TDDFT Study of the Electronic Structure,Absorption and Emission Spectra of the Light Emitters of the Amazing Firefly Bioluminescence and Solvation Effects on the Spectra

The ground and excited state properties of luciferin (LH₂) and oxyluciferin (OxyLH₂), the bioluminescent chemicals in the firefly, have been characterized using density functional theory (DFT) and time dependent DFT (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin were predicted with a self‐consistent isodensity polarized continuum model of the solvent using TDDFT. The S₀→S₁ vertical excitation energies in the gas phase and in water were obtained. Optimizations of the excited state geometries permitted the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both of the absorption and emission spectra on proceeding from the gas phase to aqueous solution were also predicted.     

A Dual‐Color Far‐Red to Near‐Infrared Firefly Luciferin Analogue Designed for Multiparametric Bioluminescence Imaging

Red‐shifted bioluminescent emitters allow improved in vivo tissue penetration and signal quantification, and have led to the development of beetle luciferin analogues that elicit red‐shifted bioluminescence with firefly luciferase (Fluc). However, unlike natural luciferin, none have been shown to emit different colors with different luciferases. We have synthesized and tested the first dual‐color, far‐red to near‐infrared (nIR) emitting analogue of beetle luciferin, which, akin to natural luciferin, exhibits pH dependent fluorescence spectra and emits bioluminescence of different colors with different engineered Fluc enzymes. Our analogue produces different far‐red to nIR emission maxima up to λ_max=706 nm with different Fluc mutants. This emission is the most red‐shifted bioluminescence reported without using a resonance energy transfer acceptor. This improvement should allow tissues to be more effectively probed using multiparametric deep‐tissue bioluminescence imaging.     

Theoretical Study of Dinoflagellate Bioluminescence

Dinoflagellates are the most ubiquitous luminescent protists in the marine environment and have drawn much attention for their crucial roles in marine ecosystems. Dinoflagellate bioluminescence has been applied in underwater target detection. The luminescent system of dinoflagellates is a typical luciferin–luciferase one. However, the excited‐state oxyluciferin is not the light emitter of dinoflagellate bioluminescence as in most luciferin–luciferase bioluminescent organisms. The oxyluciferin of bioluminescent dinoflagellates is not fluorescent, whereas its luciferin emits bright fluorescence with similar wavelength of the bioluminescence. What is the light emitter of dinoflagellate bioluminescence and what is the chemical process of the light emission like? These questions have not been answered by the limited experimental evidence so far. In this study, for the first time, the density functional calculation is employed to investigate the geometries and properties of luciferin and oxyluciferin of bioluminescent dinoflagellate. The calculated results agree with the experimental observations and indicate the luciferin or its analogue, rather than oxyluciferin, is the bioluminophore of dinoflagellate bioluminescence. A rough mechanism involving energy transfer is proposed for dinoflagellate bioluminescence.     

Mechanisms in the quantum yield of Cypridina bioluminescence

Abstract— –The influence of temperature, pH, salts, and reactant concentrations on the biolumin-escent oxidation of Cypridina luciferin catalyzed by Cypridina luciferase indicates a highest quantum yield φ (einsteins per mole of luciferin oxidized) of 0.31 in H₂O, or 0.33 in 99% D₂O. With the aid of data on fluorescence of the light-emitting oxyluciferin-luciferase complex, and of oxyluciferin in diglyme, partial explanations are suggested for the observed variations in φ, including the relatively low φ, of 0.03 for chemiluminescence of luciferin in organic solvents, wherein a different pathway of luciferin degradation, as indicated by chromatographic evidence, results in much less population of the excited state.     

A Theoretical Analysis of the Potential Role of π–π Stacking Interactions in the Photoprotolytic Cycle of Firefly Luciferin

Firefly oxyluciferin is a photoacid that presents a pH‐sensitive fluorescence, which results from pH‐dependent changes on the conformation of self‐aggregated π–π stacking complexes. Luciferin is a derivative of oxyluciferin with very similar fluorescence and photoacidic properties. This similarity indicates that luciferin is also expected to be able to form π–π stacking complexes, but no pH‐sensitive fluorescence is found for this compound. Here, a theoretical approach is used to rationalize this finding. We have found that luciferin only forms π–π stacking complexes in the ground state at acidic pH. At basic pH and in the excited state, luciferin is present as a dianion. This species is not able to self‐aggregate, owing to repulsive electrostatic interactions. Thus, this emissive species is not subject to π–π stacking interactions; this explains its pH‐insensitive fluorescence.     

