Solvent-Controlled Photoswitching of Azobenzene: An Excited State Shuttle.

The light-controlled excited state trans-cis isomerization process is a key to the development of conversion of light energy to mechanical motion at the molecular level. Considerable efforts have been made in tuning the isomerization process with electron donor and acceptor substituents by altering the excited state reaction coordinate. Here, we report a two novel push-pull series of para-diethylamino (DEA) and diphenylamino (DPA) substituted (E)-4'-((4-(diethylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (1) and (E)-4'-((4-(diphenylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (2). Compounds 1 and 2 undergo both photochemical and photophysical excited state deactivation pathways which can be controlled by varying the solvent polarity. These structural motifs of 1 and 2 would undergo torsional motions upon excitation to exhibit either trans→cis photoisomerization or to form a twisted intramolecular charge transfer state and both the process originates from the same excited state and competes with each other. Thus, alternations in the surrounding environment such as solvent polarity, solution viscosity, and protonation were employed to understand the preferential excited state deactivation pathway and thereby these systems could be employed as a new class of azobenzene-based luminescent photochromic molecules. For instance, in nonpolar solvent, toluene photoisomerization is preferred over TICT, but polar solvent, ethanol preferentially stabilizes the TICT state by virtue of N-C rotation which renders the energy barrier unfavourable for photoisomerization. The photostationary state of 1 in toluene is predominantly the Z isomer, whereas in ethanol E isomer is nearly two-fold higher than the Z isomer. These feature sets up a new approach towards the construction of multinary molecular switches and subsequent development in diverse fields.

Electrochemical Response of Redox Amino Acid Encoded Fluorescence Protein for Hydroxychloroquine Sensing.

The sudden rise in the demand has led to large-scale production of hydroxychloroquine (HCQ) in the global market for various diseases such as malaria, rheumatic arthritis, and systemic lupus erythematous and prophylactic treatment of early SARS-CoV-2 outbreak. Thorough monitoring of HCQ intake patients is in high demand; hence, we have developed a redox amino acid encoded fluorescent protein-based electrochemical biosensor for sensitive and selective detection of HCQ. This electrochemical biosensor is generated based on the two-electron transfer process between redox amino acid (3,4-dihydroxy-L-phenylalanine, DOPA) encoded bio-redox protein and the HCQ forms the conjugate. The DOPA residue in the bio-redox protein specifically binds with HCQ, thereby producing a remarkable electrochemical response on the glassy carbon electrode. Experimental results show that the developed biosensor selectively and sensitively detects the HCQ in spiked urine samples. The reagent-free bio-redox capacitor detects HCQ in the range of 90 nM to 4.4 µM in a solution with a detection limit of 58 nM, signal to noise ratio of 3:1, and strong anti-interference ability. Real-time screening, quantification, and relative mean recoveries of HCQ on spiked urine samples were monitored through electron shuttling using bio-redox protein and were found to be 97 to 101%. Overall, the developed bio-redox protein-based sensor has specificity, selectivity, reproducibility, and sensitivity making it potentially attractive for the sensing of HCQ and also applicable to clinical research.

Single-Molecule White-Light-Emitting Starburst Donor-Acceptor Triphenylamine Derivatives and Their Application as Ratiometric Luminescent Molecular Thermometers.

