Research Article

Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study

Volume: 2026 Number: 2 September 5, 2026

Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study

Abstract

Isoindigo-based donor–acceptor small molecules are promising candidates for organic photovoltaic applications because of their strong electron-accepting character and tunable π-conjugated structures. In this work, seven isoindigo-based derivatives bearing phenyl, thiophene, thiophene, benzothiophene, fluorene, carbazole, phenoxazine, and julolidine terminal donor units were computationally designed and investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Following a benchmark study, B3PW91/6-31+G(d,p) was selected for the systematic evaluation of structural, electronic, optical, and OPV-related properties. The results indicate that donor modification mainly affects the highest occupied molecular orbital (HOMO) energy, while the lowest unoccupied molecular orbital (LUMO) remains largely localized on the isoindigo acceptor core. Stronger donor units narrowed the electronic gap and induced bathochromic shifts in absorption, but excessive HOMO destabilization decreased the estimated open-circuit voltage from 1.28 to 0.59 V. Among the investigated molecules, the benzothiophene derivative I3 showed the most balanced OPV-related descriptor profile, with λmax = 672.55 nm, f = 1.67, LHE = 0.98, Eb = 0.36 eV, ΔLUMO = 0.64 eV, and an estimated Voc of 1.27 V. In contrast, phenoxazine and julolidine derivatives exhibited more strongly red-shifted absorption maxima at 803 and 838 nm, respectively, but their less favorable voltage-related descriptors and higher exciton binding energies reduced their overall OPV-related descriptor balance. These findings indicate that moderate donor strength is more favorable than excessive electron donation for achieving a balance among absorption, exciton dissociation, and voltage retention in isoindigo-based small-molecule OPV candidates.

Keywords

Ethical Statement

This study does not involve human participants or animals. Therefore, ethical approval was not required.

Thanks

The authors gratefully acknowledge the computational resources provided by TÜBİTAK ULAKBİM High Performance and Grid Computing Center (TRUBA).

References

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Details

Primary Language

English

Subjects

Optical Properties of Materials, Theory and Design of Materials, Physical Organic Chemistry, Computational Chemistry

Journal Section

Research Article

Publication Date

September 5, 2026

Submission Date

May 6, 2026

Acceptance Date

August 21, 2026

Published in Issue

Year 2026 Volume: 2026 Number: 2

APA
Seyitdanlioglu, P., & Kalkan, H. (2026). Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study. Journal of the Turkish Chemical Society Section A: Chemistry, 2026(2), 1-16. https://doi.org/10.18596/jotcsa.1945490
AMA
1.Seyitdanlioglu P, Kalkan H. Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study. JOTCSA. 2026;2026(2):1-16. doi:10.18596/jotcsa.1945490
Chicago
Seyitdanlioglu, Pinar, and Hilal Kalkan. 2026. “Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT TD-DFT Study”. Journal of the Turkish Chemical Society Section A: Chemistry 2026 (2): 1-16. https://doi.org/10.18596/jotcsa.1945490.
EndNote
Seyitdanlioglu P, Kalkan H (September 1, 2026) Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study. Journal of the Turkish Chemical Society Section A: Chemistry 2026 2 1–16.
IEEE
[1]P. Seyitdanlioglu and H. Kalkan, “Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study”, JOTCSA, vol. 2026, no. 2, pp. 1–16, Sept. 2026, doi: 10.18596/jotcsa.1945490.
ISNAD
Seyitdanlioglu, Pinar - Kalkan, Hilal. “Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT TD-DFT Study”. Journal of the Turkish Chemical Society Section A: Chemistry 2026/2 (September 1, 2026): 1-16. https://doi.org/10.18596/jotcsa.1945490.
JAMA
1.Seyitdanlioglu P, Kalkan H. Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study. JOTCSA. 2026;2026:1–16.
MLA
Seyitdanlioglu, Pinar, and Hilal Kalkan. “Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT TD-DFT Study”. Journal of the Turkish Chemical Society Section A: Chemistry, vol. 2026, no. 2, Sept. 2026, pp. 1-16, doi:10.18596/jotcsa.1945490.
Vancouver
1.Pinar Seyitdanlioglu, Hilal Kalkan. Effect of Terminal Donor Units on the Electronic and Photovoltaic Properties of Conjugated Small Molecules: A DFT/TD-DFT Study. JOTCSA. 2026 Sep. 1;2026(2):1-16. doi:10.18596/jotcsa.1945490