A new approach exploiting thermally activated delayed fluorescence molecules to optimize solar thermal energy storage.

We propose a new concept exploiting thermally activated delayed fluorescence (TADF) molecules as photosensitizers, storage units and signal transducers to harness solar thermal energy. Molecular composites based on the TADF core phenoxazine-triphenyltriazine (PXZ-TRZ) anchored with norbornadiene (NBD) were synthesized, yielding compounds PZDN and PZTN with two and four NBD units, respectively. Upon visible-light excitation, energy transfer to the triplet state of NBD occurred, followed by NBD → quadricyclane (QC) conversion, which can be monitored by changes in steady-state or time-resolved spectra. The small S1-T1 energy gap was found to be advantageous in optimizing the solar excitation wavelength. Upon tuning the molecule's triplet state energy lower than that of NBD (61 kcal/mol), as achieved by another composite PZQN, the efficiency of the NBD → QC conversion decreased drastically. Upon catalysis, the reverse QC → NBD reaction occurred at room temperature, converting the stored chemical energy back to heat with excellent reversibility.

The azatryptophan-based fluorescent platform for in vitro rapid screening of inhibitors disrupting IKKß-NEMO interaction.

The nuclear factor-κB (NF-κB) plays an important role in inflammatory and immune responses. Aberrant NF-κB signaling is implicated in multiple disorders, including cancer. Targeting the regulatory scaffold subunit IκB kinase γ (IKKγ/NEMO) as therapeutic interventions could be promising due to its specific involvement in canonical NF-κB activation without interfering with non-canonical signaling. In this study, the use of unnatural amino acid substituted IKKß with unique photophysical activity to sense water environment changes upon interaction with NEMO provides a powerful in vitro screening platform that would greatly facilitate the identification of compounds having the potential to disrupt IKKß-NEMO interaction, and thus specifically modulate the canonical NF-κB pathway. We then utilized a competitive binding platform to screen the binding ability of a number of potential molecules being synthesized. Our results suggest that a lead compound (-)-PDC-099 is a potent agent with ascertained potency to disrupt IKKß-NEMO complex for modulating NF-κB canonical pathway.

Excited-State Proton Transfer in 3-Cyano-7-azaindole: From Aqueous Solution to Ice.

We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (∼472 nm) emission bands. Detailed temperature-dependent studies showed that the values of activation free energy (Δ G‡) were similar between N-H and N-D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium ( Keq) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (∼450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water-ice interface property.

The Cyclic Hydrogen-Bonded 6-Azaindole Trimer and its Prominent Excited-State Triple-Proton-Transfer Reaction.

The compound 6-azaindole undergoes self-assembly by formation of N(1)-H⋅⋅⋅N(6) hydrogen bonds (H bonds), forming a cyclic, triply H-bonded trimer. The formation phenomenon is visualized by scanning tunneling microscopy. Remarkably, the H-bonded trimer undergoes excited-state triple proton transfer (ESTPT), resulting in a proton-transfer tautomer emission maximized at 435 nm (325 nm of the normal emission) in cyclohexane. Computational approaches affirm the thermodynamically favorable H-bonded trimer formation and the associated ESTPT reaction. Thus, nearly half a century after Michael Kasha discovered the double H-bonded dimer of 7-azaindole and its associated excited-state double-proton-transfer reaction, the triply H-bonded trimer formation of 6-azaindole and its ESTPT reaction are demonstrated.

A study of the competitive multiple hydrogen bonding effect and its associated excited-state proton transfer tautomerism.

1,8-Dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA) has been strategically designed and synthesized with the aim to study the competitive multiple hydrogen bonding (H-bonding) effect and the associated excited-state intramolecular proton transfer reaction (ESIPT). In nonpolar solvents such as cyclohexane, equilibrium exists between the two H-bonding isomers DHDA-23_OO and DHDA-23_OI, both of which possess double intramolecular H-bonds. In polar, aprotic solvents such as CH2Cl2, DHDA-23_OO becomes the predominant species. Due to various degrees of H-bond induced changes of electronic configuration each isomer reveals a distinct absorption feature and excited-state behavior, in which DHDA-23_OI in cyclohexane undergoes double ESIPT in a stepwise manner, giving the first and second proton-transfer tautomer emissions maximized at ∼500 nm and 660 nm, respectively. As for DHDA-23_OO both single and double ESIPT are prohibited, resulting in an intense normal 450 nm emission band. In a single crystal DHDA-23_OI is the dominant species, which undergoes excited state double proton transfer, giving intense emission bands at 530 nm and 650 nm. The mechanism associated with competitive multiple H-bonding energetics and ESIPT was underpinned by detailed spectroscopy/dynamics and computational approaches.

