Bridged-exciplexes are known to exhibit acceptable TADF performance; however, due to the nature of their formation, the emissive state is typically a charge transfer state, often with mediocre oscillator strength and low quantum yield. We here present anthracene-bridged exciplexes, which provide a large T1 to T2 gap and a locally excited S1 state. There is a range of donor and acceptor strengths, at which the electronic states of the formed exciplex and anthracene moiety can mix. This produces hot excited states, allowing extremely fast RISC to the locally excited state for emission. These hot-exciplex molecules exhibit high PLQY, fast delayed emission, and good EQEs in OLEDs.

Blue Phosphorescent Organic Light-Emitting Didoe (PHOLED) is a sole solution for overcoming deficit electroluminescence efficiency (EL) of blue fluorescent OLEDs. However, low device lifetime of PHOLED has been a key issue that delays its commercial debut. In this work, we demonstrate the unprecedented device lifetime of blue PHOLED which is the longest device lifetime to our best knowledge. Platinum(II) complex-based PHOLED exhibits deep blue emission with Commission International de l’Eclairage (CIE) y-value less than 0.2. Newly designed tetradentate platinum (II) complex played a critical role on improving stability along with host materials which forms intermolecular charge-transfer(CT) complex. Addition of di-tert-butylbenzene to benzimidazole not only prohibits undesired CT complex between host and guest but also enhanced material stability by having high metal centered triplet state(3MC) in the Pt complex. The both host materials of hole transporting and electron transporting materials utilized tert-phenyl-silane as a bulky unit to enhance color purity of PHOLED and material stability of host materials.

Organic light-emitting diode (OLED) has gain numerous attentions since an efficient OLED was firstly demonstrated by Tang and VanSlyke in 1987. Thanks to lots of efforts paid on their progress including materials and device architecture in past three decades, red and green OLEDs have great success in efficiency and lifetime. However, the development of high efficiency deep-blue counterparts with Commission Internationale de L'Eclairage (CIE) coordinate of y<0.1 currently remains in demand in the market of full-color display applications. For example, to realize the BT.2020 color space standard, the standard blue emission must have CIE coordinates of (0.131,0.046), and that is extremely challenging, especially in material development. Here, a new compound consisting of phenyls groups to connect a benzene core was successfully synthesized. The newly obtained compound exhibited a super wide bandgap of 3.5 eV and a deep-blue emission of approximately 397 nm as well as a photoluminescence quantum yield (PLQY) by 68% in thin film. Consequently, a non-doped OLED using the pristine new compound as emitting layer showed a peak efficiency of 4.9% in external quantum efficiency (EQE) and deep-blue emission with CIE coordinates of (0.16, 0.04). Note that the OLED configuration was bottom emission, which meant the such deep-blue emission resulted from the material itself, rather than microcavity effect. Grazing incidence wide-angle X-ray scattering (GIWAXS) of new compound displayed an order parameter (SGIWAXS) of 0.44, indicating molecules primarily aligned horizontally to the substrate, which contributed to the high efficiency.

Organic-inorganic hybrid halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, etc. However, the poor stability is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of organic-inorganic hybrid material - Organic Semiconductor-incorporated Perovskite (OSiP). Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. Furthermore, this rigid conjugated ligand design dramatically enhances materials’ chemical stability and suppresses solid-state ion migration and diffusion, making them promising for real-world applications. Using this stable hybrid materials, we demonstrate the fabrication of high quality polycrystalline thin films and highly stable and efficient LED devices with suppressed ion migration and improved external quantum efficiency, color purity, and operational lifetime. Optically-driven nano lasers will be discussed briefly as well.

We report that highly-efficient large-area PeLEDs with high uniformity can be realized by the use of colloidal perovskite nanocrystals (PNCs), which decouples the crystallization of perovskites from the film-formation process. PNCs are pre-crystallized and surrounded by organic ligands, and thus are not affected by the film formation process, so simple modified-barcoating which facilitates the evaporation of residual solvent provides uniform large-area films. PeLEDs that incorporated the uniform barcoated PNC films achieved external quantum efficiency (EQE) of 23.26% and EQE of 22.5% in a large pixel area of 102 mm2 with high reproducibility.