Structure-fluorescence relationship of push-pull 2-phenylbenzothiazole derivatives designed based on the firefly light-emitter

6-Dimethylamino-2-phenylbenzothiazole (1a) mimicking the firefly oxyluciferin structure and the derivatives with an electron-withdrawing substituent on the phenyl group were prepared, and their fluorescence properties were investigated in various solvents. 1a showed solvatochromic fluorescence with good fluorescence quantum yields (Φ_f >0.8). The introduction of an electron-withdrawing group led to a red-shift of the emission maximum. In particular, the derivatives with the 2,2-dicyanoethenyl and (1,3-dihydro-1,3-dioxo-2H-inden-2-ylidene)methyl groups showed near-infrared fluorescence in chloroform. In addition, the derivative with the phenylimine moiety showed efficient solid-state fluorescence, resulted from a molecular arrangement inhibiting intermolecular interactions for quenching the fluorescence state in crystals.     

Synthesis of Firefly Luciferin Analogues and Evaluation of the Luminescent Properties

Five new firefly luciferin ( 1 ) analogues were synthesized and their light emission properties were examined. Modifications of the thiazoline moiety in 1 were employed to produce analogues containing acyclic amino acid side chains ( 2 – 4 ) and heterocyclic rings derived from amino acids ( 5 and 6 ) linked to the benzothiazole moiety. Although methyl esters of all of the synthetic derivatives exhibited chemiluminescence activity, only carboluciferin ( 6 ), possessing a pyrroline‐substituted benzothiazole structure, had bioluminescence (BL) activity (λ_max=547 nm). Results of bioluminescence studies with AMP‐carboluciferin (AMP=adenosine monophosphate) and AMP‐firefly luciferin showed that the nature of the thiazoline mimicking moiety affected the adenylation step of the luciferin–luciferase reaction required for production of potent BL. In addition, BL of 6 in living mice differed from that of 1 in that its luminescence decay rate was slower.     

Intensely Fluorescent Azobenzenes: Synthesis,Crystal Structures,Effects of Substituents,and Application to Fluorescent Vital Stain

2‐[Bis(pentafluorophenyl)boryl]azobenzenes bearing hydrogen, methoxy, dimethylamino, trifluoromethyl, fluoro, n‐butyl, and tert‐butyldimethylsiloxy groups at the 4′‐position or methoxy and bromo groups at the 4‐position have been synthesized. The 4‐bromo group of the 2‐boryl‐4‐bromoazobenzene derivative was converted to phenyl and diphenylamino groups by palladium‐catalyzed reactions. The absorption and fluorescence properties have been investigated using UV/Vis and fluorescence spectroscopy. The 2‐borylazobenzenes emitted an intense green, yellow, and orange fluorescence, in marked contrast to the usual azobenzene fluorescence. The 4′‐siloxy derivative showed the highest fluorescence quantum yield (0.90) among those reported for azobenzenes to date. The correlation between the substituent and the fluorescence properties was elucidated by studying the effect of the substituent on the relaxation process and from DFT and TD‐DFT calculations. An electron‐donating group at the 4′‐position was found to be important for an intense emission. Application of fluorescent azobenzenes as a fluorescent vital stain for the visualization of living tissues was also investigated by microinjection into Xenopus embryos, suggesting these compounds are nontoxic towards embryos.     

Synthesis and characterization of new fluorescent styrene-containing carborane derivatives: the singular quenching role of a phenyl substituent

A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1‐Ph‐1,2‐C₂B₁₀H₁₁, 1‐Me‐1,2‐C₂B₁₀H₁₁ and 1,2‐C₂B₁₀H₁₂ with one equivalent of 4‐vinylbenzyl chloride leads to the formation of compounds 1 – 3 , whereas the reaction of the dilithium salt of 1,2‐C₂B₁₀H₁₂ with two equivalents of 4‐vinylbenzyl chloride gives disubstituted compound 4 . The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1 – 4 were analyzed by X‐ray diffraction. All compounds, except 1 , display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second C_cluster atom. In general, closo compounds 2 – 4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the C_cluster results in an excellent electron‐acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge‐separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.     

