Mechanochromism induced through the interplay between excimer reaction and excited state intramolecular proton transfer

Amination and Protonation Facilitated Novel Isoxazole Derivative for Highly Efficient Electron and Hole Separation

It is of great importance to understand the intrinsic relationship between phototautomerization and photoelectric properties for the exploration of novel organic materials. Here, in order to chemically control the protonation process, the aminated isoxazole derivative (2,2'-(isoxazolo[5,4-d]isoxazole-3,6-diyl)dibenzenaminium, DP-DA-DPIxz) with -N═ as the proton acceptor was designed to achieve the twisted intramolecular charge transfer (TICT) state which was triggered by an excited-state intramolecular proton transfer (ESIPT) process. This kind of protonation enhanced the intramolecular hydrogen bonding, conjugative effect, and steric hindrance effects, ensuring a barrierless spontaneous TICT process. Through the intramolecular proton transfer, the configuration torsion and conjugation dissociation of the DP-DA-DPIxz molecule was favored, which led to efficient charge separation and remarkable variations in light-emitting properties. We hope the present investigation will provide a new approach to design novel optoelectronic organic materials and shine light on the understanding of the charge transfer and separation process in molecular science.

Hydrogen-Bonded Thiol Undergoes Unconventional Excited-State Intramolecular Proton-Transfer Reactions

The chapter on the thiol-related hydrogen bond (H-bond) and its excited-state intramolecular proton-transfer (ESIPT) reaction was recently opened where compound 4'-diethylamino-3-mercaptoflavone (3NTF) undergoes ESIPT in both cyclohexane solution and solid, giving a 710 nm tautomer emission with an anomalously large Stokes shift of 12,230 cm-1. Considering the thiol H-bond to be unconventional compared to the conventional Pauling-type -OH or -NH H-bond, it is thus essential and timely to probe its fundamental difference between their ESIPT. However, thiol-associated ESIPT tends to be nonemissive due to the dominant nπ* character of the tautomeric lowest excited state. Herein, based on the 3-mercaptoflavone scaffold and π-elongation concept, a new series of 4'-substituted-7-diethylamino-3-mercaptoflavones, NTFs, was designed and synthesized with varied H-bond strength and 690-720 nm tautomeric emission upon ultraviolet (UV) excitation in cyclohexane. The order of their H-bonding strength was experimentally determined to be N-NTF < O-NTF < H-NTF < F-NTF, while the rate of -SH ESIPT measured by fluorescence upconversion was F-NTF (398 fs)-1 < H-NTF (232 fs)-1 < O-NTF (123 fs)-1 < N-NTF (101 fs)-1 in toluene. Unexpectedly, the strongest H-bonded F-NTF gives the slowest ESIPT, which does not conform to the traditional ESIPT model. The results are rationalized by the trend of carbonyl oxygen basicity rather than -SH acidity. Namely, the thiol acidity relevant to the H-bond strength plays a minor role in the driving force of ESIPT. Instead, the proton-accepting strength governs ESIPT. That is to say, the noncanonical thiol H-bonding system undergoes an unconventional type of ESIPT.

Interrogating the nature of aggregates formed in a model azine based ESIPT coupled AIE active probe: stark differences in photodynamics in the solid state and aggregates in water