Superfluorescence is a quantum optical phenomenon in which an initially excited ensemble of incoherent dipoles first acquire macroscopic coherence and then collectively recombine and radiate a burst of photons. This process is a symmetry breaking macroscopic quantum phase transition similar to superconductivity and Bose-Einstein condensation. Since quantum coherence is extremely fragile at high temperatures, similar to other macroscopic quantum phase transitions, superfluorescence has been almost always observed at cryogenic temperatures. In this presentation I will first present our results on room temperature superfluorescence in lead-halide perovskites and then present the mechanism that enables this exotic quantum phase transition at room temperature.

Charge-transfer excitons are named as artificially engineered excitons carrying cooperatively mutually tunable energy, polarization, and spin-orbital coupling parameters that can be conveniently formed at heterostructured interfaces for wide-range optoelectronic applications. However, charge-transfer excitons have been remaining as an explored phenomenon in the new-generation semiconductors, namely solution-processing semiconducting perovskites, limiting the tuning abilities to control optoelectronic properties. Recently, we found that charge-transfer excitons can be conveniently formed as metastable states in both linear and nonlinear polarization regimes in quasi-2D and superlattice-2D heterostructured Pb/Sn perovskites. In linear polarization regime, charge-transfer excitons have led to broad light-emitting and photo-detecting capabilities with AC operating conditions in quasi-2D heterostructured perovskites [(PEA2PbI4)x:(PEA2SnI4)1-x]. In nonlinear polarization regime, charge-transfer excitons can enable normally-difficult-observed optical phenomena such as infrared-to-visible up-conversion luminescence and X-ray scintillation with self-amplified behaviors in superlattice-2D heterostructured perovskites [(PEA)2Pb1-x SnxI4]. Clearly, artificially-engineered charge-transfer excitons provide a new platform to further advance the optoelectronic properties in 2D perovskites. This presentation will discuss the key parameters of controlling charge-transfer excitons in quasi-2D and superlattice-2D heterostructured perovskites towards generating advanced light-emitting and photo-detecting properties in both linear and nonlinear polarization regimes.

Controlling the formation, size distribution and orientation of quantum wells (QWs) in layered hybrid perovskite (LHP) thin films is foundational to their optoelectronic device applications. These applications require exquisite control of energy and charge transport which tend to be highly anisotropic in low-dimensional phases in LHP thin films. Here, we combine a powerful suite of multimodal in situ characterizations to elucidate the precise solution-to-solid conversion of the sol into the LHP thin film. We identify, for the first time, the presence of oriented colloidal transient nanostructures during spin coating well before the onset of crystallization of phases.

Top-emitting LED architectures allow for novel leverages over performance when compared to standard bottom-emitting architectures. In particular, metal-halide perovskite LEDs (PeLEDs) would benefit from increased external quantum efficiency (EQE) through proper microcavity design and an optimized outcoupling layer. We developed a top-emitting stack structure for a MAPbI3 PeLED to study how device performance can be tuned with various dielectric outcoupling layers. We show improvements of at least 2% in EQE compared to a bottom-emitting architecture by using various outcoupling layers in the top-emitting PeLED. We further matched experimental EQE trends to simulation, allowing for predictive assessment of optimal PeLED design.

Organic light-emitting diodes (OLEDs) are unique light sources with a promising potential even beyond display and lighting applications. The low-temperature processability of organic materials allows the fabrication of organic devices on plastic substrates, enabling deformable optoelectronic devices. In this talk, we present our group's progress on these deformable OLEDs. First of all, we report on a device architecture leading to fully encapsulated stretchable OLEDs with high performance. A straightforward yet highly effective fabrication method is provided based on laser-patterned substrates and stress-relieving layers. Secondly, we report on a revised architecture for OLED-based health-monitoring sensors that can measure blood oxygen level and heart rate. Finally, we introduce key engineering made for ultralow power consumption in these sensors.

