Comprehensive Model of the Photophysics of N-Phenylnaphthalimides: The Role of Solvent and Rotational Relaxation

Synthesis and characterization of 1,2,3,4-naphthalene and anthracene diimides

We report the synthesis and characterization of naphthalene and anthracene scaffolds end-capped by cyclic imides. The solid-state structures of the N-phenyl derivatives, determined by X-ray crystallography, reveal changes in packing preference based on the number of aromatic rings in the core. The optical and electronic properties of the title compounds compare favorably with other previously described isomers and expand the toolbox of electron-deficient aromatic compounds available to organic materials chemists.

Synthesis, Characterization, and Properties of Sila-Annulated Phenanthrene Imides

A series of sila-annulated phenanthrene imides were synthesized through a three-step synthetic route, which represent a hybrid class of biphenyl-based π-conjugated molecules incorporating an imide unit and silole. A comprehensive investigation of their structural, photophysical, and electronic properties was studied by experiment and theoretical calculations. Notably, sila-annulated phenanthrene imides with significant aggregation-induced emission (AIE) properties were observed.

Intrinsic Photostability in Dithiolonaphthalimide Achieved by Disulfide Bond-Induced Excited-State Quenching

Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo-N-(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism. Quantum chemical calculations show that the S-S stretching leads to a dramatic decrease in the energy gap between the S1 and S0 states, similar to the situation in 1,2-dithiane. Due to the efficient nonradiative process, the excited-state lifetime of DTDPNI resolved by ultrafast spectroscopy is determined to be ∼20 ps. It is found that the excellent photostability is achieved by ultrafast excited-state quenching induced by the S-S stretching, rather than the cleavage of the disulfide bond; even the disulfide bridge is in a very rigid aromatic molecular system.

High Electron Mobility Hot-Exciton Induced Delayed Fluorescent Organic Semiconductors

The development of high mobility emissive organic semiconductors is of great significance for the fabrication of miniaturized optoelectronic devices, such as organic light emitting transistors. However, great challenge exists in designing key materials, especially those who integrates triplet exciton utilization ability. Herein, dinaphthylanthracene diimides (DNADIs), with 2,6-extended anthracene donor, and 3'- or 4'-substituted naphthalene monoimide acceptors were designed and synthesized. By introducing acceptor-donor-acceptor structure, both materials show high electron mobility. Moreover, by fine-tuning of substitution sites, good integration with high solid state photoluminescence quantum yield of 26 %, high electron mobility of 0.02 cm²  V^-1  s^-1 , and the feature of hot-exciton induced delayed fluorescence were obtained in 4'-DNADI. This work opens a new avenue for developing high electron mobility emissive organic semiconductors with efficient utilization of triplet excitons.

Quantum simulations of thermally activated delayed fluorescence in an all-organic emitter

We investigate the prototypical NAI-DMAC thermally activated delayed fluorescence (TADF) emitter in the gas phase- and high-packing fraction limits at finite temperature, by combining first principles molecular dynamics with a quantum thermostat to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔE_ST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔE_ST vanishes in the perfect crystal, it is of the order of ∼0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between the HOMOs and LUMOs has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results on ΔE_ST and oscillator strengths, together with our estimates of direct and reverse intersystem crossing rates, show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds. Our findings highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC.

Bipolar 1,8-naphthalimides showing high electron mobility and red AIE-active TADF for OLED applications

Aiming to design bipolar organic semiconductors with high electron mobility and efficient red thermally activated delayed fluorescence (TADF), three donor-acceptor compounds were designed and synthesized selecting 1,8-naphthalimide as an acceptor and phenoxazine, 3,7-di-tert-butylphenothiazine or 2,7-di-tert-butyldimethyl-9,10-dihydroacridine as donor moieties. Aggregation induced emission enhancement was detected for the compounds causing efficient TADF in the solid-state. Photoluminescence quantum yields up to 77% were observed for the films of the compounds doped in a host. The compounds exhibited small singlet-triplet splitting (0.03-0.05 eV), and high reverse intersystem crossing rates of 2.08 × 10⁵-1.13 × 10⁶ s^-1. The compounds were characterized by satisfactory hole and electron-injecting properties with ionization potentials of 5.72-5.83 eV and electron affinities of 2.79-2.91 eV. Bipolar charge transport was revealed by time of flight measurements. Electron transport with low dispersity and mobilities exceeding 2 × 10^-3 cm² V^-1 s^-1 was observed at an electric field of 4.6 × 10⁵ V cm^-1. The compounds were used as emitters in red electroluminescent devices, which showed maximum external quantum efficiencies up to 8.2%. Utilization of host-guest systems as light-emitting materials with hosts preferably transporting holes and TADF guests which preferably transport electrons allowed maximum efficiencies to be achieved at a practical brightness of 700-2200 cd m^-2. DFT calculations of the geometry, electronic structure, absorption and photoluminescence spectra of all compounds were carried out to prove the conclusions drawn from the experiment. The results of the calculations clearly show that the first excited state for all compounds is the intramolecular charge transfer state. Quantitative analysis of the separation degree of electronic density during excitation allows the observed dependence of the blue shift value in the absorption and emission spectra on the increasing polarity of the solvent to be explained.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