A novel Schiff base 4-bromo-2-((E)-((E)-(1-(naphthalen-2-yl)ethylidene)hydrazono)methyl)phenol (BNHMP) was synthesized and characterized by NMR, ESI-MS, FTIR and single crystal X-ray diffraction studies. In the solution phase, BNHMP shows prominent emission from the keto-form, a consequence of excited state intramolecular proton transfer (ESIPT). The quantum yield and excited state lifetime decrease in polar solvent THF compared to relatively non-polar solvent DCM. Interestingly, in aqueous solution (pH 7.0), the quantum yield along with the excited state lifetime undergoes tremendous increment. Dynamic light scattering experiments and FESEM reveal the formation of aggregates in water as reflected by the increased hydrodynamic radius of BNHMP in water. Hence, aqueous phase studies revealed BNHMP to be an AIE active probe. On the other hand, BNHMP shows huge emission intensity in the solid state. Interestingly, the emission decay behavior of BNHMP changes upon excitation, as BNHMP shows very broad absorption in the solid state. Upon excitation at 360 nm, a triexponential decay pattern is found, which changes to a biexponential one upon excitation at 450 nm. Meticulous analysis of the fluorescence lifetimes led to the assignment of J and H aggregates coexisting in the solid state with the former dominating the photodynamics. A judicious comparison of the lifetime behavior in the solid state to that in water leads to the conclusion that BNHMP undergoes AIE by the formation of J and H aggregates to an equal extent, a phenomenon starkly different from the solid-state scenario. The current results hold significance as this is among a few reports where such comprehensive spectrodynamic dissection has been performed for an ESIPT-AIE active Schiff base in solution as well as in the solid phase, thereby giving a holistic vision of the nature and fate of aggregation occurring in such azine based systems and subsequently advancing the understanding of such systems in terms of their photo behavior.

Correlating structure and photophysical properties in thiazolo[5,4-d]thiazole crystal derivatives for use in solid-state photonic and fluorescence-based optical devices

There is a growing demand for new fluorescent small molecule dyes for solid state applications in the photonics and optoelectronics industry. Thiazolo[5,4-d]thiazole (TTz) is an organic heterocycle moiety which has previously shown remarkable properties as a conjugated polymer and in solution-based studies. For TTz-based small molecules to be incorporated in solid-state fluorescence-based optical devices, a thorough elucidation of their structure-photophysical properties needs to be established. Herein, we have studied four TTz-based materials functionalized with alkyl appendages of varying carbon chain lengths. We report the single crystal structures of the TTz derivatives, three of which were previously unknown. The packing modes of the crystals reveal that molecular arrangements are largely governed by a chorus of synergistic intermolecular non-covalent interactions. Three crystals packed in herringbone mode and one crystal packed in slipped stacks proving that alkyl appendages modulate structural organization in TTz-based materials. Steady state and time-resolved photophysical properties of these crystals were studied via diffuse-reflectance, micro-Raman, and photoluminescence spectroscopy. The crystals fluoresce from orange-red to blue spanning through the whole gamut of the visible spectrum. We have established that photophysical properties are a function of crystal packing in symmetrically substituted TTz-based materials. This correlation was then utilized to fabricate crystalline blends. We demonstrate, for the first time, that symmetrically substituted donor-acceptor-donor TTz-based materials can be used for phosphor-converted color-tuning and white-light emission. Given the cost effectiveness, ease of synthesis and now a structure-photophysics correlation, we present a compelling case for the adoption of TTz-based materials in solid-state photonic and fluorescence-based optical devices.

Julolidine based red emitting ESIPT/AIE active material showing luminescence beyond excimer emission: An "on-off" emission response to Cu2

A new julolidine-fluorene based excited state intramolecular proton transfer (ESIPT)/aggregate induced emission (AIE) active Schiff-base (JDF) has been synthesized and evaluated for its photophysical properties in solution and aggregated/solid states. The correlation between the emission behavior and the solid state crystal packing structure revealed the interplay of ESIPT coupled excimer reaction occurring in the solid state, which is one of the rare examples reported so far. For a comprehensive comparison, we synthesized a non-ESIPT methyl derivative (JDF-Me) of JDF capable of showing excimer emission only in the solid state. Further, JDF exhibits normal as well as keto emission in solution, upon addition of water, its poor solvent, that promotes aggregation, the fluorescence emission shows the preponderance of the excimer band in the low energy region. It was also interesting to note that in the solid state (thin films), JDF shows emission beyond the excimer emission, which is wavelength dependent. This is attributed to the formation of diverse clusters leading to the extended delocalization beyond excimers, and represents a clustering-triggered emission ascribing bright red color to the solid JDF. Such mélange of emission characteristics of JDF are responsible for the multicolor emission covering a broad range of electromagnetic spectrum, which is demonstrated by the confocal microscopy images of the JDF recorded in different states. Further, in its aggregated state, JDF recognized Cu²⁺ ions, selectively, manifested in the form of emission quenching via the interaction of Cu²⁺ ions with the oxygen and nitrogen atoms of JDF inhibiting the excimer formation.