Stretchable electroluminescent devices have been obtained via special device architecture or intrinsically stretchable materials. The challenges in fabricating intrinsically stretchable EL devices with high and robust performance are in many facets, including stretchable conductors, emissive materials, and compatible processes. For the stretchable transparent electrode, ionically conductive gel, conductive polymer coating, and conductor network in surface of elastomer have all been proven useful. The stretchable EL materials are currently limited to conjugated polymers and phosphor particles embedded in elastomer matrices. The presentation will provide an overview of the important materials and approaches and describe stretchable polymer light emitting electrochemical cells.

Integrating an organic light-emitting diode (OLED) back-to-back with an organic photodiode (OPD) is a well-known path to upconvert near-infrared light for display purposes. This talk will explore a new, positive feedback regime that arises when the OLED is a tandem device with greater-than-unity quantum efficiency, enabling the OLED emission>>OPD absorption>>OLED emission loop gain to exceed one. The runaway increase in OLED light output that occurs in this situation enables low-level light detection with upconversion gain >10,000 and millisecond dynamic response.

Colloidal quantum dot (QD) based light-emitting diodes (QLEDs) have been attracted attention as a promising candidate following organic light-emitting diodes, because of their superb optical properties such as narrow spectral bandwidths (FWHM ~30 nm), high external quantum efficiency (~20%), and solution processibility. This proceeding describes recent progress on the device structures and fabrication processes of QLEDs for their potential application to displays and lighting sources. Designing novel device architectures to improve device efficiency and stability, as well as to simplify the solution-based QD patterning processes will be also discussed.

Organic-inorganic lead halide perovskite has emerged as a remarkable gain material for nanoscale photonic devices ranging from lasers, LEDs, photodetectors, as well as photonic metamaterials. While the demonstration of electrically pumped lasing from perovskite is still elusive, optically pumped continuous wave (CW) lasing is a necessary intermediate step to achieve this goal. In this work, we show green lasing under quasi-CW optical pumping at peltier-cooling accessible 260K, from a directly patterned and encapsulated MAPbBr3 photonic crystal cavity. In the meantime, hyperbolic metamaterial (HMM) is a special class of anisotropic material that has drawn tremendous research attention recently, as it exhibits metal and dielectric features at the same time. This unique feature makes HMM’s isofrequency surface unbounded, and consequently, infinite wave vector values and optical density of states (DOS) can be achieved, leading to applications such as super resolution imaging, spontaneous emission enhancement, to topological photonics. However, only finite wave vector values and optical DOS can be experimentally achieved due to the inherent loss from the metal constituent, hindering the insertion of HMM into these applications. In order to overcome this hurdle, we experimentally demonstrated a luminescent MAPbI3 perovskite/Au HMMs, wherein the dielectric constituent is fully composed of perovskite gain, thus compensating the metal loss. Our work paves the way towards realizing nanoscale luminescent perovskite metamaterials and devices for insertion into photonic integrated circuits.

Colloidal quantum dot (QD)-based light emitting devices are poised to become the leading technology in next generation flat panel displays but their electroluminescent (EL) stability is still insufficient for commercial applications. We recently found that using a cascaded hole transport layer (HTL) structure can lead to significant EL stability enhancements, prolonging device EL lifetime by 25 times. Introducing modifications to the ZnO electron transport layer can lead to similar benefits. Investigations show that the stability enhancement in both cases is associated with a better management of charge and exciton distributions in the HTL. Results from these investigations will be discussed.

Organic exciton-polariton lasers have recently reached thresholds that are comparable to conventional photonic lasers based on microcavities filled with organic materials. However, tuning of molecular arrangement as a means to further enhance the performance of polariton lasers has remained largely unexplored. Here, we investigate how a combination of conformation and molecular alignment can help enhance lasing performances in strongly coupled microcavities.

