Molecular engineering, synthesis, and atomistic structure–property relationship of indoloquinoxaline-capped small donors for efficient organic solar cells

Abstract

The growing demand for high-performance organic photovoltaics has sparked great interest in small-molecule donor (SMD) materials that offer well-defined structures and superior batch-to-batch consistency. In this study, we report the molecular design, synthesis, and atomistic structure–property characterization of three indoloquinoxaline (IQ)-capped SMDs named DPP-Th-IQ, BT-Th-IQ, and TT-IQ for potential applications in all-small-molecule organic solar cells (ASM-OSCs). Each SMD features a distinct central core, including diketopyrrolopyrrole (DPP), benzothiadiazole (BT), or thieno[3,2-b]thiophene (TT) with thiophene as bridging units in the DPP and BT derivatives, to systematically tune electronic structures, optical profiles, and charge transport properties. Electrochemical analysis confirmed that all three SMDs possess well-aligned HOMO–LUMO levels conducive to pairing with the Y6 non-fullerene acceptor. Density functional theory (DFT) calculations revealed low hole/electron reorganization energies with extensive frontier-orbital delocalization, indicative of efficient charge transport. Photophysical experiments based on UV-vis, photoluminescence, and solvatochromic analysis and complementary computational characterization showed strong intramolecular charge transfer in SMDs. Electron density difference analysis explained that particularly the benzothiadiazole-based BT-Th-IQ donor exhibits the lowest exciton binding energy coupled with high charge transfer excitations, indicating efficient exciton dissociation. Donor–acceptor interfacial modeling further predicted robust face-on π–π stacking and favorable donor-Y6 orientations that support interfacial charge transfer. Importantly, all three SMDs demonstrated initial thin-film stability: films retained ≥90% of their initial absorbance after 30 hours of continuous AM 1.5G irradiation, and thermogravimetric analysis showed decomposition temperatures (5% weight loss) exceeding 250 °C. Overall, this study clarifies the interplay between molecular design, electronic structure, interfacial interactions, and stability, providing a strategic path toward next-generation high-efficiency ASM-OSCs based on IQ-capped donors.

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1. Introduction

The continuously growing global demand for sustainable energy has accelerated research and development in advanced photovoltaic technologies. Among the various platforms, organic solar cells (OSCs) have garnered particular attention due to their potential for lightweight, flexible, and cost-effective energy harvesting devices. In contrast to certain hybrid perovskite and dye-sensitized solar cells that may employ toxic or corrosive materials, OSCs present a more environmentally benign alternative, relying on carbon-based materials with relatively lower toxicity profiles.^1–3 Early OSCs primarily utilized fullerene acceptors paired with small molecular donors, attaining modest yet promising power conversion efficiencies (PCEs). However, limitations such as narrow absorption coefficients, constrained energy-level tunability, and suboptimal thermal stabilities restricted these devices to PCE values of about 13%.⁴

Subsequent developments focused on the integration of non-fullerene acceptors (NFAs), which helped push OSC efficiencies above 20%.^5,6 NFAs offer advantages such as wider absorption ranges, customizable bandgaps, and improved morphological stability. Yet, pairing NFAs with polymer-based donors introduces additional complexities, with most notably polydispersity and potential morphological instabilities that may limit device reproducibility and long-term performance. In response, small molecular donors (SMDs) have emerged as compelling alternatives, owing to their well-defined molecular structures, tunable electronic structures for optimal donor–acceptor alignment, and batch-to-batch consistency. Nonetheless, despite these attributes, all-small-molecule OSCs (ASM-OSCs) continue to trail polymer–NFA systems in terms of overall efficiency, with most reports hovering around 15–16% PCE.^7–9 This discrepancy underscores the need for new small molecular donor designs that not only rival polymer-based donors but also surpass existing challenges related to morphology, stability, and scalability.

Molecular design strategies for small-molecule donors often involve A–D–A or A–π–D–π–A backbones, where electron-rich (donor) and electron-deficient (acceptor) units are carefully arranged to optimize energy-level alignment and facilitate efficient charge transfer. Building blocks like benzo-[1,2-b:4,5-b′]dithiophene (BDT) have been widely adopted due to their planar geometry, which enhances molecular packing and charge mobility.¹⁰ Early BDT-containing donors achieved moderate PCEs when combined with fullerene acceptors, while subsequent efforts pairing BDT-based donors with high-performance Y6-type NFAs have driven ASM-OSC efficiencies beyond 15%.^11–13 Other core motifs, such as naphtho[1,2-b:5,6-b′]dithiophene, porphyrins, and thienobenzo-dithiophene (TBD), extend the available chemical space, enabling systematic fine-tuning of optical properties and energy levels.^14–16 Although these structural innovations have improved ASM-OSC performance, several practical issues persist. For example, many small-molecule donors lack the favorable film-forming characteristics inherent to polymers, necessitating additional processing or post-treatment steps to achieve optimal domain sizes and morphologies in the active layer. Moreover, certain small-molecule donors are prone to crystallization or self-aggregation, which, if not properly controlled, can hinder charge transport and limit device reproducibility.

Despite the advancements, the morphology of ASM-OSCs remains difficult to control. Achieving a continuous, yet finely mixed donor–acceptor network typically demands post-treatments (e.g., thermal or solvent vapor annealing) that may be unsuitable for large-scale manufacturing.¹⁶ Moreover, the low viscosity of small-molecule solutions complicates large-area printing of photoactive layers, posing a hurdle for industrial fabrication. Finally, although polymer-based materials often show greater morphological stability, questions persist regarding the long-term reliability of fully small-molecule systems.¹⁷ Systematic stability studies will be integral as the field advances. Therefore, ASM-OSCs remain in a stage of ongoing materials exploration and efficiency enhancement, and more comprehensive answers to existing challenges may only emerge as a wider variety of donor–acceptor combinations are investigated. Progress is particularly dependent on the careful molecular engineering of small-molecule donors and acceptors to optimize key photovoltaic parameters: open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF). Achieving V_OC values above 0.90 V requires precise control of the donor's highest occupied molecular orbital (HOMO) and the acceptor's lowest unoccupied molecular orbital (LUMO) energy levels, while broad absorption spectra and high charge-carrier mobility are essential to maximizing both J_SC and FF.^18–20 Consequently, the primary obstacle lies in designing donor–acceptor (D/A) systems with aligned energy levels, extended absorption across the visible to near-infrared region, and robust morphological and electronic properties. Addressing this challenge necessitates the discovery and development of efficient SMDs that not only meet these stringent performance criteria but also enhance device stability, thereby paving the way for future efficient ASM-OSCs.

This study reports the design, synthesis, and computational characterization of three new SMDs based on indoloquinoxaline (IQ) end-groups and two electron-deficient central cores, including diketopyrrolopyrrole (DPP), and benzothiadiazole (BT), one electron-rich core, thienothiophene (TT) and named as DPP-Th-IQ, BT-Th-IQ, and TT-IQ, respectively. IQ-based architecture has recently garnered increasing interest. The intrinsic donor–acceptor character of 6H-indolo[2,3-b]quinoxaline, which is derived from electron-rich indole ring and electron-deficient quinoxaline fragment facilitates efficient intramolecular charge transfer (ICT), making it a promising terminal unit in SMD design. Among central units, DPP stands out for its robust electron-transporting capacity, high photochemical stability, and strong absorption, whereas BT is a versatile acceptor fragment easily synthesized from readily available precursors. Both DPP- and BT-centered molecules are further modified with thiophene π-bridges to extend conjugation and fine-tune energy levels. By contrast, TT central unit imparts a rigid backbone that is conducive to planar packing and efficient π–π interactions, potentially boosting charge mobility and light absorption. By systematically examining the structural, optical, electrochemical, and charge transfer (CT) properties of these IQ-capped donors with distinct core units, we aim to clarify structure–property relationships at atomistic level and highlight promising design principles for next-generation ASM-OSCs.

