Converting molecular luminescence to ultralong room-temperature phosphorescence via the excited state modulation of sulfone-containing heteroaromatics

Ultrafast photophysics of an orange-red thermally activated delayed fluorescence emitter: the role of external structural restraint

The application of thermally activated delay fluorescence (TADF) emitters in the orange-red regime usually suffers from the fast non-radiative decay of emissive singlet states (kSNR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although kSNR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the kSNR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and kSNR. Spectroscopic and theoretical investigations indicated that S1/S0 ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic-vibrational coupling through the Huang-Rhys factor, while kSNR is largely affected by the reorganization energy of the promoting modes. By restraining S1/S0 ES-SR in doping films, the kSNR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S1/S0 ES-SR, promoting modes and kSNR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.

Blue-Shifted and Broadened Fluorescence Enhancement by Visible and Mode-Selective Infrared Double Excitations

Mode-selective vibrational excitations to modify the electronic states of fluorescein dianion in methanol solutions are carried out with a femtosecond visible pulse synchronized with a tunable high-power, narrow-band picosecond infrared (IR) pulse. In this work, simultaneous intensity enhancement, peak blueshift, and line width broadening of fluorescence are observed in the visible/IR double resonance experiments. Comprehensive investigations on the modulation mechanism with scanning the vibrational excitation frequencies, tuning the time delay between the two excitation pulses, theoretical calculations, and nonlinear and linear spectroscopic measurements suggest that the fluorescence intensity enhancement is caused by the increase of the Franck-Condon factor induced by the vibrational excitations at the electronic ground state. Various enhancement effects are observed as vibrations initially excited by the IR photons relax to populate the vibrational modes of lower frequencies. The peak blueshift and line width broadening are caused by both increasing the Franck-Condon factors among different subensembles because of IR pre-excitation and the long-lived processes induced by the initial IR excitation. The results demonstrate that the fluorescence from the visible/IR double resonance experiments is not a simple sum frequency effect, and vibrational relaxations can produce profound effects modifying luminescence.

Multi-Responsive Afterglows from Aqueous Processable Amorphous Polysaccharide Films

Polymer-based room-temperature phosphorescence (RTP) materials, especially polysaccharide-based RTP materials, earn sustained attention in the fields of anti-counterfeiting, data encryption, and optoelectronics owing to their green regeneration, flexibility, and transparency. However, those with both ultralong phosphorescence lifetime and excitation wavelength-dependent afterglow are rarely reported. Herein, a kind of amorphous RTP material with ultralong lifetime of up to 2.52 s is fabricated by covalently bonding sodium alginate (SA) with arylboronic acid in the aqueous phase. The resulting polymer film exhibits distinguished RTP performance with excitation-dependent emissions from cyan to green. Specifically, by co-doping with other fluorescent dyes, further regulation of the afterglow color from cyan to yellowish-green and near-white can be achieved through triplet-to-singlet Förster resonance energy transfer. In addition, the water-sensitive properties of hydrogen bonds endow the RTP property of SA-based materials with water/heat-responsive characteristics. On account of the color-tunable and stimuli-responsive afterglows, these smart materials are successfully applied in data encryption and anti-counterfeiting.

Sulfur Oxidation State and Substituents Influenced Multifunctional Organic Luminophores in BTP Core for OLEDs: A Computational Study on RTP, TADF Emitter and Sensitizer

The exploration of triplet excitons in thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) molecules has become a subject of significant attention and interest in recent studies. This study employed density functional theory (DFT) and time-dependent DFT theoretical methods to delve into the intricate relationship between the molecular structure and properties of molecules designed with the oxidation of sulfur atoms (S, SO, and SO2) in benzothiazinophenothiazine (BTP) core units. The calculations revealed that as the oxidation state of the sulfur atom increased, the BTP derivatives exhibited elevated ionization potentials (IPs), electron affinities (EAs), and triplet energies (ET), accompanied by reduced reorganization energies (λ), singlet energies (ES), and a S1-T1 energy gap (ΔEST). Additionally, the decrease in the exchange energy prompts a shift in the excited-state properties of molecules, transitioning them from hybridized local and charge transfer (HLCT) to charge transfer (CT) in the S1 state while maintaining their HLCT character in the T1 state. The sulfur oxidation process systematically decreases spin-orbit coupling magnitudes in the S1-T1 and T1-S0 pathways while increasing the KRISC rate, signifying a reduced propensity for phosphorescence radiative decay in oxidized molecules. Thorough investigations have explored the screening effect and orbital mixing of lone pair electrons in sulfur atoms, satisfying the desired criteria for a multifunctional RTP, TADF emitter and sensitizer.