Organic semiconductors lasers (OSLs) do not naturally operate in the Continuous-Wave (CW) regime due to the accumulation of long-lived triplet states. Recently, however, triplet engineering has allowed quasi-CW lasing, notably in BSBCz and its derivatives. An analysis of CW lasing conditions in organic semiconductors that includes the effects of Triplet Absorption, Singlet-Triplet-Annihilation, Triplet-Triplet Annihilation or Reverse Intersystem Crossing is presented. In addition of photophysical parameters, we show the crucial role played by the resonator Q factor in the lasing regime. This work provides a roadmap towards true-CW lasing based on a global approach that encompasses photophysics and optical design.

We realize a tunable laser based on a liquid crystal optical microcavity doped with the pyrromethene 580 organic dye. The tunable range reaches 40 nm. By transforming the system into the Rashba-Dresselhaus coupling regime, the laser action takes place from the bottoms of two oppositely polarized valleys shifted apart in reciprocal space. Measurements of emissions in real space show the persistent spin-helix lasing, which is a consequence of the spin coherence of the system. The platform that we propose can be used in quantum communication, in which information is encoded through light polarization.

Tunability is an important aspect of lasers. Nowadays there are many possible ways to achieve this advantageous property; however, dynamic tuning is limited. Red to InfraRed emissive dyes allow a direct visualization of molecular interactions, through deep tissue penetration, along with minimal tissue damage. In our studies we use simple systems based on a single dye-doped polymeric thin films for distributed feedback (DFB) and random lasing (RL) investigations. As active compounds we have applyed novel push-pull luminescent diphenylaminofluorene and tiophene derivatives, with different acceptor groups. Integration of such luminescent dyes with transparent polymeric medium allows fabricating real-time lasing tunability in the visible region and first biological window (650-950 nm). The observed spectral tuning of 150 nm is a groundbreaking value obtained in a single-dye system. Also Excited-State Intramolecular Proton Transfer (ESIPT) compounds, have attracted our considerable attention, due to their unique optical properties. In this contribution we show a novel bis-trimethylsilyl substituted 2-(2’hydroxyphenyl)benzothiazole (HBT) derivatives functionalized with a trifluoromethyl - a strong electron-withdrawing group. Such structure enabled real-time red-green-blue (RGB) switching of emission, both in solution and solid-state, providing white laser light emission. We show strong dependence on environment polarity, as well as Aggregation-Induced Emission Enhancement (AIEE) properties, and successful implementation of ESIPT molecules in DFB lasing, both in solution and solid-state.

Evaluating the lasing potential of light-emitting materials has become an important aspect of thin-film laser research. Measurement of Amplified Spontaneous Emission (ASE) thresholds is a widespread technique for this matter, but the question of whether measuring this threshold in energy or power density becomes relevant whenever pump duration and excited state lifetime share the same order of magnitude. By comparing thresholds of a DCM-based organic waveguide with 4 different pump durations, we establish that power density is the most appropriate unit even for pulses shorter than the excited state lifetime by a factor of 5.

Effect of Matrix Material on Room-Temperature Phosphorescence Characteristics of Graphene Quantum Dots

Star-shaped oligofluorene-truxene materials are excellent materials for frequency downconversion and application in organic lasers and visible light communications. To open up the potential of star-shaped structures for applications such as OLEDs and OLETs, we designed novel core systems to improve aggregation in the solid state whilst retaining respectable levels of emission. In this talk, we present the synthesis and characterisation of some new materials, including TriR and Ind3HBC, which are based on soluble fused cores of graphene fragments.

Some thermally activated delayed fluorescence (TADF) emitters have a propensity to form aggregate species. This can create challenges for device fabrication but also makes photophysical analysis and interpretation extremely complex. Recently I have led three studies into how best to probe this behaviour to create a characterization framework that can expose whether a TADF compound is susceptible to aggregation. Starting from this trilogy of work I will introduce two ongoing studies. These studies further contribute to the wider discussion in the field regarding intermolecular interactions and highlight the challenges ahead as distinguishing between monomer and aggregate species becomes harder.