2. Materials and methods

2.1. Materials and instruments

All reagents and catalysts were purchased from Sigma-Aldrich and Tokyo Chemical Industry and used without further purification. Solvents, including dichloromethane (DCM), ethyl acetate, and n-hexane, were procured from local vendors and distilled prior to use. Melting points were determined using a Jindal melting point apparatus with open capillary tubes. ¹H and ¹³C NMR spectra were recorded on a Bruker Ascend 400 MHz spectrometer, employing deuterated chloroform (CDCl₃) as the solvent and trimethylsilane (TMS) as an internal reference. The IR spectra were recorded using a Bruker Alpha Platinum ATR spectrometer. The UV-vis absorption spectra were obtained using a Shimadzu Pharmaspec UV-1700 spectrophotometer in both solutions and solid films and photoluminescence (PL) spectra were measured using a Cary Eclipse Fluorescence Spectrophotometer. Photostability tests were performed under continuous AM 1.5G illumination (100 mW cm⁻²) using a G2V Optics Pico™ LED solar stimulator. PerkinElmer USA, model STA 8000 device was used to examine the stability of compounds at varied temperatures (20–120 °C). In a nitrogen atmosphere, at a heating rate of 10 °C min⁻¹ thermogravimetric analysis (TGA) was performed in the temperature range of 30 °C to 700 °C. For cyclic voltammetry measurements, a standard three-electrode configuration was employed in a PalmSens EmStat3+ potentiostat, using Ag/AgCl as the reference electrode, fluorine-doped tin oxide (FTO) glass as the working electrode, and ferrocene as an internal reference.

2.2. Computational methodology

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out with Gaussian 09 software.^21–24 B3LYP hybrid functional combined with the 6-31G(d,p) basis set was employed to model molecular geometries and electronic structures.^25,26 Vibrational frequency calculations were performed to confirm that all geometries correspond to local minima on the potential energy surface. For electronic structures, Mulliken population analysis was employed to investigate the contributions of individual molecular fragments to the frontier molecular orbitals (FMOs). Furthermore, the density of states (DOS) and partial density of states (PDOS) of the molecular units in the donors were analyzed using PyMOlyze-1.1 software.²⁷ The solvent-phase absorption spectra was simulated by CAM-B3LYP/6-31G(d,p) method with IEFPCM model and Gibbs solvation free energy (ΔG_solv) was simulated using the SMD model by M06-2X/6-31G(d,p) method.²⁸

Electron density difference (EDD) analysis were conducted for every optimized SMD and SMD/acceptor complexes in order to visualize the redistribution of charge upon photo-excitation and to extract quantitative charge transfer descriptors.^21,29 Following Le Bahers et al. and the Multiwfn 3.8 implementation, four primary quantities given in eqn (1)–(4) were evaluated from the difference density Δρ(r) = ρ_ele(r) − ρ_hole(r):^21,30

t_index = D_CT − H_index (4)

where D_CT is the electron–hole centroid separation among the barycentre, q_CT the total transferred charge magnitude. Whereas, the indices H_index/t_index were used to evaluate charge transition behavior and the extent of charge separation. H_index is the spatial extent of the transition along the CT axis, and t_index is a separation metric that approaches zero when electron and hole strongly overlap. Through-space CT mechanism was examined based on these EDD parameters. In practice, higher D_CT together with large q_CT and low H_index is taken as an indicator of efficient spatial charge separation.

For each excited state the inter-fragment charge-transfer (IFCT) matrix was built and the relative contributions of charge-transfer and local excitations were obtained from eqn (5) and (6):

LE (%) = 100 − CT (%) (6)

where q_D→A is the electron population transferred from donor fragment D to acceptor fragment A.

A large D_CT or CT (%) combined with a small H_index, t_index and LE (%) evidences efficient charge separation and, by extension, a reduced probability of geminate and non-geminate recombination in organic photovoltaic materials. In addition, the transition density matrix (TDM) and hole/electron overlap heat maps were generated to visualize electron–hole coherence across molecular fragments in SMDs.^30,31 All EDD and IFCT quantities were obtained directly via Multiwfn 3.8 on excited-state wave-functions calculated using the range-separated hybrid (RSH) functional “CAM-B3LYP”.^32–35

The charge transport capabilities of the materials were assessed by calculating reorganization energy, a key parameter for estimating charge carrier mobility in organic materials. The adiabatic potential energy surface (PES) approach was used to compute the reorganizational energy of holes (λ_hole) and electrons (λ_ele), respectively.^36,37 These values were derived based on the relaxation of the cationic and anionic states to the neutral molecular geometry, as defined by the following eqn (7) and (8):

λ_hole = (E⁺_neutral − E_neutral) + (E_cationic − E⁰_cationic) (7)

λ_ele = (E⁻_neutral − E_neutral) + (E_anionic − E⁰_anionic) (8)

where, E_neutral represents the neutral molecule energy, E⁺_neutral and E⁻_neutral are the cationic and anionic states energies optimized from the neutral geometry, while E_cationic and E_cationic denote the cationic and anionic states energies, respectively. Whereas, E⁰_cationic and E⁰_anionic correspond to the energy of the neutral molecule optimized in the cationic and anionic geometries, respectively.

3. Results and discussion

3.1. Synthesis

The synthesis of side-peripheral IQ groups was carried out with the preparation of 8-bromo-6H-indolo[2,3-b]quinoxaline derivatives via a straightforward condensation reaction, as previously described in the literature.³⁸ This involved the reaction of 1,2-diaminobenzene with 6-bromoindoline-2,3-dione in glacial acetic acid under reflux, yielding the target compound in a high yield of 87%. To improve the solubility and processability of the rigid-structured fused-ring IQ derivatives, N-alkylation was performed using 2-ethylhexyl bromide and potassium carbonate in dimethylformamide at 80 °C overnight, resulting in the formation of 8-bromo-6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxaline (C₈IQa). Subsequent borylation was carried out using bis(pinacolato)diboron as the borylating agent, in the presence of palladium acetate, X-Phos as the catalyst, and potassium acetate as the base, resulting in 6-(2-ethylhexyl)-8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-6H-indolo[2,3-b]quinoxaline (C₈IQb), as given in Scheme 1.

Scheme 1 Synthesis pathway of diketopyrrolopyrrole (DPP), benzothiadiazole (BT), and thieno[3,2-b]thiophene (TT) core units containing small molecular donors: (a) glacial acetic acid, DMF, reflux 24 h, (yield, 87%) (b) 2-ethylhexyl bromide, K₂CO₃, DMF, 85 °C, 18 h, (yield, 80%) (c) bis(pinacolato)diboron, Pd(CH₃COO)₂, X-Phos, CH₃CO₂K, toluene, 85 °C, 14 h (yield, 75%) (d) Pd(CH₃COO)₂, X-Phos, K₂CO₃, THF/water, 85 °C, 18 h (yield, 57%) (e) Pd(CH₃COO)₂, K₂CO₃, pivalic acid, PCy₃·HBF₄ ligand, DMA, 110 °C, 5 h (yield, 78%) (f) PdCl₂(PPh₃)₂, THF, 65 °C, 24 h (yield, 87%).