Modulation of the Naked‐Eye and Fluorescence Color of a Protonated Boron‐Doped Thiazolothiazole by Anion‐Dependent Hydrogen Bonding

The reaction of a cyclic alkyl(amino)carbene (CAAC)‐stabilized thiazaborolo[5,4‐d]thiazaborole (TzbTzb) with strong Brønsted acids, such as HCl, HOTf (Tf=O₂SCF₃) and [H(OEt₂)₂][BAr^F₄] (Ar^F=3,5‐(CF₃)₂C₆H₃), results in the protonation of both TzbTzb nitrogen atoms. In each case X‐ray crystallographic data show coordination of the counteranions (Cl⁻, OTf⁻, BAr^F₄⁻) or solvent molecules (OEt₂) to the doubly protonated fused heterocycle via hydrogen‐bonding interactions, the strength of which strongly influences the ¹H NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb‐to‐CAAC charge transfer character involved in the S₀→S₁ transition, ultimately enabling fine‐tuning of their absorption and emission spectral features.

Water Molecular Bridge-Induced Selective Dual Polarization in Crystals for Stable Multi-Emitter

In the solid state, the molecular polarization of donor–acceptor (D–A) molecules can be implemented in a simple way via the use of an external polarizing source (e.g., an electric field). However, internal chemical polarization approaches are less studied due to difficulties related to controlling the charge-separation orientation in the solid state. Herein, a series of D–A molecules with both a proton donor and an acceptor were designed. Water-based molecular bridges were then established in their crystal structures, which firmly and alternately connected the proton donor of one molecule and the acceptor of another via an intermolecular H-bond network. In this way, the selective dual polarization of a phenolic hydroxyl group and a pyridinyl group could be achieved, owing to the strengthening of the charge-separation orientation upon the simultaneous deprotonation and protonation of the D–A molecules. This effect led to a 3–5-fold amplification of the molecular dipole moment in the crystal form relative to the monomeric state. On this basis, multi-excitation and multi-emission characteristics were achieved in these charge-separated crystals, endowing them with the ability to visually detect the energy of a light source, covering a wide range of the UV-Vis spectral region. This work provides a practical chemical approach for developing intrinsically polarized systems that can exhibit stable but distinct molecular photophysical properties.

In the solid state, the molecular polarization of donor–acceptor (D–A) molecules can be implemented by internal chemical polarization approaches.

Dual Solution-/Solid-State Emissive Excited-State Intramolecular Proton Transfer (ESIPT) Dyes: A Combined Experimental and Theoretical Approach

Excited-state intramolecular proton transfer (ESIPT) dyes typically show strong solid-state emission, but faint fluorescence intensity is observed in the solution state owing to detrimental molecular motions. This article investigates the influence of direct (hetero)arylation on the optical properties of 2-(2'-hydroxyphenyl)benzoxazole ESIPT emitters. The synthesis of two series of ESIPT emitters bearing substituted neutral or charged aryl, thiophene, or pyridine rings is reported herein along with full photophysical studies in solution and solid states, demonstrating the dual solution-/solid-state emission behavior. Depending on the nature of substitution, several excited-state dynamics are observed: quantitative or partially frustrated ESIPT process or deprotonation of the excited species. Protonation studies revealed that pyridine substitution triggered a strong increase of quantum yield in the solution state for the protonated species owing to favorable quinoidal stabilization. These attractive features led to the development of a second series of dyes with alkyl or aryl pyridinium moieties showing strong tunable solution/solid fluorescence intensity. For each series, ab initio calculations helped rationalize and ascertain their behavior in the excited state and the nature of the emission observed by the experimental results.