White-light emission (WLE) from a single molecule is a highly desirable alternative to a complex mixture of complementary colour emitters, which suffers from poor stability and reproducibility for potential use in organic electronic devices and lighting applications. We report single-molecule WLE both in solution and thin films by judiciously controlled π-electron delocalisation between the triarylamine subchromophoric units. Triphenylamine (TPA) forms the central core, and the phenyl rings are substituted with the electron-deficient acceptor 3-ethylrhodanine (Rh) and electron-rich donors triphenylamine or carbazole. The enforced biphenyl configuration of the TPA core and the other donors renders the π-conjugation across the entire chromophore poor, thus the individual subchromophoric units retain their individual emission characteristics, which cover all three primary colour emissions, that is, red, green and blue (RGB). TPA-Rh units exhibit broad fluorescence in the green-red region originating from the local excited (LE) state and intramolecular charge transfer state (ICT), strongly influenced by the solvent, water, and temperature. Different fluorescence parameters, including spectral maxima, ratiometric changes in ICT emission at the expense of blue emission from terminal donor units, and changes in lifetime, have a linear relationship with temperature between 180-330 K, thus the molecules can function as a multiparameter luminescent molecular thermometer. A temperature coefficient of 0.19 K-1 in ratiometric fluorescence changes along with a spectral shift of 0.3 nm K-1 and their workability over the wide temperature makes these molecules promising materials for potential applications. At lower temperatures, individual subchromophoric properties subside because of the reduced dihedral angle of biphenyl, and fluorescence from the whole molecule becomes dominant.

Excited State Electronic Interconversion and Structural Transformation of Engineered Red-Emitting Green Fluorescent Protein Mutant.

Red fluorescent proteins with a large Stokes shift offer a limited autofluorescence background and are used in deep tissue imaging. Here, by introducing the free amino group in Aequorea victoria, the electrostatic charges of the p-hydroxybenzylidene imidazolinone chromophore of green fluorescent protein (GFP) have been altered resulting in an unusual, 85 nm red-shifted fluorescence. The structural and biophysical analysis suggested that the red shift is due to positional shift occupancy of Glu222 and Arg96, resulting in extended conjugation and a relaxed chromophore. Femtosecond transient absorption spectra exhibited that the excited state relaxation dynamics of red-shifted GFP (rGFP) (τ4 = 234 ps) are faster compared to the A. victoria green fluorescent protein (τ4 = 3.0 ns). The nanosecond time-resolved emission spectra of rGFP reveal the continuous spectral shift during emission by a solvent reorientation in the chromophore. Finally, the molecular dynamics simulations revealed the rearrangement of the hydrogen bond interactions in the chromophore vicinity, reshaping the symmetric distribution of van der Waals space to fine tune the GFP structure resulting from highly red-shifted rGFP.

Synthesis and photophysics of extended π-conjugated systems of substituted 10-aryl-pyrenoimidazoles.

The synthesis of π-extended 10-aryl-pyrenoimidazoles having different substituents was realised via Ru(ii)-catalyzed oxidative annulation of 10-aryl-pyrenoimidazole with diphenylacetylene. The single crystal X-ray structure of trifluoromethyl and carboxylate substituted annulated-10-aryl-pyrenoimidazoles confirms the near coplanarity of the pyrene and imidazole moieties and the presence of twisted conformation resulting in intermolecular C-Hπ interactions. The lowest energy absorption maximum becomes red-shifted characteristic to the nature of the substituent owing to the extended π-conjugation, and specifically the nitro substituent shows intense absorption in the visible region with the maximum at 440 nm. All the molecules were found to show intense fluorescence both in solution and solid states. Strikingly, 170 nm red-shifted fluorescence with a large Stokes shift ca. 7000 cm-1 for the nitro derivative, a value nearly two-fold higher than the parent compound despite its rigid polyaromatic skeleton was observed. The combination of electron rich π-conjugated aromatic systems with electron deficient substituents induces the intramolecular charge transfer interactions, which has been corroborated with the theoretical calculations.

Tuning the Electronic Properties of 2-Cyano-3-phenylacrylamide Derivatives.

We are the first to report the synthesis of a new class of 2-cyanoarylacrylamide (2-CAA) derivatives and observe that the synthesized 2-CAA shows fluorescence properties due to the formation of a dimeric interaction of hydrogen bonds between carbonyl oxygens and amide hydrogens (C═O···H-N-C═O···H-N···); i.e., dimers are linked through dimeric N-H···O hydrogen bonds. The single-crystal X-ray structure shows molecules to be hydrogen-bonded dimers, which further form a parallel stacking arrangement, mediated by significant π-π interactions. The (1)H NMR and fluorescence spectral studies indicate the coexistence of amide and iminol tautomers in solution, which can be influenced by the nature of the solvent. Further, the excitation-wavelength-dependent fluorescence spectrum and the biexponential fluorescence decay profiles suggest the presence of more than one emitting species; i.e., amide and iminol tautomers coexists in solution. We have also shown that the equilibrium between the two tautomers can be tuned by the judicious choice of electron-donating or -withdrawing substituents.

