Tuning Domain-Based Charge Transfer in Organic Dyes: Impact of Heteroatom Doping in the pi-linker of Carbazole-Based Systems

TL;DR

This work develops and applies a pCCD-based framework to quantify domain-based charge transfer in carbazole-based D–B–A dyes with mono-, di-, and tri-doped π-bridges. By combining ground-state pCCD, EOM-pCCD+S for excited states, and a domain decomposition of excitations, it reveals that nitrogen doping, particularly in tri-doped bridges (NNN), yields the strongest forward CT ($D \rightarrow B \rightarrow A$) up to 42.6%, while overall charge separation remains weak. The methodology is benchmarked against CAM-B3LYP TD-DFT and TheoDORE analyses, highlighting consistent trends with methodological differences in CT distribution. The findings provide actionable design rules for tuning CT in organic DSSC sensitizers, emphasizing the role of bridge doping pattern and the dominance of bridge-to-acceptor coupling in governing CT pathways.

Abstract

This work presents an innovative computational study of domain-based charge transfer that leverages the localized orbitals of pair Coupled Cluster Doubles (pCCD). This method enables both directional monitoring and quantitative assessment of charge transfer among donor (D), bridge (B), and acceptor (A) moieties. We applied this approach to a series of newly designed carbazole-based prototypical organic dyes, doping the bridge at positions 1, 2, and 3 with nitrogen, oxygen, and sulfur atoms to generate mono-, di-, and tri-doped variants. Our results demonstrate a clear and progressive enhancement in charge transfer as the degree of nitrogen or oxygen doping increases from mono- to di- to tri-doped systems. For mono-doped dyes, the highest forward charge transfer from donor to bridge to acceptor (D$\xrightarrow{}$B$\xrightarrow{}$A) occurs when a heteroatom (N or O) is placed in the terminal ring of the bridge, closer to the acceptor. In di-doped dyes, the largest forward charge transfer is observed when heteroatoms occupy both terminal positions, with one atom (N or S) adjacent to the donor and the other (N) near the acceptor. Nitrogen-doped systems consistently outperform their oxygen and sulfur counterparts. Among all variants, the organic dye doped with three nitrogen atoms at the bridge exhibits the most efficient and highest directional donor-to-acceptor charge transfer (42.6\%), making it the most promising candidate for potential applications in dye-sensitized solar cells. Finally, our calculations predict weak charge separation in all systems, indicating that charge transfer predominantly occurs from the bridge to the acceptor.

Figures (6)

• Figure 1: Schematic representation of the domain-based charge-transfer analysis. All investigated dyes were divided into three domains: Donor (D), Bridge (B), and Acceptor (A). The configuration state (CI) vectors contributing to the first excited state, each describing an individual $i \rightarrow a$ transition (from an occupied orbital $i$ to an unoccupied orbital $a$), were analyzed and assigned to the corresponding domains. By summing the weights (squared amplitudes of the CI vectors) over each domain, the character of the excited state can be determined in terms of charge transfer between domains versus local excitation within a given domain.
• Figure 2: A group of 33 $\pi$-conjugated linker moieties, mono-, di-, and tri-doped with heteroatoms ($\mathrm{{\color{nitrogen}N}}$, $\mathrm{{\color{oxygen}O}}$ and $\mathrm{{\color{sulfur}S}}$), is used to design prototypical organic dyes while maintaining a common carbazole donor and cyanoacrylic acid acceptor, where structures marked with a dagger ($\dagger$) indicate systems previously reported in the literature. pi-linker4
• Figure 3: The (a) D$\rightarrow$B*, (b) B$\rightarrow$B*, (c) B$\rightarrow$A*, and (d) D$\rightarrow$B$\rightarrow$A contributions to the first excited state of the mono-, di-, and tri-doped systems, calculated using the EOM-pCCD+S/cc-pVDZ method. The data was sorted according to the forward charge transfer.
• Figure 4: The hole and electron character for each domain (donor, bridge, and acceptor marked in blue, red, and yellow, respectively), obtained by subtracting the hole population from the electron population from the CAM-B3LYP (dash line) and EOM-pCCD+S calculations obtained for dyes with (a) mono-, (b) di-, and (c) tri-doped bridges. Positive values indicate a greater hole character, whereas negative values suggest a greater electron character.
• Figure 5: (a,b) HOMO–LUMO gaps and (c,d) first excitation energies calculated with (a, c) EOM-pCCD+S/cc-pVDZ and (b, d) TD-CAM-B3LYP/cc-pVDZ methods. The gray line is the reference value calculated for the undoped (CCC) system.