Small molecular donors with DPP, BT, and TT cores were synthesized with IQ moieties as terminal groups through different synthesis pathways. Thiophene was used as a bridging unit in the DPP- and BT-based donor derivatives, while the TT-based derivative synthesised without the bridging thiophene units, as illustrated in Scheme 1. The donor molecule 2,5-bis(2-ethylhexyl)-3,6-bis(5-(6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxalin-8-yl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP-Th-IQ) was synthesized starting from 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione. Initially, N-alkylation of the DPP core was achieved by reacting 2-ethylhexyl bromide with potassium carbonate in DMF at 80 °C for 24 hours, producing 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione. Bromination of this intermediate with N-bromosuccinimide (NBS) in DMF at 60 °C overnight yielded (1). Finally, the target DPP-Th-IQ donor was synthesized through Suzuki–Miyaura cross-coupling reaction between 1 and C₈IQb in the presence of a palladium catalyst, resulting in a 57% yield after purification. The BT-based donor, 4,7-bis(5-(6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxalin-8-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BT-Th-IQ), was synthesized starting from 4,7-dibromobenzo[c][1,2,5]thiadiazole (DBBT). Initially, a Stille coupling reaction was performed between DBBT and tributyl(thiophen-2-yl)stannane in the presence of dichlorobis(triphenylphosphine)palladium(II) as the catalyst and tetrahydrofuran (THF) as the solvent, yielding (2). Direct arylation (C–H activation) of 2 with C₈IQa yielded the final BT-Th-IQ donor in a 78% yield. Whereas, the TT-based donor, 2,5-bis(6-(2-ethylhexyl)-6H-indolo[2,3-b]quinoxalin-8-yl)thieno[3,2-b]thiophene (TT-IQ), was synthesized in a single step via Stille coupling. This involved the reaction of 2,5-bis(tributylstannyl)thieno[3,2-b]thiophene (3) with the prepared brominated IQ derivative C₈IQa using dichlorobis(triphenylphosphine)palladium(II) as the catalyst in THF. The reaction afforded TT-IQ in high yield (87%) after purification. The chemical structures of synthesized intermediates and compounds were confirmed by ¹H NMR and ¹³C NMR spectroscopy, and the corresponding spectra are provided in the SI (Fig. S1–S9), along with detailed reaction kinetics.

3.2. Energy levels and estimated driving-force offsets

An understanding of the HOMO and LUMO energies is paramount when designing efficient D/A systems for BHJ-OSCs. In particular, the donor's HOMO must be sufficiently deep to maximize the V_OC of OSCs, while the donor's LUMO should be higher than that of the acceptor to facilitate efficient electron transfer and prevent back-electron transfer.^39,40Table 1 provides the electrochemical properties of the newly synthesized IQ-capped SMDs based on the CV experiments alongside the benchmark Y6 NFA. For insights into the redox behavior and energy levels estimation of the synthesized SMDs, CV measurements were carried out in anhydrous dichloromethane (5 mL) containing 2 mM of the analyte and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte, at a scan rate of 50 mV s⁻¹. The solutions were purged with dry N₂ for 5 min prior to measurements. Oxidation (E^onset_ox) and reduction (E^onset_red) onsets were referenced to the ferrocene/ferrocenium redox couple (Fc/Fc⁺), assuming an absolute HOMO of 4.88 eV for Fc/Fc⁺. CV profiles and an energy-level diagrams comparing SMDs to Y6 are shown in Fig. 1a–d. Whereas, the CV profiles of the SMDs with the labelled oxidation and reduction potential values are given in the SI, Fig. S10a–c.

Table 1 Electronic structure parameters and energy-driving potential of designed DPP-, BT-, and TT-based donor molecules when blended against the benchmark Y6 acceptor. The V_OC and driving-force offsets for Y6 acceptor were evaluated against polymer donor PM6⁴¹

Molecule	E H (eV)	E L (eV)	E ^CVg ^b (eV)	E ^optg ^c (eV)	V OC (V)	ΔEL–L^e (eV)	ΔEH–H^e (eV)
a HOMO and LUMO energy level (EH and EL, respectively) as determined by the oxidation (E^onsetox) and reduction onsets (E^onsetred) by CV via relation EHOMO = −(E^onsetox − E^onsetFc + 4.8) eV and ELUMO = −(E^onsetred − E^onsetFc + 4.8) eV. b Energy bandgap (E^CVg) determined from the CV calculations via relation E^CVg = EH − EL. c Energy bandgap (E^optg) determined by the onset spectra. d Open-circuit voltage (VOC) evaluated via Scharber relation as where EL is LUMO of acceptor, EH is HOMO level of donor, e is the elementary charge, and 0.3 V is an empirical factor for charge separation. e ΔEL–L is the energy difference between EL of acceptor to EL of the donor, while ΔEH–H is the energy difference from EH of acceptor to EH of the donor.
DPP-Th-IQ	−5.35	−3.70	1.65	1.71	0.95	0.4	0.35
BT-Th-IQ	−5.13	−3.35	1.78	1.85	0.73	0.75	0.57
TT-IQ	−5.30	−3.44	1.86	1.96	0.9	0.66	0.4
Y6	−5.65	−4.10	1.60	1.51	1.1	0.54	0.2

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Fig. 1 Cyclic voltammograms of SMDs measured in anhydrous dichloromethane (5 mL) containing 2 mM of the analyte and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte, at a scan rate of 50 mV s⁻¹. (b) Energy levels diagram representing HOMO/LUMO energy levels of donor (H_D/L_D) and HOMO/LUMO energy levels acceptor (H_A/L_A) for feasible charge transfer mechanism in photoactive configuration. (c) Energy levels diagram and (d) estimated open-circuit voltage (V_OC) based on Scharber relation with respective driving-force offsets of the SMDs against benchmark Y6 NFA.

From the CV results, we inferred that DPP-Th-IQ exhibited an oxidation onset (E_ox) of approximately 1.05 V and a reduction onset (E_red) of −0.67 V, leading to a calculated HOMO energy (E_H) of −5.35 eV and LUMO energy (E_L) of −3.70 eV. The electrochemical bandgap (E^CV_g) was thus 1.65 eV, which is almost consistent with optical bandgap (E^opt_g) of 1.71 eV, TT-IQ showed a slightly lower oxidation potential but a more negative reduction potential of −0.87 V, indicative of the electron-rich thieno[3,2-b]thiophene core's influence on the redox behavior. The E_H and E_L were determined to be −5.30 eV and −3.44 eV, respectively, and the E^CV_g was observed as 1.86 eV. Whereas BT-Th-IQ, incorporating the electron rich BT fragment, exhibited the lowest oxidation onset at 0.91 V and a reduction onset of −0.95 V, resulting in E_H of −5.13 eV and E_L of −3.35 eV. While the E^CV_g of BT-Th-IQ SMD was estimated as 1.78 eV, almost consistent with the E^opt_g 1.85 eV based on the onset absorption spectra (Table 1). Small disparity in bandgaps reflect intra- and intermolecular interactions that shift the absorption edge in thin films. Overall, the deeper E_H (around −5.3 to −5.4 eV) of DPP-Th-IQ and TT-IQ suggest a higher attainable V_OC compared to BT-Th-IQ, yielding a shallower E_H (−5.13 eV) which could reduce the maximum achievable voltage in cell device. Meanwhile, all three SMDs have E_L well above that of Y6 (−4.1 eV),⁴¹ ensuring a sufficient offset for exciton dissociation at D/A interface as shown in Fig. 1c and d.