Charge instability of symmetry broken dipolar states in quadrupolar and octupolar triphenylamine derivatives.

The quadrupolar and octupolar cyano triphenylamines shows symmetry broken dipolar charge transfer state, however, its stability can be controlled by the rotation of N-C bond of amino and phenylene moiety.

Colorimetric detection of fluoride ion by 5-arylidenebarbituric acids: dual interaction mode for fluoride ion with single receptor.

Two 5-arylidenebarbituric acid derivatives (IH and IM) have been synthesized by the Knoevenagel condensation of barbituric acid with 4-N,N-dimethylamino benzaldehyde and studied for anion sensing activities. Both receptors sense fluoride ion with high selectivity and sensitivity and the sensing action has been demonstrated by naked eye detection, UV-visible absorption, and fluorescence spectral changes in the presence of F(-). Indeed, the F(-) sensing mechanism for receptor IH depends on F(-) ion concentration. While at higher concentrations F(-) forms strong hydrogen bonding interaction with the N-H proton of the receptor, at lower concentrations sensing is influenced by the deprotonation of the methylene proton, followed by the chemical reaction, which is also confirmed by the (1)H-NMR technique. On the other hand, when replacing the N-H proton with a methyl group, IM does not show any concentration dependent behaviour with F(-). The F(-) concentration dependent sensing is attributed to the changes in the receptor-anion interaction equilibrium, where at higher F(-) concentrations, F(-) interacts with the receptor through hydrogen bonding and at lower concentrations it induces a chemical reaction.

Benzoselenadiazole fluorescent probes--near-IR optical and ratiometric fluorescence sensor for fluoride ion.

A highly selective and sensitive near-IR optical sensor, benzoselenadiazole based diarylamine (TBS-HN), for fluoride (F(-)) has been designed and synthesized. TBS-HN also shows turn-on ratiometric fluorescence signaling in the presence of F(-) by inhibiting the excited state intramolecular proton transfer (ESIPT) processes.

Synthesis and photophysical properties of N-fused tetraphenylporphyrin derivatives: near-infrared organic dye of [18]annulenic compounds.

A variety of N-fused porphyrin derivatives were prepared and their photophysical properties were investigated. Although intact N-fused tetraarylporphyrins showed almost no emission, introduction of electron-withdrawing groups such as a nitro group and a cyano group on the macrocycles caused significant refinements in their emission efficiency. Long emission wavelengths (900-1000 nm) as well as fairly large Stokes shifts (∼1200 cm(-1)) are exceptionally unique photophysical properties among [18]annulenic compounds, which could be rationalized by the excited state intramolecular proton transfer (ESIPT) process. Relatively weak emission quantum yields (∼5.0 × 10(-4)) and unusually short S(1) state lifetimes (∼13.5 ps) are in good agreement with the ESIPT process. The solvent and substituent effects on the photophysical properties are also discussed in conjunction with the theoretical studies, where the mesityl groups at the meso-positions play a unique role.

Crystallographic, photophysical, NMR spectroscopic and reactivity manifestations of the "8-heteroaryl effect" in 4,4-difluoro-8-(C(4)H(3)X)-4-bora-3a,4a-diaza-s-indacene (X = O, S, Se) (BODIPY) systems.