Environment‐Sensitive Probes Containing a 2,6‐Diethynylpyridine Motif for Fluorescence Turn‐On Detection and Induction of Nanoarchitectures of Human Telomeric Quadruplex

Bis(phenylethynyl)pyridylcarboxamides with amide side chains at the para position of the NH₂ group possess strong solvatochromic properties compared with the meta analogues. Fluorescence binding titrations show that these probes exhibit remarkable fluorescence turn‐on responses upon interacting with the human telomeric G‐quadruplex (h‐TELO). Förster resonance energy transfer melting analysis shows the high selectivity of these probes for h‐TELO over duplex DNA. Isothermal titration calorimetry, as well as UV/Vis and fluorescence spectroscopy studies, show that the meta analogue has a twofold binding affinity for h‐TELO over the para analogue. The noncovalent interaction of these small‐molecule probes with h‐TELO has been used to regulate the assembly of novel supramolecular nanoarchitectures.     

Asymmetrical Fluorene[2,3‐b]benzo[d]thiophene Derivatives: Synthesis,Solid‐State Structures,and Application in Solution‐Processable Organic Light‐Emitting Diodes

A series of novel asymmetrical fused compounds containing the backbone of fluorene[2,3‐b]benzo[d]thiophene (FBT) were effectively synthesized and fully characterized. Single‐crystal X‐ray studies demonstrated that the length of the substituent side chains greatly affects the solid‐state packing of the obtained fused compounds. DFT, photophysical, and electrochemical studies all showed that the FBTs have large band gaps, low‐lying HOMO energy levels, and therefore good stability toward oxidation. Moreover, the substituents strongly influence the fluorescence properties of the resulting FBT derivatives. The di‐n‐hexyl compound exhibits intense fluorescence in solution with the highest quantum yield of up to 91 %. Solution‐processed green phosphorescent organic light‐emitting diodes with the di‐n‐butyl derivative as the host material exhibited a maximum brightness of 14 185 cd m^?2 and a luminescence efficiency of 12 cd A^?1.     

Photophysical Properties and Electronic Structure of Bacteriochlorin–Chalcones with Extended Near‐Infrared Absorption

Synthetic bacteriochlorins enable systematic tailoring of substituents about the bacteriochlorin chromophore and thereby provide insights concerning the native bacteriochlorophylls of bacterial photosynthesis. Nine free‐base bacteriochlorins (eight prepared previously and one prepared here) have been examined that bear diverse substituents at the 13‐ or 3,13‐positions. The substituents include chalcone (3‐phenylprop‐2‐en‐1‐onyl) derivatives with groups attached to the phenyl moiety, a “reverse chalcone” (3‐phenyl‐3‐oxo‐1‐enyl), and extended chalcones (5‐phenylpenta‐2,4‐dien‐1‐onyl, retinylidenonyl). The spectral and photophysical properties (τ_s, Φ_f, Φ_ic, Φ_isc, τ_T, k_f, k_ic, k_isc) of the bacteriochlorins have been characterized. The bacteriochlorins absorb strongly in the 780–800 nm region and have fluorescence quantum yields (Φ_f) in the range 0.05–0.11 in toluene and dimethylsulfoxide. Light‐induced electron promotions between orbitals with predominantly substituent or macrocycle character or both may give rise to some net macrocycle ? substituent charge‐transfer character in the lowest and higher singlet excited states as indicated by density functional theory (DFT) and time‐dependent DFT calculations. Such calculations indicated significant participation of molecular orbitals beyond those (HOMO ? 1 to LUMO + 1) in the Gouterman four‐orbital model. Taken together, the studies provide insight into the fundamental properties of bacteriochlorins and illustrate designs for tuning the spectral and photophysical features of these near‐infrared‐absorbing tetrapyrrole chromophores.     

Structures and excitation energies of Zn–tetraarylporphyrin analogues: A theoretical study

The photophysical properties of β-substituted Zn–tetraarylporphyrin (ZnTAP) analogues used as dyes in dye-sensitized solar cells were studied using density functional theory (DFT). Singlet-excitation energy calculations of ZnTAP analogues were performed using time-dependent DFT with B3LYP, B3PW91, PBE0 exchange–correlation functionals at 6-31G(d) and 6-31+G(d) basis sets using B3LYP/6-31G(d) geometries. The PBE0 functional at 6-31+G(d) basis set provided a better correlation with the experimental data for both B- and Q-bands. The inclusion of solvation effect in the calculations provided a good agreement in terms of B:Q_ave ratio of the oscillator strengths for both analogues with the experimental values. Analogue 2 has a higher and a more balanced charge-carrier transport rates than analogue 1. In general, the addition of an electron-donating group in the meso-substituent (analogue 2) resulted in a narrower band gap, higher oscillator strength, a more red-shifted absorption spectra, and better charge-transfer characteristics than analogue 1.     