Mechanochromic Luminescence of 2,6-Bis(4-biphenyl)isonicotinic Acid via Interconversion of Classical/Frustrated Brönsted Pair

A practicable strategy to a reversible mechanochromic material featuring interconversion of classical/frustrated Brönsted pairs has been established. We report the mechanochromic property of 2,6-bis(4-biphenyl)isonicotinic acid (1), which features a frustrated Brönsted pair in the crystalline form and a classical Brönsted pair after grinding. A large mechanochromic shift was found from 428 to 505 nm. In addition, compound 1 also exhibits acidochromic behavior, which further proves that the formation of an acid-base interaction is responsible for the mechanochromic phenomenon.

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S₁ absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm^-1. Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible nπ* contribution to the S₁' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S₁' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

New [2,2]Fluorenophanes Give Insights into Asymmetric Charge Transfer-Mediated Exciton Delocalization along the π-π Packing Direction

A series of new [2,2]fluorenophanes has been synthesized and characterized; among them, molecules of crystallographically asymmetric anti-[2.2](1,4)(4,1)fluorenophane (K2C-2) aggregate to form one-dimensional supramolecular chain structures through effective intermolecular π-π overlapping. This, in combination with the synergistic intramolecular π-π interaction, leads to prominent dual emission mediated by charge transfer (CT) exciton delocalization. Support of this new insight is given by mapping the transition density along the π-π packing direction where the intramolecular excitation and intermolecular CT coexist in K2C-2.

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (g_abs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (g_PL) of the order of 10⁻³ were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of g_abs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (g_EL) of 1.09 × 10⁻⁴ and −1.02 × 10⁻⁴ at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications.

Mechanically induced single-molecule white-light emission of excited-state intramolecular proton transfer (ESIPT) materials

Described herein is the first example of mechanically induced single-molecule white-light emission based on excited-state intramolecular proton transfer (ESIPT) materials. The mechanism of mechanochromism is clearly disclosed by powder and single crystal X-ray diffraction (XRD) data, infrared spectroscopy, and fluorescence up-conversion measurement, etc. 2-(2'-Hydroxyphenyl)oxazole (6b) with a herringbone packing motif exhibits a predominant keto-form emission, giving off yellowish-green fluorescence. Mechanical grinding transforms the herringbone packing motif into a brickwork packing motif, decreases the intermolecular distances, which results in an enhanced intermolecular charge-transfer interaction, and therefore changes the ESIPT dynamics, leading to an enhanced enol-form emission and white fluorescence. Herringbone-packing 6b is thermodynamically more stable than brickwork-packing 6b. Thus, the latter can convert to the former by solvent fuming or thermal annealing.

Impact of Heteroatom Substitution on Dual-State Emissive Rigidified 2-(2'-hydroxyphenyl)benzazole Dyes: Towards Ultra-Bright ESIPT Fluorophores*

2-(2'-Hydroxyphenyl)benzazole (HBX) fluorophores are well-known excited-state intramolecular proton transfer (ESIPT) emitters largely studied for their synthetic versatility, photostability, strong solid-state fluorescence and ability to engineer dual emission, thus paving the way to applications as white emitters, ratiometric sensors, and cryptographic dyes. However, they are heavily quenched in solution, due to efficient non-radiative pathways taking place as a consequence of the proton transfer in the excited-state. In this contribution, the nature of the heteroring constitutive of these rigidified HBX dyes was modified and we demonstrate that this simple structural modification triggers major optical changes in terms of emission color, dual emission engineering, and importantly, fluorescent quantum yield. Investigation of the photophysical properties in solution and in the solid state of a series of ethynyl-TIPS extended HBX fluorophores, along with ab initio calculations demonstrate the very promising abilities of these dyes to act as bright dual-state emitters, in both solution (even in protic environments) and solid state.

