This article presents general concepts that have guided important developments in our recent research progress regarding room‐temperature phosphorescent dyes and their potential applications. We ...first elaborate the theoretical background for emissive metal complexes and the strategic design of the chelating C‐linked 2‐pyridylazolate ligands, followed by their feasibility in functionalization and modification in an aim to fine‐tune the chemical and photophysical properties. Subsequently, incorporation of 2‐pyridylazolate chromophores is illustrated in the synthesis of the highly emissive, charge‐neutral Os, Ru, Ir, and Pt complexes. Insights into their photophysical properties are gained from spectroscopy, relaxation dynamics, and theoretical approaches, from which the lowest‐lying excited states, competitive radiative decay, and radiationless processes are then analyzed in detail. In view of applications, their potentials for OLEDs have been evaluated. The results, in combination with the fundamental basis, give a conceptual design contributed to the future advances in the field of OLEDs.
Glow in the dark: Pyridylazolate chelating ligands can form rigid molecular frameworks with heavy transition‐metal ions to give rise to the desirable absorption and efficient emission under optimum chemical modification. More specifically, these chelate complexes display bright phosphorescent emission spanning the whole visible spectrum, making them suitable to serve as ideal phosphors for OLED applications.
This paper provides a systematic review and analysis of different phenomena that violate a basic principle, Kasha’s rule, when applied to photochemical reactions. In contrast to the classical route ...of ultrafast transition to the lowest energy excited state and photochemical reaction starting therein, in some cases, these reactions proceed directly from high-energy excited states. Nowadays, this phenomenon can be observed for a number of major types of excited-state reactions: harvesting product via intersystem crossing; photoisomerizations; bond-breaking; and electron, proton, and energy transfers. We show that specific conditions for their observation are determined by kinetic factors. They should be among the fastest reactions in studied systems, competing with vibrational relaxation and radiative or nonradiative processes occurring in upper excited states. The anti-Kasha effects, which provide an important element that sheds light on the mechanisms of excited-state transformations, open new possibilities of selective control of these reactions for a variety of practical applications. Efficient utilization of excess electronic energy should enhance performance in the systems of artificial photosynthesis and photovoltaic devices. The modulation of the reporting signal by the energy of excitation of light should lead to new technologies in optical sensing and imaging.
Circularly polarized thermally activated delayed fluorescence (CP‐TADF) and multiple‐resonance thermally activated delayed fluorescence (MR‐TADF), which exhibit novel circularly polarized ...luminescence and excellent color fidelity, respectively, have gained immense popularity. In this study, integrated CP‐TADF and MR‐TADF (CPMR‐TADF) are prepared by strategic design and synthesis of asymmetrical peripherally locked enantiomers, which are separated and denoted as (P,P″,P″)‐/(M,M″,M″)‐BN4 and (P,P″,P″)‐/(M,M″,M″)‐BN5 and exhibit TADF and circularly polarized light (CPL) properties. As the entire molecular frame participates in the frontier molecular orbitals, the resulting helical chirality of (+)/(−)‐BN4‐ and (+)/(−)‐BN5‐based solution‐processed organic light‐emitting diodes (OLEDs) helps in achieving a narrow full width at half maximum (FWHM) of 49/49 and 48/48 nm and a high maximum external quantum efficiency (EQE) of 20.6%/19.0% and 22.0%/26.5%, respectively. Importantly, unambiguous circularly polarized electroluminescence signals with dissymmetry factors (gEL) of +3.7 × 10−3/−3.1 × 10−3 (BN4) and +1.9 × 10−3/−1.6 × 10−3 (BN5) are obtained. The results indicate successful exploitation of CPMR‐TADF‐emitter‐based OLEDs to exhibit three characteristics: high efficiency, color purity, and circularly polarized light.
Circularly polarized thermally activated delayed fluorescence (CP‐TADF) and multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) properties are integrated into a new advanced material, a CPMR‐TADF material. OLEDs based on these CPMR‐TADF emitters show excellent performance, attaining a three‐in‐one advantage: high efficiency, color purity, and circular polarized light simultaneously.
Charge and proton transfer reactions in the excited states of organic dyes can be coupled in many different ways. Despite the complementarity of charges, they can occur on different time scales and ...in different directions of the molecular framework. In certain cases, excited-state equilibrium can be established between the charge-transfer and proton-transfer species. The interplay of these reactions can be modulated and even reversed by variations in dye molecular structures and changes of the surrounding media. With knowledge of the mechanisms of these processes, desired rates and directions can be achieved, and thus the multiple emission spectral features can be harnessed. These features have found versatile applications in a number of cutting-edge technological areas, particularly in fluorescence sensing and imaging.
Tunable electron‐accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π‐conjugated motifs, define ...the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π‐extended molecules functionalized with P‐cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation‐anion interactions, which can drastically change physical behavior of these two‐component systems.