The exciplex has attracted immense attention as a highly efficient organic light-emitting diode (OLED) emitter owing to the inclusion of a thermally activated delayed fluorescence (TADF) process. Moreover, the exciplex can exhibit long-persistent luminescence (LPL) over several hours by optimization of the donor and acceptor concentrations. However, there have been no reports of LPL from exciplex OLEDs, and the correlation between OLEDs and OLPLs has not yet been clarified. In this study, we confirmed the presence of OLPL under electrical excitation. We elucidated the detailed emission processes of OLPL systems including the thermal activation process such as TADF.

Recently, we reported the accurate rate equation set to analyze the kinetics of three-state photophysics for the thermally activated delayed fluorescence (TADF) materials. The accurate rate constant could be estimated with the simple equation approximated to be no phosphorescence in emission, and the non-radiative decay rate was resulted to be 0. Therefore, the rate constants should be provided as the possible ranges when it is necessary to consider the non-radiative decay from triplet etc. In here, we introduce how analyze the actual emission decays profile which be fit with not only the bi- but also tri-exponential curves to analyze TADF.

Currently, the major problem of most thermally activated delayed fluorescence (TADF) emitters is low rate of reverse intersystem crossing (rISC), a crucial process, responsible for conversion of dark triplet excitons into emissive singlet ones. One of the solutions to accelerate rISC is to increase spin-orbit coupling (SOC) between triplet and singlet states. It can be achieved by heavy-atom effect (HAE). Described here research is aimed to verify HAE concept for TADF materials through detailed experimental and DFT investigations conducted on blue and red/NIR derivatives of TADF emitters with high potential for application [1,2].

It is strongly believed that further progress in organic light emitting technologies depends on if we can develop heavy-metal-free materials with fast reverse intersystem crossing (rISC) and fluorescence rates in subnanosecond domain. Nowadays, the most promising uprising all-organic OLED technologies including those using thermally activated delayed fluorescence (TADF) phenomenon and TADF combined with Förster resonance energy transfer (FRET), a so-called “hyperfluorescence”, rely on the donor-acceptor TADF materials with the fastest rISC. However, understanding of mechanism of basic phostophysical processes in such materials still remains poorly investigated. Obviously, general theory of TADF is also highly required. This presentation will focus on the features of popular and most demanded blue and red TADF emitters, which deviate from our understanding within the classic photophysical model. An alternative TADF model will be described [1, 2], which explains these deviations suggesting that spin-flip transitions between the singlet (1CT) and triplet (3CT) states of the charge-transfer character are actually not as “forbidden” as stated by selection rules. The presented model emphasizes the importance of the 3CT-1CT transition, which molecular vibrations/rotations are crucial for rISC and which aspects of molecular design can improve TADF materials. Bibliography [1]DOI: 10.1039/d2tc00476c; [2]DOI: 10.1021/acs.jpcb.0c10605 Financial support: LIDER XI grant (LIDER/47/0190/L-11/19/NCBR/2020) and CHEMFIZ program (WND-POWR.03.02.00-00-I059/1) of National Centre for Research and Development. Sonata 16 project (UMO-2020/39/D/ST5/03094) of National Science Centre, Poland. DFT calculations were performed on the computers of the Wroclaw Centre for Networking and Supercomputing (WCSS), Poland.

In this study, we examined the correlations between the accumulated charge density and triplet-polaron quenching (TPQ) using simultaneous measurement of displacement current and photoluminescence intensity. We applied this technique to metal-insulator-semiconductor devices where a model structure of an Ir(ppy)3-based OLED was involved, in order to investigate the contribution of unipolar charge accumulation. This technique allows us to investigate not only the TPQ rate constant but also the detailed charge distribution around the EML that is influenced by SOP. The results suggest that the SOP management is an important issue to optimize the device performance of OLEDs.

In vacuum-deposited film composed of polar OLED materials such as Alq3 and TPBi, the permanent dipole moment of the molecules spontaneously orders surface normal on average, resulting in the formation of polarization charge on the film surface and reverse sides. These polarized charges attract charge carriers of the opposite sign because of the Coulomb interaction: They are an essential factor determining the dynamic behavior of charge carriers in the device, such as injection from the electrode to organic material and accumulation at the heterointerface. In this talk, we will mainly discuss the role of the polarization charge in OLEDs.