The V_OC was estimated by pairing the SMDs with benchmark Y6 acceptor based on the Scharber relation, which incorporates the donor's E_H and the acceptor's E_L along with an empirical offset of 0.3 V for charge separation. Under these assumptions, DPP-Th-IQ and TT-IQ both resulted in relatively high predicted V_OC values of 0.95 and 0.90 V, respectively. By contrast, BT-Th-IQ exhibited a lower estimated V_OC of 0.73 V, owing to its upshifted HOMO and comparatively smaller HOMO–LUMO offset with Y6. Efficient exciton dissociation in OSCs typically requires a minimum energy offset of ∼0.2–0.3 eV between the donor and acceptor LUMOs (ΔE_L–L) and often an analogous offset in HOMOs (ΔE_H–H).⁴² Here, all three SMDs demonstrated sufficient LUMO energy offsets with Y6 NFA, such as DPP-Th-IQ, BT-Th-IQ, and TT-IQ showcased the offsets of 0.40 eV, 0.75 eV, and 0.66 eV, respectively. Although a larger offset favors electron transfer, it can also lead to energy losses that lower the V_OC as demonstrated in interfacial CT mechanism in Fig. 1b. This trade-off is exemplified by BT-Th-IQ, which has the largest ΔE_L–L of 0.75 eV but the lowest predicted V_OC value of 0.73 V. In contrast, DPP-Th-IQ exhibited a balanced approach maintaining a sufficient LUMO offset of 0.40 eV for electron transfer while preserving a relatively high potential for V_OC. From device engineering perspective, the ideal D/A pair balances a minimal energy offset to reduce voltage losses with sufficient offset to drive exciton dissociation and minimize charge recombination. Thus, while BT-Th-IQ can provide robust electron-driving potential, its upshifted HOMO level poses a challenge for maximizing V_OC. Conversely, DPP-Th-IQ and TT-IQ exhibited deeper HOMOs with adequate LUMO offsets, indicating promise for higher V_OC alongside efficient charge generation.

3.3. Electronic structure properties

To elucidate how molecular geometry and electronic configuration influence the performance of the synthesized IQ-capped SMDs, DFT calculations were performed at B3LYP/6-31G(d,p) level of theory. Fig. 2a presents the optimized geometry, highlighting the dihedral angles between the IQ end-groups, any bridging thiophene rings, and the central cores. In DPP-Th-IQ, the IQ-thiophene and thiophene-DPP core dihedral angles are 25.2° and 11.1°, respectively. In BT-Th-IQ, the IQ-thiophene dihedral measures 25.7°, whereas the thiophene–BT core dihedral is 8.5°. Larger angels between side IQs and thiophene bridges are due to steric repulsions among neighboring hydrogen atoms of fused benzene IQ units and thiophenes. The TT-IQ, which lacks bridging thiophene rings, exhibited IQ-TT core dihedral of 26.1°. Overall, the SMDs exhibited quasi-coplanar arrangements which can partially reduce π-conjugation but geometry is sufficient for effective ICT and electronic delocalization.

Fig. 2 (a) Optimized molecular geometries, (b) molecular electrostatic potential (MESP) surfaces, (c) frontier molecular orbitals delocalization, and (d) partial DOS (PDOS) graphs based on core, bridging, and end-capped fragments in the SMDs simulated by B3LYP/6-31G(d,p) method.

Molecular electrostatic potential (MESP) maps given in Fig. 2b further revealed distinct regions of positive (blue) and negative (red) potential, corresponding to electron-deficient and electron-rich moieties, respectively.⁴³ Across the three SMDs, the electronegative atoms (nitrogen in IQ and sulfur in the core/thiophene) tend to attract electron density to produce negative potential localized pockets. Simultaneously, the alkyl-substituted fragments and less electronegative regions appeared as positive potential zones. This spatial variation in electrostatic potential not only corroborates the partial donor–acceptor character inherent to each molecule but also rationalize the observed CT pathways. Regions exhibiting strongly negative potential often overlap with segments that accept electron density in the LUMO, whereas areas of positive potential align with electron-rich sites in the HOMO and provide a complementary perspective on how holes/electrons may be distributed/transferred upon excitation.

Furthermore, the FMOs analysis based on delocalization of HOMOs and LUMOs given in Fig. 2c and PDOS graphs based on Mulliken analysis in Fig. 2d showed that the HOMOs of DPP-Th-IQ and TT-IQ delocalize on electron-rich cores (≈58% for DPP-Th-IQ and ≈52% for TT-IQ). Whereas, the BT-Th-IQ's HOMO is more evenly distributed between the core (≈25%) and thiophenes (≈49%). Such a broad HOMO delocalization can improve hole mobility, an essential feature for efficient donors in OSCs.^44,45 In contrast, the LUMOs of the SMDs exhibited varied localization. BT-Th-IQ's LUMO (≈68%) on the core reflects the strong electron-withdrawing character of benzothiadiazole, while TT-IQ's LUMO lies predominantly on the IQ end-groups (≈80%). DPP-Th-IQ displayed a more balanced LUMO distribution across the central DPP core, bridging thiophenes, and IQ terminal. These results are summarized in SI, Table S1. Although torsional angles introduce partial breaks in π-conjugation, the inherent donor–acceptor motif promotes a robust ICT pathway, as evidenced by the even frontier-orbital delocalization and PDOS patterns. Moreover, comparing the DFT-derived HOMO–LUMO gaps with experimentally measured electrochemical bandgaps (Table 1) shows consistent trends and validates the computational approach. Slight deviations are attributable to inherent DFT approximations and excitonic effects in the solid state. Overall, these findings underscore that a careful balance of backbone planarity, donor–acceptor segmentation, and electron-rich bridging motifs can be harnessed to fine-tune HOMO–LUMO energy levels, thereby enhancing both charge transport and device performance in OSCs.

3.4. Optical properties and solvatochromism

Optical characteristics of synthesized SMDs were evaluated to assess their suitability in OSCs. Table 3 summarizes the experimental absorption and emission spectral characteristics, along with respective theoretical parameters derived from TD-DFT calculations. Steady state optical spectroscopy was employed to examine the SMDs in dilute chloroform solutions (2.5 μg mL⁻¹). The UV-vis spectra of the representative samples are shown in Fig. 3a, with the absorption covering the spectral range of 500 nm, 570 nm and 740 nm. The latter sample exhibited extended absorption and broader spectrum compared to its counterparts. Whilst TT-IQ yielded an absorption peak value at 435 nm, BT-Th-IQ displayed absorption peaks at 386 nm and 480 nm.

Fig. 3 (a) Normalized absorption (b) normalized emission spectra upon optical excitation at 390 nm, 415 nm and 385 nm for DPP-Th-IQ, BT-Th-IQ and TT-IQ respectively in chloroform (2.5 μg mL⁻¹) solution (c) thin-films absorption spectra profile of the synthesized SMDs.