Highly Fluorescent Green Fluorescent Protein Chromophore Analogues Made by Decorating the Imidazolone Ring

The synthesis and photophysical behavior of an unexplored family of green fluorescent protein (GFP)‐like chromophore analogues is reported. The compound (Z)‐4‐(4‐hydroxybenzylidene)‐1‐propyl‐2‐(propylamino)‐1H‐imidazol‐5(4 H)‐one (p‐HBDNI, 2 a ) exhibits significantly enhanced fluorescence properties relative to the parent compound (Z)‐5‐(4‐hydroxybenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one (p‐HBDI, 1 ). p‐HBDNI was considered as a model system and the photophysical properties of other novel 2‐amino‐3,5‐dihydro‐4H‐imidazol‐4‐one derivatives were evaluated. Time‐dependent DFT calculations were carried out to rationalize the results. The analogue AIDNI ( 2 c ), in which the 4‐hydroxybenzyl group of p‐HBDNI was replaced by an azaindole group, showed improved photophysical properties and potential for cell staining. The uptake and intracellular distribution of 2 c in living cells was investigated by confocal microscopy imaging.     

Photomagnetic and nonlinear optical properties in cis‐trans green fluoroprotein chromophore coupled Bis‐imino nitroxide diradicals

The current study extends an earlier investigation (Bhattacharya, et al., Phys. Chem. Chem. Phys. 2012, 14, 6905) to further explore various photomagnetic and optical properties of bis‐imino nitroxide, that is, (IN)₂‐based green fluorescent protein (GFP) chromophore coupled diradicals revealing new significant features. The conversion mechanisms of selected trans‐isomers into their corresponding cis‐conformers are discussed in detailed using a number of recently‐developed density functional theory (DFT) functionals based on the Minnesota suite of DFT‐models as well as using some other DFT functionals developed earlier. To provide a more in‐depth analysis of variations in magnetic properties as trans‐conformers (singlet ground‐state) convert into their cis‐analogues (triplet ground‐state), the changes in exchange magnetic coupling constants J are compared with the variation of the selected aromaticity indices. The aromaticity indices include the nuclear independent chemical shift [NICS(0)] values calculated at the center of ring structures and the harmonic oscillator model of aromaticity. Furthermore, the investigation of static nonlinear optical response properties in the (IN)₂‐based GFP chromophore coupled diradicals reveal unusually large static first hyperpolarizabilities for these systems which is highly significant for practical applications in optics and optoelectronics. © 2015 Wiley Periodicals, Inc.     

Ab initio investigation on the structures and spectra of the firefly luciferin

The ground state (S₀) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (T _v) of three low-lying states (S₁, S₂, and S₃) were calculated by TD-DFT B3LYP//CASSCF method. The S₁ geometry was optimized by CASSCF method. Its T _v and the transition energy (T _e) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S₁ state T _v values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated T _v values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired. Supported by the National Natural Science Foundation of China (Grant No. 20673012) and the Major State Basic Research Development Programs (Grant No. 2004CB719903)     

Photophysical Analysis of 1,10‐Phenanthroline‐Embedded Porphyrin Analogues and Their Magnesium(II) Complexes

The synthesis, characterization, photophysical properties, and theoretical analysis of a series of tetraaza porphyrin analogues ( H? Pn : n=1–4) containing a dipyrrin subunit and an embedded 1,10‐phenanthroline subunit are described. The meso‐phenyl‐substituted derivative ( H? P1 ) interacts with a Mg²⁺ salt (e.g., MgCl₂, MgBr₂, MgI₂, Mg(ClO₄)₂, and Mg(OAc)₂) in MeCN solution, thereby giving rise to a cation‐dependent red‐shift in both the absorbance‐ and emission maxima. In this system, as well as in the other H? Pn porphyrin analogues used in this study, the four nitrogen atoms of the ligand interact with the bound magnesium cation to form Mg²⁺–dipyrrin–phenanthroline complexes of the general structure MgX? Pn (X=counteranion). Both single‐crystal X‐ray diffraction analysis of the corresponding zinc‐chloride derivative ( ZnCl? P1 ) and fluorescence spectroscopy of the Mg‐adducts that are formed from various metal salts provide support for the conclusion that, in complexes such as MgCl? P1 , a distorted square‐pyramidal geometry persists about the metal cation wherein a chloride anion acts as an axial counteranion. Several analogues ( H? Pn ) that contain electron‐donating and/or electron‐withdrawing dipyrrin moieties were prepared in an effort to understand the structure–property relationships and the photophysical attributes of these Mg–dipyrrin complexes. Analysis of various MgX? Pn (X=anion) systems revealed significant substitution effects on their chemical, electrochemical, and photophysical properties, as well as on the Mg²⁺‐cation affinities. The fluorescence properties of MgCl? Pn reflected the effect of donor‐excited photoinduced electron transfer (d‐PET) processes from the dipyrrin subunit (as a donor site) to the 1,10‐phenanthroline acceptor subunit. The proposed d‐PET process was analyzed by electron paramagnetic resonance (EPR) spectroscopy and by femtosecond transient absorption (TA) spectroscopy, as well as by theoretical DFT calculations. Taken together, these studies provide support for the suggestion that a radical species is produced as the result of an intramolecular charge‐transfer process, following photoexcitation. These photophysical effects, combined with a mixed dipyrrin–phenanthroline structure that is capable of effective Mg²⁺‐cation complexation, lead us to suggest that porphyrin‐inspired systems, such as H? Pn , have a role to play as magnesium‐cation sensors.     