Amorphous films with preferential molecular alignment can exhibit spontaneous orientation polarization (SOP). In OLEDs, SOP is frequently observed in the electron-transport layer (ETL), leading to exciton-polaron quneching and peak efficiency loss. We find that the efficiency reduction scales directly with the degree of ETL SOP. We also show that quenching can be tuned by mixing the polar ETL with a non-polar host. Finally, we demonstrate how film processing conditions can serve as an additional axis of control, with elevated substrate temperature and reduced deposition rate during film growth leading to more isotropic molecular orientation, and a reduction in SOP-induced quenching.

One of the key factors that determines organic device performance is the specific molecular structure and function of organic materials in thin films, strongly governed by molecular conformational changes during operation. In this talk, the formation and dynamics of molecular conformation in blue light-emitting polymers will be first discussed. By probing in real time the evolution of fluorescence spectra of single polymer chains, we identify dynamic molecular conformation-controlled photophysical processes and their impact on device performance. Second, the importance of molecular conformational changes in organic photoconversion small molecules will be presented. Their effects on energetics, intermolecular interactions, and hence device performance and stability will be highlighted.

The complex nature of the emissive layers makes it difficult to gain a fundamental understanding of the host-matrix effects on the luminescence properties of the emitters. Here, we present a computational workflow to investigate the impact of molecular packing configurations on electronic transitions in emitters. This workflow provides a framework for the systematic development and application of OLED materials. The results of this study highlight the significant impact of host–emitter interactions on radiative and nonradiative recombination processes and offer guidelines to tune these interactions for advancing OLED devices.

Perovskite light-emitting diodes (PeLEDs) are considered as a promising candidate for next-generation solution-processed full-color displays. However, the external quantum efficiency as well as the operational stability of deep-blue (< 460 nm) PeLEDs still lag far behind compared to their red and green counterparts. Herein, the rapid crystallization method based on antisolvent bathing is proposed for trealization deep-blue PeLEDs. By promoting the immediate removal of precursor solvents from the wet perovskite films, the development of the quasi-2D Ruddlesden‒Popper perovskite (2D-RPP) crystals with n value of < 3 was completely hampered, so that phase-pure 2D-RPP films whose bandgap is suitable for deep-blue PeLEDs were successfully obtained.

For passive matrix organic light emitting diodes, crosstalk effect is a serious problem. This problem adversely affects the image quality of display with driver circuit operation. In this study, 16x16 pixel monochrome green emissive passive matrix (PM) OLED Display Module was successfully fabricated which exhibits high efficiency, high luminance uniformity and MIL-STD-3009 night vision (NVIS) compatibility. Current efficiency, power efficiency and external quantum efficiency has been obtained as 22.79 cd/A, 10.23 lm/W and 4.61%, respectively. Additionally, crosstalk effect was removed by adjusting spin rate of the hole injection layer so that display image quality optimally has been achieved.

While open-shell luminophores with doublet spin properties are considered highly promising for next generation organic-light emitting devices, most radicals are associated with poor photostability and photoluminescence quantum yield (PLQYs). We establish certain structure-performance interrelations to improve the optical properties of radicals specifically. Two series of trityl radicals functionalized with one to three 2,7-disubstituted carbazole units are studied, carrying either nitriles or bromines as substituents. The electron-withdrawing substituents in 2,7-position induce a blue-shift of emission and exceptional PLQYs up to 87 %. Quantum mechanical calculations further elucidate the electronic and steric properties of the molecules responsible for the outstanding optical performance.