The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution.

3-Me-2,6-diazaindole ((2,6-aza)Ind) was strategically designed and synthesized to probe water molecule catalyzed excited-state proton transfer in aqueous solution. Upon electronic excitation (λmax ∼ 300 nm), (2,6-aza)Ind undergoes N(1)-H to N(6) long-distance proton transfer in neutral H2O, resulting in normal (340 nm) and proton-transfer tautomer (480 nm) emissions with an overall quantum yield of 0.25. The rate of the water-catalyzed proton transfer shows a prominent H/D kinetic isotope effect, which is determined to be 8.3 × 108 s-1 and 4.7 × 108 s-1 in H2O and D2O, respectively. Proton inventory experiments indicate the involvement of two water molecules and three protons, which undergo a relay type of excited-state triple proton transfer (ESTPT) in a concerted, asynchronous manner. The results demonstrate for the first time the fundamental of triple proton transfer in pure water for azaindoles as well as pave a new avenue for 2,6-diazatryptophan, an analogue of tryptophan exhibiting a similar ESTPT property with (2,6-aza)Ind, to probe biowaters in proteins.

The In Situ Tryptophan Analogue Probes the Conformational Dynamics in Asparaginase Isozymes.

Dynamic water solvation is crucial to protein conformational reorganization and hence to protein structure and functionality. We report here the characterization of water dynamics on the L-asparaginase structural homology isozymes L-asparaginases I (AnsA) and II (AnsB), which are shown via fluorescence spectroscopy and dynamics in combination with molecular dynamics simulation to have distinct catalytic activity. By use of the tryptophan (Trp) analog probe 2,7-diaza-tryptophan ((2,7-aza)Trp), which exhibits unique water-catalyzed proton-transfer properties, AnsA and AnsB are shown to have drastically different local water environments surrounding the single Trp. In AnsA, (2,7-aza)Trp exhibits prominent green N(7)-H emission resulting from water-catalyzed excited-state proton transfer. In stark contrast, the N(7)-H emission is virtually absent in AnsB, which supports a water-accessible and a water-scant environment in the proximity of Trp for AnsA and AnsB, respectively. In addition, careful analysis of the emission spectra and corresponding relaxation dynamics, together with the results of molecular dynamics simulations, led us to propose two structural states associated with the rearrangement of the hydrogen-bond network in the vicinity of Trp for the two Ans. The water molecules revealed in the proximity of the Trp residue have semiquantitative correlation with the observed emission spectral variations of (2,7-aza)Trp between AnsA and AnsB. Titration of aspartate, a competitive inhibitor of Ans, revealed an increase in N(7)-H emission intensity in AnsA but no obvious spectral changes in AnsB. The changes in the emission profiles reflect the modulation of structural states by locally confined environment and trapped-water collective motions.

Insight into the mechanism and outcoupling enhancement of excimer-associated white light generation.

Fundamental insight into excimer formation has been gained by using 9,10-bis[4-(9-carbazolyl)phenyl]anthracene] (Cz9PhAn) as a probe. Cz9PhAn exhibits a highly emissive blue fluorescence in solution and is found to emit a panchromatic white light spectrum (400-750 nm) in film, powder and single crystal, in which an additional excimer band appears at ∼550 nm. Detailed structural analyses, emission relaxation dynamics and a theoretical approach conclude the formation of an anthracene*/phenyl ring excimer through an overlap between π* (anthracene) and π (phenyl ring) orbitals in a face-to-edge stacking orientation. The rate of excimer formation is determined to be 2.2 × 109 s-1 at room temperature, which requires coupling with lattice motion with an activation energy of 0.44 kcal mol-1. Exploiting Cz9PhAn as a single emitter, a fluorescent white organic light emitting diode (WOLED) is fabricated with a maximum external quantum efficiency (ηext) of 3.6% at 1000 cd m-2 (4.2 V) and Commission Internationale de L'Eclairage (CIE) coordinates of (0.30, 0.33). The white-light Cz9PhAn reveals a preferred orientation of the transition dipole moment in the emitting layer to enhance light outcoupling. This non-doped, single component (Cz9PhAn) WOLED greatly reduces the complexity of the fabrication process, rendering a green and cost-effective alternative among the contemporary display/lighting technologies.

Excited-state intramolecular proton-transfer reaction demonstrating anti-Kasha behavior.