DPP-Th-IQ exhibited the most red-shifted absorption maxima (λ^abs_max) in solution at 630 nm with an additional peak at 400 nm. The 630 nm peak reflects significant ICT attributed to its DPP core and extended conjugation through bridging thiophenes. By contrast, λ^abs_max in thin-film revealed a notable red shift to 710 nm, indicative of altered intermolecular interactions or π–π stacking in the solid state as shown in Fig. 3c. Whereas BT-Th-IQ SMD showed a most pronounced gap between its solution λ^abs_max 386 nm and thin-film λ^abs_max at 515 nm, underscoring the strongest influence of molecular packing on these materials.

The PL spectra of the SMDs were measured in dilute chloroform solutions and given in Fig. 3b. DPP-Th-IQ exhibited the most red-shifted emissions maxima λ^emi_max at 664 nm due to enhanced ICT.⁴⁶ Whereas, BT-Th-IQ and TT-IQ showed consistent emission maxima at 493 nm and 495 nm respectively. In Table 2 we further show the respective Stokes shifts (λ_st) for the SMDs calculated based on λ^abs_max and λ^emi_max differences. A relatively small λ_st of 34 nm in DPP-Th-IQ suggests minimal structural rearrangement in the excited state which is indicative that the chromophore can reduce the non-radiative energy losses and favor higher photocurrent generation.^47–50 Whereas, the large λ_st of 107 nm suggests a substantial reorganization of the BT-Th-IQ framework upon photoexcitation, which may offer avenues for exciton management. Meanwhile, TT-IQ SMD, featuring a TT core having λ^abs_max at 435 nm and λ^emi_max at 495 nm in chloroform with respective λ_st of 60 nm has a lower spectral coverage, suggesting that strategic combination with complementary acceptors could exploit TT-IQ's slightly higher energy gap to maximize overall solar absorption. The photoexcitation-states based charge transfer is further discussed in the subsequent sections.

Table 2 Optical characteristics of the synthesized DPP-Th-IQ, BT-Th-IQ, and TT-IQ SMDs

Molecule	Experimental				Theoretical
	λ ^abssol. ^a (nm)	λ ^absfil. ^a (nm)	λ ^emisol. ^a (nm)	λ st (nm)	λ ^abstheo. ^c (nm)	E X (eV)	μ tr (a.u.)	f	C.I.^e
a Wavelengths of the absorption/emission maxima (λ^abssol./λ^emisol.) in chloroform solutions (2.5 μg mL^−1) and in film (λ^emifil.) at wavelengths of 410 nm. b Stokes shift (λst) calculated from absorption and emission maxima difference. c Theoretically calculated absorption maxima (λ^abstheo.). d S0–S1 state excitation energy (EX), transition electric dipole moment (μtr), and oscillatory strength (f). e Orbitals charge transfer configuration interactions (C.I.), simulated by TD-DFT/CAM-B3LYP/6-31G(d,p) method.
DPP-Th-IQ	630	710	664	34	543	2.27	28.1	1.57	H → L (+68%)
									H → L+2 (+11%)
BT-Th-IQ	386	515	493	107	503	2.46	31.5	1.87	H → L (+67%)
									H−4 → L (+10%)
TT-IQ	435	460	495	60	388	3.19	38.5	2.85	H → L (+63%)
									H−3 → L+1 (+20%)

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Solvatochromic effect was investigated by recording UV-vis absorption and photoluminescence spectra in chlorobenzene, tetrahydrofuran, toluene, and methanol. The spectra are given in SI, Fig. S11 and detailed in Tables S2–S4. Among the synthesized SMDs, DPP-Th-IQ displayed the largest bathochromic response, with λ^abs_max shifting from 598–610 nm in non-polar media to 660 nm in methanol and λ^emi_max red-shifting from 660–665 nm to 800 nm with λ_st ≈ 140 nm. The pronounced shift highlights strong ICT stabilization in polar environments.^51,52BT-Th-IQ showed smaller absorption changes, such as from 380–387 nm to ∼513 nm in methanol. Yet a sizable PL red-shift, which resulted in λ_st in the range 92–133 nm, indicated the polarity-induced excited-state reorganization. Whereas, TT-IQ SMD exhibited minimal solvent-dependent shifts in λ^abs_max (425–430 nm), yet its λ^emi_max exhibited considerable sensitivity, shifting from 485–490 nm in nonpolar solvents to 550 nm in methanol. This resulted in a significant increase in λ_st from about 57–65 nm in lower polarity solvents to up to 121 nm in highly polar methanol. The combined trends confirm that solvent polarity predominantly tunes excited-state relaxation, with DPP-Th-IQ experiencing the greatest ICT-driven stabilization.

Additionally, the TD-DFT simulations with CAM-B3LYP/6-31G(d,p) method were carried out and given in Table 2. Computational results support the experimental findings by predicting λ^abs_theo values that mirror the observed red-blue absorption trends. Specifically, DPP-Th-IQ and BT-Th-IQ reveal relatively low-energy electronic transitions at 543 nm and 503 nm, respectively, while TT-IQ appears more blue-shifted at 388 nm. The S₀–S₁ excitation energy (E_X), transition dipole moment (μ_tr), and oscillator strength (f) offer additional insights into each SMD's propensity for strong light absorption and effective exciton generation which is a critical parameter for high photocurrent in OSC. Notably, TT-IQ exhibits the largest μ_tr (38.5 a.u.) and highest f (2.85), underscoring a particularly intense transition. Molecular orbitals configuration interaction (C.I.) analyses further confirmed that these prominent absorptions arise primarily from the HOMO → LUMO transitions, supplemented by minor contributions from nearby orbitals such as HOMO → LUMO+2 in DPP-Th-IQ and HOMO−4 → LUMO in BT-Th-IQ. Overall, these photophysical characteristics highlight the importance of structural tuning in SMDs. DPP-Th-IQ leverages a strongly red-shifted profile that can capture lower-energy photons, BT-Th-IQ gains considerable absorption enhancement in the solid state, and TT-IQ offers robust transition strengths and moderate film-phase shifts.

Based on the above experimental and computational optoelectronic analysis, the synthesized SMDs showcase varied trends in molecular interactions, processing conditions, and CT behavior. For instance, the DPP-Th-IQ SMD, with its highly sensitive ICT response and red-shifted absorption, suggests strong D/A interactions in BHJ blends. Its ability to maintain broad absorption in various solvent environments makes it an excellent candidate for low-bandgap OSCs. BT-Th-IQ, showing high film-phase aggregation and moderate solvatochromic effects, suggests potential as a highly tunable donor, where solvent engineering can be leveraged to optimize film morphology and charge mobility. TT-IQ, with high oscillator strength and moderate solvatochromic shifts, could serve as a complementary donor in multi-component blends, where its higher transition dipole moment may enhance CT at D/A interfaces.

3.5. Thermal and photostability

The photostability of the IQ-based SMDs was assessed in the thin-film state. Thin films were prepared by spin-coating (0.1 mg mL⁻¹) chlorobenzene solutions onto pre-cleaned quartz substrates (1 × 1 cm²) at 2000 rpm for 60 s, followed by thermal annealing at 100 °C for 20 minutes under vacuum to remove residual solvent and enhance molecular ordering. The films were then exposed to continuous AM 1.5G solar irradiation (100 mW cm⁻²) for 30 h under ambient conditions. Photo degradation was monitored by UV-vis absorption spectroscopy at regular intervals. The evolution of normalized absorbance over time is presented in SI, Fig. S13a–c, with comparative stability data summarized in Table S5. All SMDs demonstrated excellent photo stability up to 90% retained absorbance after exposure for 30 hours.