Rational molecular and doping strategies to obtain organic polymers with ultralong RTP

Organic-doped polymers and room-temperature phosphorescence (RTP) mechanisms have been widely reported. However, RTP lifetimes >3 s are rare and RTP-enhancing strategies are incompletely understood. Herein, we demonstrate a rational molecular doping strategy to obtain ultralong-lived, yet bright RTP polymers. The n-π* transitions of boron- and nitrogen-containing heterocyclic compounds can promote a triplet-state population, and the grafting of boronic acid onto polyvinyl alcohol can inhibit molecular thermal deactivation. However, excellent RTP properties were achieved by grafting 1-0.1% (N-phenylcarbazol-2-yl)-boronic acid rather than (2-/3-/4-(carbazol-9-yl)phenyl)boronic acids to afford record-breaking ultralong RTP lifetimes up to 3.517-4.444 s. These results showed that regulation of the interacting position between the dopant and matrix molecules to directly confine the triplet chromophore could more effectively stabilize triplet excitons, disclosing a rational molecular-doping strategy for achieving polymers with ultralong RTP. Based on the energy-donor function of blue RTP, an ultralong red fluorescent afterglow was demonstrated by co-doping with an organic dye.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

Carbonyl-Containing Thermally Activated Delayed Fluorescence Emitters for Narrow-Band Electroluminescence

Carbonyl-containing derivatives show enduring vitality in the field of thermally activated delayed fluorescence (TADF) materials; they can realize high device efficiency by using both singlet and triplet excitons for electroluminescence. Recently, a system based on fused ketone/amine exhibited huge potential for constructing multi-resonance TADF (MR-TADF) emitters, which exhibit higher narrow-band emission than conventional TADF emitters with twisted donor-acceptor (D-A) structure. Herein, we summarize current research progress in both traditional and MR-type ketone derivatives with TADF characteristics for introducing the molecular design strategy of maintaining high device efficiency while keeping narrow-band emission profile. We hope this review can inspire the emergence of more high-performance narrow-band materials.

Diboraanthracene-Doped Polymer Systems for Colour-Tuneable Room-Temperature Organic Afterglow

Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far-reaching application potential and fundamental interest. Here, two laterally expanded 9,10-dimesityl-dihydro-9,10-diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue-green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron-donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long-lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue-green emitters, respectively) are attributed to the properties of single molecules.

A Class of Organic Units Featuring Matrix-Controlled Color-Tunable Ultralong Organic Room Temperature Phosphorescence

A novel class of organic units (N-1 and N-2) and their derivatives (PNNA-1 and PNNA-2) with excellent property of ultralong organic room temperature phosphorescence (UORTP) is reported. In this work, N-1, N-2, and their derivatives function as the guests, while organic powders (PNCz, BBP, DBT) and polymethyl methacrylate (PMMA) serve as the host matrixes. Amazingly, the color of phosphorescence can be tuned in different states or by varying the host matrixes. At 77 K, all molecules show green afterglow in the monomer state but yellow afterglow in the aggregated state because strong intermolecular interactions exist in the self-aggregate and induce a redshift of the afterglow. In particular, PNNA-1 and PNNA-2 demonstrate distinctive photoactivated green UORTP in the PMMA film owing to the generation of their cation radicals. Whereas the PNNA-1@PNCz and PNNA-2@PNCz doping powders give out yellow UORTP, showing matrix-controlled color-tunable UORTP. In PNCz, the cation radicals of PNNA-1 and PNNA-2 can stay stably and form strong intermolecular interactions with PNCz, leading to a redshift of ultralong phosphorescence.

Facile construction of C,N-disulfonated 5-amino pyrazoles through an iodine-catalyzed cascade reaction

A green and facile synthesis of previously unreported C,N-disulfonated 5-amino pyrazoles was established through an iodine-catalyzed cascade reaction.