We report unusual photophysical properties observed on two newly designed 3-hydroxychromone derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT) reaction. The efficiency of ESIPT reaction is greatly enhanced upon excitation with high energy quanta to S n (n > 1) levels in low-polarity solvents. Based on detailed analyses of excitation and emission spectra as well as time-resolved emission kinetics we derive that conditions, in which this phenomenon contradicting Kasha's rule is observed, are quite different from that for observation of anti-Kasha emission.

Probing the polarity and water environment at the protein-peptide binding interface using tryptophan analogues.

7-Azatryptophan and 2,7-diazatryptophan are sensitive to polarity changes and water content, respectively, and should be ideal for studying protein-protein and protein-peptide interactions. In this study, we replaced the tryptophan in peptide Baa (LKWKKLLKLLKKLLKLG-NH2) with 7-azatryptophan or 2,7-diazatryptophan, forming (7-aza)Trp-Baa and (2,7-aza)Trp-Baa, to study the calmodulin (CaM)-peptide interaction. Dramatic differences in the (7-aza)Trp-Baa and (2,7-aza)Trp-Baa fluorescence properties between free peptide in water and calmodulin-bound peptide were observed, showing a less polar and water scant environment at the binding interface of the peptide upon calmodulin binding. The affinity of the peptides for binding CaM followed the trend Baa (210±10 pM)<(7-aza)Trp-Baa (109±5 pM)<(2,7-aza)Trp-Baa (45±2 pM), showing moderate increase in binding affinity upon increasing the number of nitrogen atoms in the Trp analogue. The increased binding affinity may be due to the formation of more hydrogen bonds upon binding CaM for the Trp analogue with more nitrogen atoms. Importantly, the results demonstrate that (7-aza)Trp and (2,7-aza)Trp are excellent probes for exploring the environment at the interface of protein-peptide interactions.

One-Pot Dichotomous Construction of Inside-Azayohimban and Pro-Azayohimban Systems via an Enantioselective Organocatalytic Cascade; Their Use as a Model to Probe the (Aza-)Indole Local Solvent Environment.

A one-pot enantioselective synthesis of 7-azaindole-octahydroisoquinolin-3-one and an inside-aza-yohimbane system containing five contiguous stereogenic centers with high enantioselectivities (>99% ee) was achieved. The prepared highly functionalized polycyclic system provides a model for probing the solvent catalyzed proton transfer reaction and mimicking the local environment of the tryptophan moiety in proteins.

Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde.

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S0 →S1 (π-π*) absorption at ∼400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA* → TB*) and backward (TA* ← TB*) proton transfer were determined to be (1.7 ps)(-1) and (3.6 ps)(-1), respectively. The fast equilibrium leads to identical population lifetimes of ∼54 ps for both TA* and TB* tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.

Water-catalyzed excited-state proton-transfer reactions in 7-azaindole and its analogues.

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pKa, pKa*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with -H (7AI) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)-H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)-H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)-CH3 tautomer species, for which pKa* of the cationic form, that is, the N(7)-CH3N(1)-H(+) species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0-14.3 (H- 14.2).

A new series of fluorescent indicators for super acids.

The photophysical properties of fluorescent Hammett acidity indicator derived from 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3-a:3',2'-j]acridine (1a), 6-bis(pyrido[3,2-g]indol-2'-yl)pyridine (1b) and their analogues have been investigated in sulfuric acid solutions by means of absorption, fluorimetry, relaxation dynamics and computational approach. These new indicators undergo a reversible protonation process in the Hammett acidity range of H0 < 0, accompanied by a drastic increase of the bright blue-green (1a) or yellow (1b) fluorescence intensity upon increasing the acidity. For 1a in H2 SO4 , the emission yield increases as large as 200 folds from pH = -0.41 to the Hammett acidity range of -5.17, the results of which are rationalized by a much increase of the steric hindrance upon third protonation toward the central pyridinic site, together with their accompanied changes of electronic configuration from charge transfer to a delocalized ππ* character in the lowest lying excited state. The combination of 1a and 1b renders a wide and linear range of H0 measurement from -1.2 to -5.1 detected by highly intensive fluorescence.

Probing water environment of Trp59 in ribonuclease T1: insight of the structure-water network relationship.