To investigate thermal behavior and molecular interactions, temperature-dependent UV-vis absorption measurements were conducted in chlorobenzene (0.1 mg mL⁻¹) over the range of 20 °C to 120 °C. Spectra are shown in SI, Fig. S13d–f. DPP-Th-IQ and BT-Th-IQ showed minimal bathochromic shifts and modest intensity changes with temperature which indicates stable π–π interactions/aggregation states that persist upon heating. In contrast, TT-IQ displayed more noticeable intensity variations at elevated temperature, pointing to comparatively weaker intermolecular interactions. Thermal stability was further evaluated by TGA under nitrogen atmosphere at a heating rate of 10 °C min⁻¹. As shown in the TGA profiles in SI, Fig. S14, the 5% mass-loss temperatures (T₅%) exceeded 250 °C for all SMDs and were highest for BT-Th-IQ ∼410 °C and DPP-Th-IQ ∼400 °C, with TT-IQ the lowest ∼270 °C. The high decomposition thresholds of DPP-Th-IQ and BT-Th-IQ define comfortable processing windows and support their robustness during solution deposition and post-deposition annealing.

3.6. Reorganization energy and solvation free energy

Charge transport in OSCs is governed by the ability of the donor and acceptor molecules to efficiently facilitate charge hopping. This efficiency is quantified by the reorganization energy, which represents the energy associated with structural relaxation upon CT.^53,54 Herein, λ_hole and λ_ele were calculated for the SMDs and the Y6 NFA via B3LYP/6-31G(d,p) method to determine hole and electron mobility, respectively. The results are summarized in Table 3.

Table 3 Hole/electron reorganization energy (λ_hole/λ_ele) calculated by B3LYP/6-31G(d,p) method and dipole moment in ground-state (μ_g) and excited-state (μ_e) with respective Gibbs solvation-free energy (ΔG_solv) in chloroform solvent calculated using M06-2X/6-31G(d,p) method via SMD solvation model

Molecule	λ hole (eV)	λ ele (eV)	μ g (Debye)	μ e (Debye)	Δμ (Debye)	ΔGsolv (kcal mol^−1)
DPP-Th-IQ	0.288	0.213	2.55	3.22	0.67	−40.84
BT-Th-IQ	0.255	0.237	3.43	4.47	1.04	−37.94
TT-IQ	0.258	0.241	2.44	3.09	0.67	−31.92
Y6	0.160	0.150	1.07	1.89	0.82	−14.61

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Among the SMDs, BT-Th-IQ exhibited the lowest λ_hole (0.255 eV), followed closely by TT-IQ (0.258 eV), and DPP-Th-IQ (0.288 eV). The relatively low λ_hole of BT-Th-IQ suggests that it possesses the most favorable hole transport properties among the three donors, as a lower hole reorganization energy reduces charge-trapping losses and enhances carrier mobility. The BT core likely contributes to this reduced reorganization energy by minimizing geometric relaxation upon oxidation, leading to more efficient hole transport. In contrast, DPP-Th-IQ exhibited the highest λ_hole (0.288 eV), suggesting that it experiences greater structural distortion upon hole injection, potentially due to localized charge distribution in the oxidized state. For electron transport, DPP-Th-IQ exhibited the lowest λ_elec (0.213 eV), which is favorable for robust electron transport due to the electron-deficient nature of the DPP core, which stabilizes the negative charge upon reduction and reduces the structural reorganization for CT. On the other hand, BT-Th-IQ and TT-IQ have slightly higher λ_ele (0.237 eV and 0.241 eV, respectively), indicating marginally less electron transport efficiency than DPP-Th-IQ. Moreover, for efficient charge separation and transport in photoactive layer, the reorganization energies of donors and acceptors should be well balanced.^55,56 The benchmark NFA Y6 exhibits significantly lower λ_hole (0.160 eV) and λ_ele (0.150 eV), reinforcing its role as an efficient charge transporter. The lower reorganization energy of Y6 confirms that it can readily accept electrons from SMDs, minimizing energy loss during charge separation and facilitating long-lived charge carriers in photoactive layer. Comparatively the SMDs have higher λ_hole values than Y6, suggesting that hole transport is slightly less efficient than electron transport. However, the relatively small gap between λ_hole and λ_ele in the three SMDs suggests that these molecules can still achieve balanced charge transport, an important factor in reducing charge recombination and increasing FF in OSCs. These results show that the IQ-capped molecules are excellent candidates for high-performance OSCs, particularly when paired with low-reorganization-energy acceptors like Y6.

Additionally, the molecular polarity, charge redistribution, and solubility are the other key parameters which are influenced by the dipole moment and solvation free energy and greatly affect film morphology and charge transport in OSCs.^43,57 Herein, the dipole moment and Gibbs solvation-free energy (ΔG_solv) were further computed via M06-2X/6-31G(d,p) method in chloroform solvent. Results given in Table 3 show that among SMDs, BT-Th-IQ exhibited the highest ground-state dipole moment (μ_g = 3.43 D) and excited-state dipole moment (μ_e = 4.47 D), with the largest dipole moment variation (Δμ = 1.04 D). This indicates strong charge redistribution upon excitation, consistent with its high CT character. DPP-Th-IQ and TT-IQ exhibited lower dipole moment variation (Δμ = 0.67 D), suggesting moderate ICT contributions, while Y6 displays a dipole moment variation (Δμ = 0.82 D), indicating a moderate degree of charge separation. Whereas, DPP-Th-IQ exhibited the most negative ΔG_solv (−40.84 kcal mol⁻¹), followed by BT-Th-IQ (−37.94 kcal mol⁻¹) and TT-IQ (−31.92 kcal mol⁻¹), indicating that DPP-Th-IQ has the highest solubility in chloroform, which may aid in better thin-film formation.

3.7. Intramolecular charge transfer and transition density analysis

ICT and exciton dissociation play a pivotal role in the performance of the OSCs.⁵⁴ To comprehend the photoexcitation dynamics of the synthesized SMDs, a detailed electron transition density analysis was performed based on TD-DFT/CAM-B3LYP/6-31G(d,p) calculations in MultiWfn 3.8. The analysis includes key electron transition properties such as CT distance (D_CT), H_index, t_index, intrinsic CT excitation, intrinsic local excitation (LE), transferred charge (q_CT), hole/electron delocalization indices (HDI/EDI), and exciton binding energy (E_B), as summarized in Table 4.^21,29 To visualize the charge distribution, electron density difference (EDD) contours, hole/electron heat maps, and transition density matrix (TDM) heat maps were also generated and given in Fig. 4.