Excited-state intramolecular proton transfer in the kinetic-control regime

A new series of molecules bearing a 2,11-dihydro-1H-cyclopenta[de]indeno[1,2-b]quinoline (CPIQ) chromophore with the N-HN type of intramolecular hydrogen bond are strategically designed and synthesized, among which CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs in solution exhibit a single emission band with an anomalously large Stokes shift, whereas CPIQ-NH2 and CPIQ-NHMe show apparent dual-emission property. This, in combination with time-resolved spectroscopy and the computational approach, leads us to conclude that CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs undergo ultrafast, highly exergonic excited-state intramolecular proton transfer (ESIPT), while a finite rate of ESIPT is observed for CPIQ-NH2 and CPIQ-NHMe with a time constant of 117 ps and 39 ps, respectively, in acetonitrile at room-temperature. Further temperature-dependent studies deduce an appreciable ESIPT barrier for CPIQ-NH2 and CPIQ-NHMe. Different from most of the barrier associated ESIPT molecules that are commonly in the thermodynamic-control regime, i.e. found in the thermal pre-equilibrium between excited normal and proton-transfer tautomer states, CPIQ-NH2 and CPIQ-NHMe cases are in the kinetic-control regime where ESIPT is irreversible with a significant barrier. The barrier is able to be tuned by the electronic properties of the -R group in the NR-H proton donor site, resulting in ratiometric fluorescence for normal versus tautomer emission.

The endeavor of vibration-induced emission (VIE) for dynamic emissions

Organic chromophores with large Stokes shifts and dual emissions are fascinating because of their fundamental and applied interest. Vibration-induced emission (VIE) refers to a tunable multiple fluorescence exhibited by saddle-shaped N,N'-disubstituted-dihydribenzo[a,c]phenazines (DHPs), which involves photo-induced configuration vibrations from bent to planar form along the N-N axis. VIE-active molecules show intrinsic long-wavelength emissions in the unconstrained state (planar state) but bright short-wavelength emissions in the constrained state (bent state). The emission response for VIE-active luminogens is highly sensitive to steric hindrance encountered during the planarization process such that a tiny structural variation can induce an evident change in fluorescence. This can often be achieved by tuning the intensity ratio of short- and long-wavelength bands. In some special cases, the alterations in the emission wavelength of VIE fluorophores can be achieved step by step by harnessing the degree of bending angle motion in the excited state. In this perspective, we summarize the latest progress in the field of VIE research. New bent heterocyclic structures, as novel types of VIE molecules, are being developed, and the general features of the chemical structures are also being proposed. Technologically, novel emission color-tuning approaches and VIE-based probes for visualizing biological activity are presented to demonstrate how the dynamic VIE effect can be exploited for cutting-edge applications.

Persistent organic room temperature phosphorescence: what is the role of molecular dimers?

Molecular dimers have been frequently found to play an important role in room temperature phosphorescence (RTP), but its inherent working mechanism has remained unclear. Herein a series of unique characteristics, including singlet excimer emission and thermally activated delayed fluorescence, were successfully integrated into a new RTP luminogen of CS-2COOCH₃ to clearly reveal the excited-state process of RTP and the special role of molecular dimers in persistent RTP emission.

The first purely organic room temperature phosphorescence (RTP) luminogen, with singlet excimer emission and thermally activated delayed fluorescence (TADF) effect, was successfully developed.

Phenothiazine Scope: Steric Strain Induced Planarization and Excimer Formation

Phenothiazine derivatives based on the 10-phenyl-10H-phenothiazine (NAS) chromophore, namely 7-phenyl-7H-benzo[c]phenothiazine (NAS-1) and 12-phenyl-12H-benzo[a]phenothiazine (NAS-2), were designed and synthesized. NAS-1 and NAS-2 are constitutional isomers with different steric strains imposed on the phenothiazine core moiety. In solution, the more-strained NAS-2 possesses a bent structure and undergoes photoinduced structural planarization (PISP). In the crystal, despite the absence of PISP, bent NAS-2 exhibits prominent excimer emission as well as emission mechanochromism, which is not observed in the planar-like NAS and NAS-1. This unconventional observation results from the bent core structure facilitating π-π stacking of the peripheral naphthalene moieties. Two-photon-coupled depth-dependent emission shows spectral differences between the surface and kernel of the NAS-2 crystal, and is believed to be a general phenomenon, at least in part, for materials exhibiting emission mechanochromism.