## 3 Result and Discussion
### Time-Correlated Single Photon Counting (TCSPC) Studies  
Electron injection efficiencies were estimated from time-correlated single photon counting (TCSPC) studies with **AP9** and **AP12** to compare with the reported values for **AP6** and **AP8**[24]. The experiments were conducted for each dye in DCM solution, on TiO\({}_{2}\), on TiO\({}_{2}\) with chenodeoxycholic acid (CDCA) as a deaggregating agent, and on TiO\({}_{2}\) with CDCA and LiI [26; 40; 41]. Due to the short excited-state lifetimes of the dyes in solution combined with instrument response function limitations (>150 ps timescale), injection efficiencies can only be reported as a lower limit and are likely faster than that reported. The equation \(\eta_{\text{eff}}=1-(\tau_{\text{TiO}_{2}}/\tau_{\text{sol}})\) is used to estimate the photoinduced dye electron injection efficiency to the TiO\({}_{2}\) CB where the overall efficiency (\(\eta_{\text{eff}}\)) is calculated based on the fluorescence decay lifetime (\(\tau\)) of the dye in different environments. The solution lifetimes of the dyes ranged from 0.34 to 0.79 ns in the following order: **AP12 > AP9 > AP6 > AP8** (Figure 6 and Table 3). The lifetimes of the \(\pi\)-extended **AP9** and the fused ring **AP12** were significantly longer than that of derivatives **AP8** and **AP6**, respectively (\(\sim\)0.78 ns vs. \(\sim\)0.36 ns). On TiO\({}_{2}\) with no additives, an emission can still be observed, but lifetimes were shorter than observed in solution ranging from 0.29-0.55 ns  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Dye** & \(\tau_{\text{sol}}\) **(ns) 1 ** & \(\tau_{\text{TiO}_{2}}\) **(ns) 1 [FOOTNOTE:1]Footnote 1: In dichloromethane (DCM),2 [FOOTNOTE:1]Footnote 1: In dichloromethane (DCM),2 [FOOTNOTE:1]Footnote 1: On TiO\({}_{2}\), 4 On TiO\({}_{2}\), 4 On TiO\({}_  
with \(\eta_{\text{eff}}\) values of 27-42%. In this environment, **AP9** showed the highest injection efficiency, with **AP6** and **AP8** showing the lowest \(\eta_{\text{eff}}\) values of \(\sim\)28%. The addition of the deaggregating agent CDCA to the dyeing solution resulted in significantly increased charge injection efficiencies for **AP9** and **AP12** from 42% to 58% and 30% to 68%, respectively. **AP6** and **AP8** remained relatively unchanged, which suggests these dyes have fewer dye-dye energy transfer pathways and excited-state deactivation pathways than **AP9** and **AP12**. **AP9** and **AP12** inject best in isolation from other dye molecules when the CDCA insulator is added. The addition of Li resulted in fluorescence lifetimes shorter than the response function of the instrument, leading to the highest injection efficiencies observable for each dye ranging from >54% (**AP8**) to >81% (**AP12**). Given that the dye excited-state lifetimes exceeded the response time of the instrument with the LiI additive, no further additives were evaluated. This does suggest that the low peak IPCE observed for **AP12** was not due to electron injection into TiO\({}_{2}\) and was more likely a combination of results from a low molar absorptivity and possibly a relatively slow regeneration event, as has been previously observed for DD-\(\pi\)-AA dyes [26]. However, the >81% electron injection efficiency from the novel **AP12** anchor/acceptor group with a very low driving force for electron transfer (160 mV) strongly suggests this group is valuable for the future design of NIR organic dyes for DSCs. It is noteworthy that the excited-state lifetimes for the dyes evaluated in this series are short relative to typical D-\(\pi\)-A dye designs, which are often \(\geq\)1-2 ns. This may be an inherent limitation to the DD-\(\pi\)-AA design, as short lifetimes have also been observed for a thienopyrazine-based DD-\(\pi\)-AA dye **AP3** (0.36 ns in DCM) [26]. However, **AP3** gave DSC devices with >10% PCE and >80% peak IPCE values in low light in the prior study. **AP6** also showed a reasonable peak IPCE of \(\sim\)70% despite having one of the shortest excited-state lifetimes of the series.