow{2}{*}{**E\({}_{\text{1/2}}\) * (V vs. NHE)**} \\  & & \multicolumn{2}{c}{**(c \(\times\) 10\({}^{4}\) M\({}^{-1}\) cm\({}^{-1}\))**} & & **Emission \(\lambda_{\text{max}}\) nm** & & \\ \hline
**1a** & 285 (2.75) & & 480 (2.43) & 640 & & 1.22 \\
**1b** & & 337 (3.85) & 496 (3.77) & 635 & 0.66 & 1.34 \\
**1c**\({}^{\text{b}}\) & & 345 (4.22) & 505 (3.37) & 690 & 0.65 & 1.32 \\ \hline
**2a** & 307 (2.34) & 400 (2.68) & 462 (3.34) & 571 & & 1.14 & 1.36 \\
**2b** & & 361 (0.97) & 460 (0.58) & 510 & & & \\
**2c** & 342 (2.66) & 408 (sh, 2.50) & 465 (3.09) & 509 & & & \\
**2d** & & 352 (1.23) & 450 (0.92) & 509 & & & \\
**2e** & 301 (1.82) & 370 (3.32) & 441 (1.67) & 504 & & & \\ \hline
**3a** & 240 (2.65) & 365 (4.66) & & 483 & & 1.12 & 1.35 \\
**3b** & 240 (3.50) & 370 (5.10) & & 474 & 0.65 & 1.22 & 1.38 \\
**3c** & 245 (5.05) & 370 (6.15) & & 486 & 0.66 & 1.23 & 1.39 \\
**3d** & 245 (5.50) & 370 (8.25) & & 490 & 0.77 & 1.25 & 1.40 \\
**3e** & 245 (2.27) & 370 (3.32) & & 476 & 0.77 & 1.26 & 1.40 \\ \hline
**5d** & & 320 (3.81) & 440 (0.25) & 507 & 0.77 & & 1.35 \\
**5e** & & 330 (3.54) & 450 (0.24) & 508 & 0.78 & & 1.38 \\ \hline
**7**\({}^{\text{c}}\) & & 325 (2.31) & 415 (2.48) & 657 & 0.67 & & 1.34 \\ \hline \end{tabular}
\end{table} 
Table 1: Physicochemical properties of DSSC dyes and novel aldehyde precursors reported herein. basis set and functional used in the TDDFT accurately predicted the optical properties of dye **2a**, but were less effective at predicting the spectra for the ferrocene containing dyes.