Table 4 Electron transition density properties of the designed SMDs including charge transfer distance (D_CT), H_index, t_index, intrinsic charge transfer (CT) excitation, intrinsic local excitation (LE), transferred charge (q_CT), hole/electron delocalization index (HDI/EDI), and exciton binding energy (E_B) simulated by TD-DFT/CAM-B3LYP method

Molecule	D CT (Å)	H index (Å)	t index (Å)	CT (%)	q CT (e^−)	LE (%)	HDI	EDI	E B (eV)
DPP-Th-IQ	0.06	4.59	−0.88	57.75	0.38	42.25	5.78	5.68	0.41
BT-Th-IQ	0.87	4.84	−0.98	65.22	0.48	34.77	5.00	5.70	0.31
TT-IQ	0.11	6.18	−0.76	48.95	0.43	51.05	4.83	4.20	0.44

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Fig. 4 (a) Electron density difference (EDD) surface contours, (b) hole/electron heat maps with percentage delocalization and spatial overlap within core units, thiophene bridges, and end-group IQs, (c) and transition density matrix (TDM) heat maps for atoms and molecular fragments such as core units (1), thiophene bridges (2), and side IQs units (3), simulated via TD-DFT/CAM-B3LYP method.

As shown in the EDD graphs showcasing holes (cyan) and electrons (blue) delocalization in Fig. 4a and respective heat maps in Fig. 4b, the holes (59.1%) and electrons (50.8%) are predominantly localized on the DPP core, with moderate contributions from the thiophene bridges (29.5% for holes, 36.5% for electrons) and minimal distribution in the side IQ units (11.4% for holes, 12.6% for electrons) in DPP-Th-IQ SMD. This suggests strong electronic coupling between the DPP core and thiophene bridges, which enhances charge delocalization. In BT-Th-IQ, electrons are mostly concentrated on the BT core (66.2%), whereas holes are mainly distributed over the thiophene bridges (51.4%), indicating a well-defined charge-separated state that facilitates charge extraction. In TT-IQ, holes (46.7%) and electrons (34.1%) are more evenly distributed across the molecular skeleton, particularly between the TT core and IQ end-groups. This widespread delocalization enhances charge transport but may also increase hole/electron recombination. TDM heat maps in Fig. 4c further illustrate the extent of hole/electron spatial overlap within different molecular segments. Regular hole and electron generation patterns observed across all three SMDs indicate efficient charge separation, particularly in BT-Th-IQ and DPP-Th-IQ. These findings suggest that all three donors exhibited well-defined ICT characteristics, which are favorable for charge transport and exciton dissociation in OSCs.

D _CT provides a measure of the spatial charge separation in the excited state, influencing exciton dissociation efficiency. Among the three SMDs, BT-Th-IQ exhibited the largest D_CT (0.87 Å), followed by TT-IQ (0.11 Å) and DPP-Th-IQ (0.06 Å), indicating that BT-Th-IQ exhibits the most significant charge separation upon photoexcitation. A higher D_CT value suggests stronger charge separation, which is beneficial for reducing recombination and promoting efficient charge extraction in OSCs. Conversely, DPP-Th-IQ shows a lower D_CT, suggesting a more localized excitonic state. CT and LE components further characterize the nature of the electronic transitions. BT-Th-IQ exhibits the highest CT contribution (65.2%), indicating a highly polarized charge distribution that favors efficient charge transfer. In comparison, DPP-Th-IQ (57.7%) and TT-IQ (48.9%) exhibit lower CT contributions, with TT-IQ displaying the highest LE component (51.1%), suggesting a more localized charge distribution. These findings confirm that BT-Th-IQ is the most promising donor for achieving long-range charge separation, while TT-IQ, with its higher LE component, may rely more on intermolecular interactions to facilitate charge transport.

The parameter q_CT further provides quantitative measure of charge redistribution in the excited state. From the results, BT-Th-IQ exhibited the highest q_CT (0.48e⁻), followed by TT-IQ (0.43e⁻) and DPP-Th-IQ (0.38e⁻). The larger q_CT of BT-Th-IQ confirms strong ICT behavior, which is beneficial for reducing energy loss during charge transport. HDI and EDI further indicate the extent of charge delocalization across the molecular framework. DPP-Th-IQ SMD having HDI/EDI of 5.78/5.68 and BT-Th-IQ with HDI/EDI 5.00/5.70 exhibited widespread charge delocalization, facilitating charge hopping and improving charge mobility. Whereas, TT-IQ, with lower HDI/EDI of 4.83/4.20, suggests a more localized charge transport pathway. Furthermore, the BT-Th-IQ SMD exhibited the lowest E_B (0.31 eV), followed by DPP-Th-IQ (0.41 eV) and TT-IQ (0.44 eV). The reduced E_B in BT-Th-IQ is attributed to its high CT character and significant charge transfer distance, further reinforcing its higher charge transport characteristics. The slightly higher E_B in TT-IQ suggests a more tightly bound excitonic state, which may require an optimized D/A interface to enhance charge dissociation.

3.8. Donor/acceptor interfacial charge transfer properties

Efficient interfacial CT at the D/A interface plays a pivotal role in determining the performance of OSCs, as it critically affects CT state dissociation and free-charge generation.⁵⁸ To explore CT characteristics, the interfacial packing, excited-state configurations, and CT mechanisms of complexes formed between the synthesized SMDs and the benchmark Y6 NFA were studied. Initially, optimized face-on orientation configurations of D/A complexes were constructed with an initial donor–acceptor separation of 3.5 Å. Geometry optimization was carried out at the B3LYP/6-31G(d,p) level, followed by TD-DFT calculations with the RSH functional CAM-B3LYP and the CT analysis based on EDD method via MultiWfn 3.8.^59,60

The optimized D/A complex geometries given in Fig. 5a and b, demonstrated distinct intermolecular arrangements for each donor when combined with Y6. All three complexes DPP-Th-IQ/Y6, BT-Th-IQ/Y6, and TT-IQ/Y6 exhibited a preferred face-on π–π stacking orientation, facilitating strong intermolecular electronic interactions. Notably, the TT-IQ/Y6 complex displayed the largest intermolecular π–π stacking area, indicative of extensive D/A interfacial overlap. However, despite the smaller stacking areas, the DPP-Th-IQ/Y6 and BT-Th-IQ/Y6 complexes exhibit significantly shorter centroid-to-centroid distances of 5.13 Å and 5.22 Å, respectively, compared to TT-IQ/Y6 at 6.27 Å. These shorter distances imply stronger intermolecular interactions and closer packing arrangements driven by the favorable electronic coupling of the DPP and BT cores with Y6. Further structural dimensions were measured to analyze the molecular packing in D/A complexes. As shown in Fig. 5c, the lengths of the DPP-Th-IQ/Y6, BT-Th-IQ/Y6, and TT-IQ/Y6 complexes are 33.9 Å, 34.8 Å, and 29.7 Å, respectively, with widths of 17.8 Å, 17.6 Å, and 16.3 Å, and heights of 11.7 Å, 10.1 Å, and 8.1 Å, respectively. Side-view packing configurations and dimensional data revealed well-defined and stable intermolecular stacking patterns as characterized by parallel alignment and slightly curved “half-bowl” conformations.

Fig. 5 (a) and (b) Top and side views of the optimized face-on D/A complexes (DPP-Th-IQ/Y6, BT-Th-IQ/Y6, and TT-IQ/Y6) obtained at the DFT/B3LYP/6-31G(d,p) level, illustrating the π–π stacking orientation and centroid-to-centroid distances. (c) Dimensional analysis of length, width, and height of each simulated D/A complex. (d) Low-lying excited-state properties (S₁–S₃) derived from TD-DFT/CAM-B3LYP/6-31G(d,p) calculations based on EDD method, highlighting excitation energies, oscillator strengths (green), transferred charge (blue), charge transfer distance index (red), and CT excitation percentages (black) for each donor/Y6 complex and their respective EDD isosurfaces.