Thermally activated delayed fluorescent (TADF) emitters are promising organic materials for application in OLED. The most demanded OLED emitters are the blue ones due to low stability of devices of this color. The main problem is the rate of spin flip transition from the lowest excited triplet to singlet state called reverse intersystem crossing (rISC). The rISC efficiency has the direct influence on the external quantum efficiency (EQE) of OLED. In organic blue emitters the spin-orbit coupling (SOC) is low which causes low rISC and low EQE. In our work, we are looking for the way to increase SOC and rISC in blue TDAF emitters. Previously we found that interactions of triplet and singlet states of charge transfer (CT) nature are playing the key role in rISC [1,2]. The influence of different substituents and their position on 3CT-1CT transition in blue TADF emitter DMAC-DPS was studied. We introduced different substituents at donor, acceptor and linker fragment of DMAC-DPS molecule. The substituent effects on the geometry (torsion angle over the σ-bound between donor and acceptor fragments), energy of states, SOC and 3CT-1CT gaps were analyzed. The impact of triplet states localized on donor and acceptor fragments was also analyzed. Our conclusions are helpful in further understanding of different rISC transitions. References: 1) DOI: 10.1039/d2tc00476c; 2) DOI: 10.1021/acs.jpcb.0c10605. Financial support: V.I is grateful to the National Science Centre, Poland within the Sonata 16 project No. UMO-2020/39/D/ST5/03094. Quantum chemical calculations were performed on the computers of the Wroclaw Centre for Networking and Supercomputing (WCSS), Poland.

Perylenediimides (PDIs) are versatile organic compounds. Broad and strong absorption of visible light, semiconducting properties, easiness of structural modification and high stability make PDIs good candidates for novel optoelectronic and photocatalytic applications. PDIs can find use in fields where photosensitizing properties are desirable, like medicine, photodynamic therapy, photocatalysis, photopolymerization and air purification. Key features of PDIs, like absorption spectrum, HOMO and LUMO energy, emission spectrum, solubility and aggregability, can be modified according to the intended use. In our work, we investigate effects of various substituents with different electronic effects on photophysical properties of PDIs. Such approach allows to alter HOMO and LUMO energy according to needs. Our aim was to optimize structure of PDI derivatives for photosensitizing purposes. Density functional theory calculations and photophysical measurements were used to determine best PDI derivatives. Selected PDI derivatives were synthesized in two steps with good yields from commercially available substrates. Absorption and emission spectra of obtained PDIs were measured. We found PDI derivatives based on a Donor-Acceptor structure had superior photophysical properties. Moreover some of Donor-Acceptor based PDIs most likely exhibit Thermally Activated Delayed Fluorescence (TADF) phenomenon. Donor-Acceptor based PDI derivatives might be a solution for low efficiency of organic photosensitizers.

The organic dyes showing thermally activated delayed fluorescence (TADF) can be very promising photosensitizers as they store absorbed energy in triplet states up to milliseconds, have high PLQY and are chemically stable. Putting heavy atom in donor part of the molecule causes elongation the emissive of fluorescensce. To compare external and internal heavy-atom effect in TADF emiters in water solutions, hosts with a different types of polarity were used to separate dye-molecules embedded in micelles and basic photophysical properties were checked using steady-state and time-resolved measurements.

The thermally activated delayed fluorescence (TADF) emitters are currently a hotspot in organic light-emitting diode (OLED) research. Although significant progress has been made, the performance of the red / NIR devices is still insufficient. Therefore, the development of efficient fluorescent materials still remains a major challenge. TADF materials enable the conversion of non-emission triplets (T1) to emission singlet (S1) of a molecule in the form of efficient fluorescence. This transformation occurs via the reverse intersystem crossing (rISC). Acceleration of rISC while maintaining a fast fluorescence rate is supposed to be a solution that minimizes the quantum efficiency (EQE) with low stability of OLED devices. In this study, we wanted to find a molecular strategy to modify the HA of the organic TADF emiter, so that it can maintain the fluorescence ratio and increase the rISC ratio. For this purpose, we designed and synthesized red emitters with the heavy atom(s) introduced into different positions into donor. Bromine atom was chosen as the heavy atom due to the ease of introducing it by common synthetic methods. The results of photophysical studies and quantum-chemical calculations indicate that it is possible to selectively accelerate rISC by HA.