Surface Loading Dictates Triplet Production via Singlet Fission in Anthradithiophene Sensitized TiO2 Films

Singlet fission, the process of transforming a singlet excited state into two lower energy triplet excited states, is a promising strategy for improving the efficiency of dye-sensitized solar cells. The difficulty in utilizing singlet fission molecules in this architecture is understanding and controlling the orientation of dyes on mesoporous metal oxide surfaces to maximize triplet production and minimize detrimental deactivation pathways, such as electron injection from the singlet or excimer formation. Here, we varied the concentration of loading solutions of two anthradithiophene dyes derivatized with either one or two carboxylic acid groups for binding to a metal oxide surface and studied their photophysics using ultrafast transient absorption spectroscopy. For the single carboxylic acid case, an increase in dye surface coverage led to an increase in apparent triplet excited-state growth via singlet fission, while the same increase in coverage with two carboxylic acids did not. This study represents a step toward controlling the interactions between molecules at mesoporous interfaces.

Table S1.Summary of time constants corresponding to the cation growth at λprobe = 1030 nm for TiO2-1 and TiO2-2.All measured surface coverages were fit to shared time constants.Table S2.Summary of time constants corresponding to the singlet excited state at λprobe = 1300 nm for all measured surface loadings for TiO2-1 and TiO2-2.All measured surface coverages were fit to shared time constants.

Figure S3 .
Figure S3.Transient absorption spectra of TiO2-1(200 µM) after 505 nm excitation at varying powers of A. 30 nJ/pulse B. 50 nJ/pulse and C. 100 nJ/pulse.Pump-probe delays are shown in the legend.D. Normalized transient absorption kinetics of TiO2-1 at varying powers probed at 570 nm, the triplet excited state feature.The behavior of this feature does not change with increasing power.

Figure S6 .
Figure S6.Relative yield of triplets based on global fit amplitudes of S1 and T1 in Figure S5 plotted vs surface coverage.This method produces similar results to Figure 3 in the main text showing a steady increase in triplet excited states with an increase in surface coverage.

Figure S10 .
Figure S10.Normalized transient absorption kinetics of TiO2-1 (200 µM) at 1030 nm, where the cation signal appears, at two different excitation powers: 30 nJ(red) and 150 nJ (black).Binding SimulationsA grid of 100x100 sites emulating a TiO2 surface was constructed in MATLAB, and sites were randomly populated for different loadings from 1-90%.1 The DBSCAN cluster analysis was performed on the result, yielding number of clusters of up to 8 (representing a full set of nearest neighbors in the unit cell).The occurrences as a function of loading were extracted and are shown in FigureS11.

Figure S11 .
Figure S11.Simulation results for 2% dye occupation of a roughly 400x400 Angstrom lattice of TiO2 sites (10000 sites).The occupation is generated by choosing random x and y positions within the lattice, with no cooperativity.Colors indicate clusters of (a) two and (b) three dyes identified through the DBSCAN algorithm in MATLAB.

Figure S12 .
Figure S12.Occurrences for dimer (N=2) and larger clusters from DBSCAN analysis of a grid of 10000 sites with increasing loading.X-axis has been translated to surface coverage to match Figure 3 of the main text.