Ionic fifteenth: The diverse synthetic pathways to π‐conjugated systems with quaternary phosphorus, its unique connectivity and tunable electron deficient character, along with the non‐covalent cation‐anion interactions, introduce new photophysical and electrochemical features to ionic chromophores.
Through the incorporation of various halogen‐substituted chiral organic cations, the effects of chiral molecules on the chiroptical properties of hybrid organic–inorganic perovskites (HOIPs) are ...investigated. Among them, the HOIP having a Cl‐substituted chiral cation exhibits the highest circular dichroism (CD) and circular polarized luminescence (CPL) intensities, indicating the existence of the largest rotatory strength, whereas the F‐substituted HIOP shows the weakest intensities. The observed modulation can be correlated to the varied magnetic transition dipole of HOIPs, which is sensitive to the d‐spacing between inorganic layers and the halogen–halogen interaction between organic cations and the inorganic sheets. These counteracting effects meet the optimal CD and CPL intensity with chlorine substitution, rendering the rotatory strength of HOIPs arranged in the order of (ClMBA)2PbI4>(BrMBA)2PbI4>(IMBA)2PbI4>(MBA)2PbI4>(FMBA)2PbI4.
Through the incorporation of Cl‐substituted chiral organic cations, the chiroptical properties of 2D chiral perovskites can be significantly enhanced. The observed circular dichroism and circular polarized luminescence intensities are found to be associated with the d‐spacing of hybrid organic–inorganic perovskites and the strength of the halogen–halogen interaction within the system.
Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (−NO2) appears on countless molecules in photochemical research owing to its ...unique characteristics such as a strong electron‐withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3‐nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro‐containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure‐photophysics relationship and hence gain insights for the strategic design of nitro chromophores.
Say NO2 to quenching: The nitro group is a well‐known fluorescence quencher, however, nitro‐substituted chromophores with considerable photoluminescence quantum yields have been reported. This Review systematically organizes a number of reports concerning fluorescent nitro compounds with detailed explanations of their photophysical properties, which provides guidelines for designing emissive nitro‐containing molecules.
The correlation between molecular packing structure and its room-temperature phosphorescence (RTP), hence rational promotion of the intensity, remains unclear. We herein present racemism enhanced RTP ...chiral chromophores by 2,2-bis-(diphenylphosphino)-1,1-napthalene (rac-BINAP) in comparison to its chiral counterparts. The result shows that rac-BINAP in crystal with denser density, consistent with a long standing Wallach's rule, exhibits deeper red RTP at 680 nm than that of the chiral counterparts. The cross packing between alternative R- and S- forms in rac-BINAP crystal significantly retards the bimolecular quenching pathway, triplet-triplet annihilation (TTA), and hence suppresses the non-radiative pathway, boosting the RTP intensity. The result extends the Wallach's rule to the fundamental difference in chiral-photophysics. In electroluminescence, rac-BINAP exhibits more balanced fluorescence versus phosphorescence intensity by comparison with that of photoluminescence, rendering a white-light emission. The result paves an avenue en route for white-light organic light emitting diodes via full exploitation of intrinsic fluorescence and phosphorescence.
ortho‐Methyl effects are exploited to tune steric hindrance between side‐chain N,N′‐diaryls and polycyclic dihydrodibenzoa,cphenazine, and in turn control the conformations of ...N,N′‐diphenyl‐dihydrodibenzoa,cphenazine (DPAC) and its ortho‐methyl derivatives Mx‐My (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho‐positiond of N,N′‐diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V‐shaped dihydrodibenzoa,cphenazine skeleton is gradually tuned from a bent (DPAC) to planar (M2‐M2) structure in the ground state. As a result, the relaxation of the excited‐state structure of DPAC and its numerous analogues could be mimicked by model structures Mx‐My, demonstrating for the first time the the conformation change from bent‐to‐planar and hence a large range of energy‐gap tuning of polycyclic conjugated structures controlled by the steric hindrance.
Six (D)PAC: ortho‐Methyl effects are exploited to tune the steric hindrance between side‐chain N,N′‐diaryls and polycyclic dihydrodibenzoa,cphenazine, and in turn control the conformation and color of N,N′‐diphenyl‐dihydrodibenzoa,cphenazine (DPAC) and its ortho‐methyl derivatives. The results show that simple insertion of a small methyl group overcomes the energy barrier to planarization in the ground state.
The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho‐methoxy group (NM) at its bay ...positions (PBI‐2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol−1) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co‐planarization of the different aromatic units, resulting in delocalization of the π‐electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co‐planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H‐bond‐driven co‐planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.
Weak bond, positive effect: A unique positive temperature effect in the single fluorescence emission of a fluorescent dyad containing both donor and acceptor moieties was observed. The origin is ascribed to the presence of coplanar and twisted structures owing to the driving of weak intramolecular hydrogen bonding.
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