An "off-on-off" fluorescence chemosensor for the sensitive detection of Cu2+ in aqueous solution based on multiple fluorescence emission mechanisms

A new organosiloxane precursor ((E)-3-hydroxy-4-((2-(2-hydroxy-4-(3-(3-(triethoxysilyl)propyl)ureido)benzoyl)hydrazono)methyl)phenyl(3-(triethoxysilyl)propyl)carbamate, hereinafter referred to as AHBH-Si) and tetraethylorthosilicate (TEOS) were mixed as the mixed Si source, and bridged periodic mesoporous organic silica (AHBH-PMOs) nanoparticles were obtained through the co-condensation reaction. AHBH-PMO nanoparticles possess mechanisms of "Aggregation Induced Emission" (AIE) and "Intramolecular Charge Transfer" (ICT), which originate from the molecular structure of AHBH having "C[double bond, length as m-dash]N" bond, ortho hydroxyl groups, etc.. Therefore, the optical properties of AHBH are excellent with respect to the solvent effect and enhanced fluorescence. For hybrid materials, the silica framework provides a rigid environment that restricts the rotation of AHBH, thereby turning on the fluorescence of AHBH due to the regulation by the AIE effect. In particular, AHBH-PMOs are no longer restricted by organic solvents and could really achieve the response to Cu2+ with high sensitivity and selectivity in aqueous solutions of a wide pH range. In addition, the detection limit is as low as 3.26 × 10-9 M. Methods such as Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and high-resolution mass spectrometry have shown the coordination interaction between AHBH and Cu2+. The Gaussian 09 software of density functional theory to calculate the reducing changes of energy gaps among AHBH and AHBH-Si before and after the addition of Cu2+ showed that coordination interaction exists in the system. These results indicate that AHBH-PMO hybrid materials have potential applications in the field of environmental monitoring.

Ultrafast Spectroscopic Analysis of Pressure-Induced Variations of Excited-State Energy and Intramolecular Proton Transfer in Semi-Aliphatic Polyimide Films

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm^-1) through an excited-state intramolecular proton transfer (ESIPT) induced by keto-enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky -CH₃ and -CF₃ side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Förster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole-dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Spin-Polarized Molecular Triplet States as Qubits: Phosphorus Hyperfine Coupling in the Triplet State of Benzoisophosphinoline

Advances in quantum information science (QIS) require the development of new molecular materials to serve as microwave addressable qubits that can be read out optically. Laser photoexcitation of organic π-conjugated molecules often results in spin-polarized phosphorescent triplet states that can be readily observed and manipulated using time-resolved electron paramagnetic resonance (EPR) techniques. Photoexcitation of N-mesityl-1,8-naphthalimide (M-NMI) and its phosphorus analogues, 2-mesitylbenzoisophosphinoline (M-BIPD) and 2-mesitylbenzoisophosphinoline oxide (M-BIPDO) results in ultrafast spin-orbit charge-transfer intersystem crossing to form the corresponding phosphorescent triplet states M-^3*NMI, M-^3*BIPD and M-^3*BIPDO. The ultrafast triplet formation dynamics, phosphorescence, and spin-polarized EPR spectra of these triplet states were examined. The most promising qubit candidate, M-^3*BIPD, was examined using pulse-EPR to measure its spin relaxation times, and pulse electron-nuclear double resonance spectroscopy to perform a two-qubit CNOT gate using the phosphorus nuclear spin.

Dual Emission: Classes, Mechanisms, and Conditions

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.

1,8-Naphthalimide-Based Polymers Exhibiting Deep-Red Thermally Activated Delayed Fluorescence and Their Application in Ratiometric Temperature Sensing

A series of naphthalimide (NAI)-based red-emissive thermally activated delayed fluorescence (TADF) acrylic monomers has been designed and synthesized. When copolymerized with a host material by Cu(0)-reversible deactivation radical polymerization (Cu(0)-RDRP), polymers exhibiting orange to deep-red TADF were obtained with quantum yields of up to 58% in solution and 31% in the solid state. These emitters exhibit dual emission consisting of high-energy prompt fluorescence from the NAI acceptor (λ_max = 340 nm in toluene) and red-delayed fluorescence from the charge-transfer process (λ_max = 633-711 nm in toluene). This dual emissive property was utilized to create red-to-blue temperature-responsive polymers by copolymerization of NAI-DMAC with N-isopropylacrylamide and a blue fluorescent dopant. These polymers exhibit red TADF at room temperature and blue fluorescence at 70 °C, with a high ratiometric fluorescent thermal response of 32 ± 4% K^-1. Such systems are anticipated to have utility in bioimaging, drug delivery, and temperature sensing, further expanding the range of applications for red TADF materials.

Aromatic-fused diketophosphanyl-core organic functional materials: phosphorus mimics of imides or beyond?