In this study, we used the tryptophan analogue, (2,7-aza)Trp, which exhibits water catalyzed proton transfer isomerization among N(1)-H, N(7)-H, and N(2)-H isomers, to probe the water environment of tryptophan-59 (Trp59) near the connecting loop region of ribonuclease Tl (RNase T1) by replacing the tryptophan with (2,7-aza)Trp. The resulting (2,7-aza)Trp59 triple emission bands and their associated relaxation dynamics, together with relevant data of 7-azatryptophan and molecular dynamics (MD) simulation, lead us to propose two Trp59 containing conformers in RNase T1, namely, the loop-close and loop-open forms. Water is rich in the loop-open form around the proximity of (2,7-aza)Trp59, which catalyzes (2,7-aza)Trp59 proton transfer in the excited state, giving both N(1)-H and N(7)-H isomer emissions. The existence of N(2)-H isomer in the loop-open form, supported by the MD simulation, is mainly due to the specific hydrogen bonding between N(2)-H proton and water molecule that bridges N(2)-H and the amide oxygen of Pro60, forming a strong network. The loop-close form is relatively tight in space, which squeezes water molecules out of the interface of α-helix and ß2 strand, joined by the connecting loop region; accordingly, the water-scant environment leads to the sole existence of the N(1)-H isomer emission. MD simulation also points out that the Trp-water pairs appear to preferentially participate in a hydrogen bond network incorporating polar amino acid moieties on the protein surface and bulk waters, providing the structural dynamic features of the connecting loop region in RNase T1.

New six- and seven-membered ring pyrrole-pyridine hydrogen bond systems undergoing excited-state intramolecular proton transfer.

New molecules, and , possessing six- and seven-membered ring pyrrole-pyridine hydrogen bonds, respectively, are designed and synthesized, which undergo excited-state intramolecular proton transfer with distinct reaction dynamics.

Locked ortho- and para-core chromophores of green fluorescent protein; dramatic emission enhancement via structural constraint.

We report the design strategy and synthesis of a structurally locked GFP core chromophore p-LHBDI, its ortho-derivative, o-LHBDI, and H2BDI possessing both para- and ortho-hydroxyl groups such that the inherent rotational motion of the titled compounds has been partially restricted. o-LHBDI possesses a doubly locked configuration, i.e., the seven-membered ring hydrogen bond and five-membered ring C(4-5-10-13-14) cyclization, from which the excited-state intramolecular proton transfer takes place, rendering a record high tautomer emission yield (0.18 in toluene) and the generation of amplified spontaneous emission. Compared with their unlocked counterparts, a substantial increase in the emission yield is also observed for p-LHBDI and H2BDI in anionic forms in water, and accordingly the structure versus luminescence relationship is fully discussed based on their chemistry and spectroscopy aspect. In solid, o-LHBDI exhibits an H-aggregate-like molecular packing, offers narrow-bandwidth emission, and has been successfully applied to fabricate a yellow organic light emitting diodes (λmax = 568 nm, ηext = 1.9%) with an emission full width at half-maximum as narrow as 70 nm.

3-Iodo-1H-pyrazolo-[3,4-b]pyridine.

The title compound, C6H4IN3, is essentially planar, with a dihedral angle of 0.82 (3)° between the planes of the pyridine and pyrazole rings. In the crystal, pairs of mol-ecules are connected into inversion dimers through N-H⋯N hydrogen bonds. C-I⋯N halogen bonds link the dimers into zigzag chains parallel to the b-axis direction. The packing also features π-π stacking inter-actions along (110) with inter-planar distances of 3.292 (1) and 3.343 (1) Å, and centroid-centroid distances of 3.308 (1) and 3.430 (1) Å.

Probing water micro-solvation in proteins by water catalysed proton-transfer tautomerism.

Scientists have made tremendous efforts to gain understanding of the water molecules in proteins via indirect measurements such as molecular dynamic simulation and/or probing the polarity of the local environment. Here we present a tryptophan analogue that exhibits remarkable water catalysed proton-transfer properties. The resulting multiple emissions provide unique fingerprints that can be exploited for direct sensing of a site-specific water environment in a protein without disrupting its native structure. Replacing tryptophan with the newly developed tryptophan analogue we sense different water environments surrounding the five tryptophans in human thromboxane A2 synthase. This development may lead to future research to probe how water molecules affect the folding, structures and activities of proteins.

4-(1H-Pyrrolo-[2,3-b]pyridin-2-yl)pyridine.

The asymmetric unit of the title compound, C12H9N3, contains two independent mol-ecules in which the dihedral angle between the pyridine and aza-indole rings are 8.23 (6) and 9.89 (2)°. In the crystal, both types of mol-ecule are connected by pairs of N-H-N hydrogen bonds into inversion dimers.