Thesis Defense: Dissipative Mechanisms in Organic Light-emitting Diodes: Role of Intramolecular Charge Transfer and Delayed Fluorescence

Organic light-emitting diodes (OLEDs) are emerging to replace conventional lighting technology due to their flexible device structures, multicolour emission, and ease of fabrication.^[1] However, one of the key challenges in developing efficient emitters for OLEDs is overcoming the dissipative channel of triplet excitons.^[2] A common approach to mitigate this challenge is to employ emitter molecules optimized for either thermally activated delayed fluorescence (TADF) or triplet-triplet annihilation (TTA) to enhance the external quantum efficiency (EQE).^[3,4] Another alternative strategy to improve the quantum efficiency is deploying TADF chromophores with fluorescence emitters by recycling dark triplet excitons in a process, namely hyperfluorescence, which also retains narrow emission bandwidth.^[5] Therefore, understanding the intricate photophysics of donor-acceptor (D-A) chromophores by exploiting the intramolecular charge transfer (ICT) state to unravel their multifarious applications attracts widespread attention.[6]

In my thesis, I have rationally designed a series of aromatic imide-based D-A chromophores that display the TADF and TTA phenomenon, where the suitable choice of donor or acceptor governs the dominant delayed emission pathways by manipulating ICT state.^[7,8] Further, by combining a TADF chromophore with a series of diketopyrrolopyrrole-based fluorescence emitters, efficient energy transfer could be facilitated, thereby enhancing the emission intensity of the fluorescence emitters.^[9] Nevertheless, in the quest to design new D-A chromophores, I made a serendipitous observation of stable radical cation formation in carbazole-based diketopyrrolopyrrole derivatives, which offers a plethora of promising applications.^[10] Our in-depth photophysical studies provide insights into the importance of developing new chromophores, which offer myriad applications in optoelectronics.

[1] Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley,D. D. C.; Santos, D. A. D.; Brédas, J. L.; Lögdlund, M.; Salaneck, W. R. Electroluminescence in Conjugated Polymers. Nature1999, 397,121– 128.

[2]Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated DelayedFluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017,29,1605444.

[3] Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-emitting Diodes from Delayed Fluorescence. Nature2012, 492, 234– 238.

[4] Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P. Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters.Adv. Mater.2013,25,3707– 3714.

[5] Nakanotani, H.; Higuchi, T.; Furukawa, T.; Masui, K.; Morimoto, K.; Numata, M.; Tanaka, H.; Sagara, Y.; Yasuda, T.; Adachi, C. High-efficiency Organic Light-Emitting Diodes with Fluorescent Emitters. Nat. Commun.2014, 5, 4016.

[6] Debnath, S.; Mohanty, A.; Naik, P.; Salzner, U.; Dasgupta, J.; Patil, S. Deciphering Intramolecular Charge Transfer in Fluoranthene Derivatives. J. Mater. Chem. C2024, 12, 9200− 9209.

[7] Debnath, S.; Ramkissoon, P.; Vonder Haar, A. L.; Salzner, U.; Smith, T. A.; Musser, A. J.; Patil, S. A Twist in Biphthalimide-Based Chromophores Enables Thermally Activated Delayed Fluorescence. Chem. Mater.2024, 36, 4607− 4615.

[8]Debnath, S.; Ramkissoon, P.; Salzner, U.; Hall, C. R.; Panjwani, N. A.; Kim, W.; Smith, T. A.; Patil. S. Modulation of Delayed Fluorescence Pathways via Rational Molecular Engineering. (Manuscript under revision)

[9] Debnathet al. Hyperfluorescence from Diketopyrrolopyrrole Derivatives. (Manuscript under preparation)

[10]Debnath et al.Enhanced Chemical Stability of Radical Cations in Carbazole-basedDiketopyrrolopyrrole Derivatives. (Manuscript under review)