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Phenothiazine-based TADF emitters with dual conformations for single-molecule white OLEDs
Pavithra V Prabhu1,2, Vibhu Darshan2,3, Indira S Divya1,2
1Chemical Sciences & Technology Division (CSTD), CSIR-National Institute for Interdisciplinary Science & Technology, CSIR-NIIST Thiruvananthapuram Kerala 695019 India joshy@niist.res.in.
This study introduces novel phenothiazine emitters (NTPH and NTPCF) with switchable conformers for advanced optoelectronic applications. These materials exhibit tunable, stimuli-responsive optical properties, including mechanochromism and potential for high-performance organic light-emitting diodes (OLEDs).
Area of Science:
- Materials Science
- Organic Chemistry
- Photophysics
Background:
- Donor-π-acceptor (D-π-A) phenothiazine derivatives are versatile materials for sensors and optoelectronics.
- Molecular conformation significantly influences photophysical properties, leading to tunable material characteristics.
- Stimuli-responsive materials with switchable conformers offer dynamic optical properties.
Purpose of the Study:
- To design and synthesize novel phenothiazine-based D-π-A emitters (NTPH and NTPCF).
- To investigate the structure-property relationships, focusing on conformational changes and their impact on photophysical behavior.
- To explore the potential of these materials in stimuli-responsive applications, including mechanochromism and organic light-emitting diodes (OLEDs).
Main Methods:
- Synthesis of phenothiazine-based D-π-A emitters (NTPH and NTPCF).
- Single-crystal X-ray diffraction analysis to determine molecular conformations (quasi-axial and quasi-equatorial).
- Photophysical characterization (emission spectra, fluorescence maxima) and theoretical studies (singlet-triplet energy gap).
- Investigation of mechano-responsive properties and fabrication of organic light-emitting diodes (OLEDs).
Main Results:
- NTPCF exhibits a quasi-axial conformation with green emission (545 nm), while NTPH shows polymorphism with quasi-axial (cyan emission, 463 nm) and quasi-equatorial conformers (TADF orange emission, 585 nm).
- Low energy barriers (<3 kcal mol⁻¹) allow dynamic conformational flexibility at room temperature, influencing the singlet-triplet energy gap.
- NTPCF demonstrates reversible cyan-to-orange mechanochromism (Δλem ∼ 135 nm), suitable for anticounterfeiting.
- Solution-processed orange OLEDs using NTPCF achieved high luminance (8400 cd m⁻²).
- NTPCF enabled single-molecule white OLEDs with tunable color temperatures and acceptable efficiency (EQE up to 4.25%).
Conclusions:
- Molecular conformation plays a critical role in tuning the photophysical properties and TADF behavior of phenothiazine derivatives.
- The synthesized emitters exhibit significant mechanochromism and are promising candidates for anticounterfeiting applications.
- NTPCF demonstrates potential for high-performance orange OLEDs and tunable white OLEDs, showcasing its multifunctional capabilities.