We have synthesized and fully characterized three novel, yet closely related, heterocyclically meso-substituted (BODIPY) fluorophores 4,4-difluoro-8-(C(4)H(3)X)-4-bora-3a,4a-diaza-s-indacene (X = O, 2-/3-furyl (7/10); Se, 2-selenenyl (9)) through the use of 2-D NMR (COSY, HSQC, and HMBC), single crystal X-ray diffraction, mass spectrometry, elemental analysis, UV-vis spectroscopy, and fluorescent decay lifetimes, for comparison to the previously reported thienyl species (X = S, 2-/3-thienyl (8/11)). Specifically, 7-11 differ formally by chalcogen (O, S, or Se) or chalcogen placement. Solid state comparisons reveal major effects stemming from subtle structural differences which allows for insights into fluorescent crystal engineering. For the 2-heteroatom substitution, an increase in molecular weight (7 < 8 < 9) correlates with an increasing unit cell-volume, a greater orthogonality for the C(4)H(3)X group, and a lower value for Phi(F). Solution and density functional theory (DFT) results reveal interesting platforms for potential in fluorescent studies for neurology. 2-Heterocyclic species show larger lambda(abs,max/em,max) values relative to 3-heterocyclic ones, based on electron withdrawing effects. 10 has the greatest Phi(F) value herein (0.25, toluene). Fluorescence lifetimes were found to be 2.60 (7), 0.74 (8), 0.27 (9), 4.26 (10), and 1.86 ns (11); lambda(em,max) decay was studied for 8. Heterocyclic differences give rise to somewhat different pyrrolic NMR spectroscopic shifts as well. These compounds resist decomposition as seen from titrations with H(2)O(2), and uniformly undergo lambda(abs,max) red-shifting and lowered Phi(F) values as they become brominated with Br(2).

Excess polarizability reveals exciton localization/delocalization controlled by linking positions on porphyrin rings in butadiyne-bridged porphyrin dimers.

We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly beta-to-beta (1Zn) and doubly beta-to-beta (2Zn) positions using the transient dc photoconductivity (TDCP) technique. The excess polarizability volumes of the singlet exciton for 1Zn and 2Zn are 110 and 270 A(3), respectively, while those of the triplet exciton are approximately 100 A(3) for both dimers. Our measurements suggest that the singlet exciton is mainly localized on one porphyrin subunit for 1Zn, similar to the case for the porphyrin monomer. While the exciton is fully delocalized on two porphyrin subunits in the case of meso-to-meso linked dimer (3Zn), the extent of exciton localization/delocalization for doubly beta-to-beta linked dimer lies between those of singly beta-to-beta and meso-to-meso linked dimers. Electronic structure calculations show that the dramatically different extents of exciton localization/delocalization are the results of frontier orbital coefficients being small at beta positions but large at meso positions. Two butadiyne linkages between the porphyrins at beta positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity.

Synthesis of arylated perylene bisimides through C-H bond cleavage under ruthenium catalysis.

Treatment of perylene bisimide (PBI) with various arylboronates in the presence of a ruthenium catalyst provides tetraarylated PBIs at 2,5,8,11-positions in good yields with perfect regioselectivity. The electronic nature of the introduced aryl substituents has a significant impact on their optical and electronic properties. This protocol has been applied to the synthesis of a water-soluble emissive PBI derivative.

Versatile photophysical properties of meso-aryl-substituted subporphyrins: dipolar and octupolar charge-transfer interactions.

Donor-acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso-phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types of charge-transfer interactions, that is, from center to periphery and periphery to center depending on the peripheral substitution, in which the subporphyrin moiety plays a dual role as both donor and acceptor. Based on the solvent-polarity-dependent photophysical properties, we have shown that the fluorescence emission of para isomers originates from the solvatochromic, dipolar, symmetry-broken, and relaxed excited states, whereas the non-solvatochromic fluorescence of meta isomers is of the octupolar type with false symmetry breaking. The restricted meso-(4-aminophenyl) rotation at low temperature prevents the intramolecular charge-transfer (ICT)-forming process. The two-photon absorption (TPA) cross-section values were determined by photoexcitation at 800 nm in nonpolar toluene and polar acetonitrile solvents to see the effect of ICT on the TPA processes. The large enhancement in the TPA cross-section value of approximately 3200 GM (1 GM = 10(-50) cm(4) s photon(-1)) with donor-acceptor substitution has been attributed to the octupolar effect and ICT interactions. A correlation was found between the electron-donating/-withdrawing abilities of the peripheral groups and the TPA cross-section values, that is, p-aminophenyl > m-aminophenyl > nitrophenyl. The increased stability of octupolar ICT interactions in highly polar solvents enhances the TPA cross-section value by a factor of approximately 2 and 4, respectively, for p-amino- and m-nitrophenyl-substituted subporphyrins. On the other hand, the stabilization of the symmetry-broken, dipolar ICT state gives rise to a negligible impact on the TPA processes.