Recently, missing pieces of organophosphorus compounds, i.e., aromatic-fused diketophosphanyl compounds, have attracted much attention as promising scaffolds of building blocks for functional organic materials. In this review, the brief historical background, synthetic methods, structures, and optoelectronic aspects of aromatic-fused diketophosphanyls are overviewed.

Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores

We present the synthesis, structural characterization, electronic structure calculations, and the ultrafast and supra-nanosecond photophysical properties of a series of five bichromophores of the general structural formula [Re(5-R-phen)(CO)₃(dmap)](PF₆), where R is a naphthalimide (NI), phen = 1,10-phenanthroline, and dmap is 4-dimethylaminopyridine. The NI chromophores were systematically modified at their 4-positions with -H (NI), -Br (BrNI), phenoxy (PONI), thiobenzene (PSNI), and piperidine (PNI), rendering a series of metal-organic bichromophores (Re1-Re5, respectively) featuring variability in the singlet and triplet energies in the pendant NI subunit. Five closely related organic chromophores as well as [Re(phen)(CO)₃(dmap)](PF₆) (Re6) were investigated in parallel to appropriately model the photophysical properties exhibited in the bichromophores. The excited state processes of all molecules in this study were elucidated using a combination of transient absorption spectroscopy and time-resolved photoluminescence (PL) spectroscopy, revealing the kinetics of the energy transfer processes occurring between the appended chromophores. The spectroscopic analysis was further supported by electronic structure calculations which identified the origin of many of the experimentally observed electronic transitions.

Achieving Nearly 30% External Quantum Efficiency for Orange-Red Organic Light Emitting Diodes by Employing Thermally Activated Delayed Fluorescence Emitters Composed of 1,8-Naphthalimide-Acridine Hybrids

The combination of rigid acridine donor and 1,8-naphthalimide acceptor has afforded two orange-red emitters of NAI-DMAC and NAI-DPAC with high rigidity in molecular structure and strongly pretwisted charge transfer state. Endowed with high photoluminescence quantum yields (Φ_PL ), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record-high orange-red TADF organic light-emitting diodes (OLEDs) with external quantum efficiencies of up to 21-29.2%, significantly surpassing all previously reported orange-to-red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record-high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal Φ_PL s.

Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups

A significant difference in fluorescence properties between 3- and 4-substituted phthalimides demonstrates the formation of excited TICT and planar ICT states, respectively.

One-pot syntheses of blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones by T3P® activation of 3-arylpropiolic acids

In situ activation of 3-arylpropiolic acids with T3P^® (n-propylphosphonic acid anhydride) initiates a domino reaction furnishing 4-arylnaphtho[2,3-c]furan-1,3-diones in excellent yields. Upon employing these anhydrides as reactive intermediates blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones are formed by consecutive pseudo three-component syntheses in a one-pot fashion. The Stokes shifts correlate excellently with the Hammett-Taft σ_R parameter indicating an extended degree of resonance stabilization in the vibrationally relaxed excited singlet state.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Studies of the fluorescence light-up effect of amino-substituted benzo[b]quinolizinium derivatives in the presence of biomacromolecules

A comparative study of the ability of amino-substituted benzo[b]quinolizinium derivatives to act as DNA- or protein-sensitive fluorescent probes is presented. Spectrophotometric titrations, DNA denaturation studies and viscometric titrations showed that all tested aminobenzo[b]quinolizinium derivatives intercalate into DNA with binding constants K(b) = 10(4)-10(5) M(-1). The intense fluorescence of the 9-aminobenzo[b]quinolizinium (Φ(fl) = 0.41) as well as the intrinsically very weak emission of the 7-aminobenzo[b]quinolizinium (Φ(fl) < 0.005) are quenched by the addition of DNA, most likely caused by a photoinduced electron transfer (PET) between the excited intercalated ligand and the DNA bases. The 6-aminobenzo[b]quinolizinium (1b) and the 6-amino-9-bromobenzo[b]quinolizinium (1c) exhibit very low fluorescence intensity in water (Φ(fl) < 0.005). However, in water-glycerol mixtures the emission intensity increases by factors of 56 (1b) and 27 (1c) with increasing glycerol content of the solution (0-100 wt%), which indicates the radiationless deactivation of the excited state of 1b and 1c due to a torsional relaxation, i.e. rotation about the exocyclic C(ar)-NH(2) bond. In the case of the bromo-substituted derivative 1c, a viscosity-independent heavy-atom-effect of the bromo substituent leads to additional quenching. The association of 1b and 1c with ds DNA leads to a restricted conformational flexibility of the intercalated ligand and results in an increase of fluorescence intensity. This effect is particularly strong in the presence of poly[dA-dT]-poly[dA-dT]. Upon association with ct DNA or poly[dG-dC]-poly[dG-dC] only very small enhancement of emission intensity (1b) or even a slight quenching (1c) of the fluorescence was observed because of the interfering PET reaction with the guanine residues. Preliminary experiments reveal that the 6-aminobenzo[b]quinolizinium derivatives 1b and 1c may also be employed as protein-sensitive probes, because their emission intensity increases upon association with selected albumins.