Excited-state vertical transitions and interfacial CT behaviors for the lowest three excited states (S₁–S₃) were thoroughly evaluated for the D/A complexes and described in Fig. 5d. The EDD method allows clear visualization of electron transition processes as “hole → electron” distributions, categorizing excited states into Frenkel excitons (localized) and pure/hybrid CT states (electron and hole spatially separated across donor and acceptor). For the DPP-Th-IQ/Y6 complex, the lowest-energy excited state (S₁ at 2.11 eV, f = 1.2) shows moderate charge transfer (CT ≈ 18%, q_CT = 0.53e⁻, D_index = 2.88 Å), predominantly acceptor-localized with ≈90% hole/electron localization on Y6. However, higher-energy states such as S₂ (2.35 eV, 56% CT, 4.73 Å D_index) and S₃ (2.56 eV, 46% CT, 4.36 Å D_index) exhibit increased charge separation, suggesting an enhanced likelihood of exciton dissociation at elevated excitation energies. EDD contours confirm this, demonstrating progressively stronger donor-involved CT contributions at these higher states. In contrast, the S₁ excitation in TT-IQ/Y6 at 2.18 and subsequent excited state S₂ at 2.60 eV shows minimal charge separation (0–2.5% CT), indicating the presence of localized Frenkel-type excitations at lowest and intermediate states. However, the third excited state S₃ at 3.0 eV recovers robust CT characteristics (97% CT, q_CT = 0.94e⁻, D_index = 4.63 Å). These findings confirm that TT-IQ/Y6 also presents suitable excited-state charge-transfer dynamics conducive to efficient exciton dissociation.

Overall, the interfacial mechanistic analysis suggests that among the modelled SMDs/Y6 complexes, BT-Th-IQ/Y6 exhibits the high CT percentages, substantial transferred charges, and significant spatial electron–hole separation distances, all beneficial for promoting efficient CT state dissociation and charge generation. DPP-Th-IQ/Y6, despite its smaller π–π stacking distance, efficiently achieves notable CT character and robust excited-state interactions. Whereas, the TT-IQ/Y6 complex, with broader π–π stacking and moderate interfacial CT distances, also demonstrates effective charge separation dynamics. Thus, all three donor complexes demonstrate favorable excited-state and structural characteristics at the molecular interface with the Y6 acceptor, highlighting their potential in the design and fabrication of future ASM-OSCs.

4. Conclusions

In this work, three SMDs named DPP-Th-IQ, BT-Th-IQ, and TT-IQ, were successfully designed, synthesized, and characterized for potential application in next-generation ASM-OSCs. Incorporating easily synthesized IQ-terminal moieties, combined with electron-deficient diketopyrrolopyrrole and benzothiadiazole cores, as well as an electron-rich thienothiophene core, we systematically explored the impact of structural motifs on their electronic, optical, charge transport, interfacial charge transfer properties and molecular stability. The strategically designed SMDs featuring a streamlined cost-efficient synthesis is beneficial for scalability, paving the way for potential commercial applications in future ASM-OSCs.

Electrochemical and DFT calculations revealed well-matched HOMO–LUMO offsets with the Y6 acceptor, indicating sufficient driving force for exciton dissociation into free charges. Notably, DPP-Th-IQ exhibited significantly improved optical properties, including redshift in λ^abs_max (630 nm) and λ^abs_emi (664 nm), the smallest electrochemical bandgap (1.65 eV), offering extended absorption in the visible–near-infrared range, in addition to improved solvation properties (ΔG_solv = −40.84 kcal mol⁻¹), which can facilitate solution-processing and film formation. Moreover, the lowest electron reorganization energy (0.213 eV) of DPP-Th-IQ, highlighting its capacity for swift electron transfer. Meanwhile, BT-Th-IQ demonstrated the highest charge transfer character (CT = 65.22%) and the longest charge transfer distance (D_CT = 0.87 Å), accompanied with the lowest exciton binding energy (0.31 eV) and low hole reorganization energy (0.255 eV) for improved hole mobility positioning it as the most promising donor for long-range charge separation. Moreover BT-Th-IQ showed a notable red shift in thin film (386–515 nm) reflecting film-phase interactions to effective charge transport, while TT-IQ exhibited balanced hole/electron delocalization, making it a useful complementary donor in OSC architectures. Whereas, the interfacial analysis of optimized donor/Y6 complexes demonstrated stable face-on π–π stacking configurations with robust D/A interactions. BT-Th-IQ/Y6 complex yielded efficient CT states, underpinning their potential for high J_SC. DPP-Th-IQ/Y6 complex showed enhanced interfacial CT indicating favorable structural attributes for achieving high charge transport mobilities. Although TT-IQ/Y6 demonstrated extensive interfacial stacking, its variable CT efficiency across excitation energies highlighted the need for precise morphological optimization to ensure consistent device performance. Importantly, the materials exhibited encouraging durability. Thin films retained ≥90% of their initial absorbance after 30 h of continuous AM 1.5G irradiation (100 mW cm⁻²), evidencing good photo-stability under operationally relevant conditions. Thermogravimetric analysis indicates T_d, 5% > 250 °C for all donors, underscoring robust thermal stability.

Overall, our findings provide valuable molecular-design insights. Among the three donors, BT-Th-IQ showed the highest intrinsic charge-separation fraction, lowest exciton-binding energy, and the most balanced hole/electron reorganization energies, together with robust face-on π–π stacking with Y6. These traits favor efficient CT state dissociation and balanced carrier transport. DPP-Th-IQ and TT-IQ exhibited complementary strengths, such as deeper HOMO for higher V_OC and symmetric hole delocalization. The present work therefore establishes a detailed atomistic structure–property framework that sets critical benchmarks for subsequent optimization. Full photovoltaic device fabrication and characterization (V_OC, J_SC, fill factor and PCE measurements) are proposed as the logical next step to realize the performance potential of these IQ-capped small-molecule donors in all-small-molecule OSC architectures.

Author contributions

Maryam Javed: synthesis, purification, experimental characterization, formal analysis, investigation, writing – original draft. Waqas Akram: synthesis, experimental characterization, computational methodology, formal analysis, data curation, visualization, investigation, writing – original draft. Zeeshan Ali: experimental methodology, synthesis, characterization. Nabeel Shahzad: software, computational analysis, data curation, visualization. Munazza Shahid: conceptualization, writing – review & editing. Ghayoor Abbas Chotana: resources, software, methodology. Jafar Iqbal Khan: methodology, validation, writing – review & editing. Jie Min: data curation, validation, resources. Muhammad Altaf: methodology, validation, data curation. Christian B. Nielsen: methodology, writing – review & editing. Raja Shahid Ashraf: conceptualization, supervision, resources, methodology, funding accusation, project administration.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information: Detailed experimental procedures for the synthesis of the new SMDs, including characterization data and simulation results supporting the findings in the main manuscript. See DOI: https://doi.org/10.1039/d5tc02318a.

Acknowledgements

We are grateful to the Higher Education Commission Pakistan's NRPU project (17546). We also acknowledge Office of Research and Innovation Commercialization (ORIC) Department, Government College University and Department of Chemistry and Chemical Engineering, Lahore University of Management Sciences for supporting this research.

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