Meso-beta doubly linked Zn(II) porphyrin trimers: distinct anti-versus-syn effects on their photophysical properties.

Meso-beta doubly linked Zn(II) porphyrin dimer 1 and anti- and syn-trimers 2 and 3 were synthesized by DDQ-Sc(OTf)(3) oxidation of the corresponding meso-beta singly linked porphyrin precursors. The absorption and fluorescence spectra and TPA cross-section values of 2 and 3 are distinctly different, reflecting their molecular shapes.

Peripheral hexabromination, hexaphenylation, and hexaethynylation of meso-aryl-substituted subporphyrins.

Effective peripheral fabrication methods of meso-aryl-substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki-Miyaura coupling and Stille coupling, respectively, in good yields. X-ray crystal structures of 2 b, 3 b, 4 b, and 5 a revealed preservation of the bowl-shaped bent structures with bowl depths similar to that of 1. Hexaethynylated subporphyrins exhibit large two-photon-absorption cross-sections due to effective delocalization of the conjugated network to the ethynyl substituents.

meso-(4-(N,N-dialkylamino)phenyl)-substituted subporphyrins: remarkably perturbed absorption spectra and enhanced fluorescence by intramolecular charge transfer interactions.

A series of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subporphyrins was synthesized by means of Buchwald-Hartwig amination protocol. Substitution of the amino group at the 4-position of the meso-phenyl substituent resulted in a remarkable red shift in the absorption spectra and drastic enhancement of fluorescence intensity probably as a consequence of intramolecular CT interaction. These characteristics have been utilized to construct a cation-sensing system by appending a 1-aza-15-crown-5 unit to subporphyrin that displays large spectral changes upon cation binding.

Structure-property relationship for two-photon absorbing multiporphyrins: supramolecular assembly of highly-conjugated multiporphyrinic ladders and prisms.

Two-photon absorption (TPA) phenomena of a series of single-strand as well as supramolecular self-assembled ladders and prisms of highly conjugated ethyne bridged multiporphyrin dimer, trimer, and star shaped pentamer have been investigated. The ligand mediated self-assembled supramolecular structures were characterized by UV-visible spectroscopy and small- and wide-angle X-ray scattering (SAXS/WAXS) analysis. The TPA cross section values of multiporphyrins increase nonlinearly from approximately 100 to approximately 18000 GM with an increased number of porphyrin units and elongated pi-conjugation length by virtue of charge transfer and excited-state cumulenic configurations. The observed opposite TPA behavior between their supramolecular ladder and prism configurations necessitates the importance of interstrand interactions between the multiporphyrinic units and the overall shape of the assembly. Furthermore, the diminished TPA cross section of the pentamer, despite the increased pi-conjugation resulting from duplex formation suggests that destabilizing the essential functional configurations at the cost of elongation of pi-delocalization pathway must cause unfavorable effects. We have also shown that one- and two-photon allowed energy-levels of linear multiporphyrins are nearly isoenergetic and the latter transition originates exclusively from the extent of pi-delocalization within the molecule. The identical TPA maximum position of the trimer and pentamer indicates that the TPA of the pentamer arises only from its basic trimer unit in spite of its extended two-dimensional pi-conjugation pathway involving five porphyrinic units.