Absorption and emission properties of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (M = Al,Ga, In) and correlations with molecular electrostatic potential

Substituent effect for a series of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (Mq3) of aluminum, gallium, and indium are investigated using density functional theory (DFT) for the ground state properties and the time-dependent version of DFT (TDDFT) for their absorption and emission properties. A comparison between the ground state energy of mer and fac isomers of all the complexes revealed that the mer configuration is always more stable than fac. The substituent effect is significantly reflected at the fluorescence maximum (λ_F) values whereas the effect is moderate at the absorption maximum (λ_abs) values. The molecular electrostatic potential (MESP) at the metal center (V_M) and the most electron rich region indicated by MESP minimum (V_min), located at the oxygen of phenoxide ring exhibit excellent correlations with the λ_F and Stokes shift (λ_F−λ_abs) values. The study suggests the use of Stokes shift as an experimental quantity to measure the excited state substituent effect while the V_min or V_M emerge as theoretical quantities to measure the same.     

Color‐Tunable Solid‐State Fluorescence Emission from Carbazole‐Based BODIPYs

Several carbazole‐based boron dipyrromethene (BODIPY) dyes were synthesized by organometallic approaches. Thiazole, benzothiazole, imidazole, benzimidazole, triazole, and indolone substituents were introduced at the 1‐position of the carbazole moiety, and boron complexation of each dipyrrin generated the corresponding compounds 1 , 2 a , and 3 – 6 . The properties of these products were investigated by UV/Vis and fluorescence spectroscopy, cyclic voltammetry, X‐ray crystallography, and DFT calculations. These compounds exhibited large Stokes shifts, and compounds 1 , 2 a , and 3 – 5 fluoresced both in solution and in the solid state. Complex 2 a showed the highest fluorescence quantum yield (Φ_F) in the solid state, therefore boron complexes of the carbazole–benzothiazole hybrids 2 b – f , which had several different substituents, were prepared and the effects of the substituents on the photophysical properties of the compounds were examined. The fluorescence properties showed good correlation with the results of crystal‐packing analyses, and the dyes exhibited color‐tunable solid‐state fluorescence.     

Photoinduced Excited State Intramolecular Proton Transfer of New Schiff Base Derivatives with Extended Conjugated Chromophores: A Comprehensive Theoretical Survey

This paper presented comprehensive theoretical investigation of excited state intramolecular proton transfer (ESIPT) of four new large Schiff base derivatives with extended conjugated chromophores. The properties of the ground state and the excited state of phototautomers of C1 to C4 [ C1 : 2‐(4′‐nitro‐stilbene‐4‐ylimino)methylphenol; C2 : 2‐(4′‐cyano‐stilbene‐4‐ylimino)methylphenol; C3 : 2‐(4′‐methoxyl‐stilbene‐4‐ylimino)methylphenol; C4 : 2‐(4′‐N,N‐diethylamino‐stilbene‐4‐ylimino)methylphenol], which included geometrical parameter, energy, rate constant, frontier orbit, Mulliken charge, dipole moment change, were studied by DFT (density functional theory), CIS (configuration interaction singles‐excitation), TDDFT (time‐dependent DFT) methods to analyze the effects of chromophore part on the occurrence of ESIPT and the role of substituent groups. The structural parameter calculation showed that the shorter R_{H? N} and larger R_{O? H} from enol to enol* form, and less twisted configuration in the excited state implied that these molecules could undergo ESIPT as excitation. Stable transition states and a low energy barrier were observed for C1 to C4 . This suggested that chromophore part increased some difficulty to undergo ESIPT for these molecules, while the possibility of occurrence of ESIPT was quite high. The negative ΔE^* (?9.808 and ?9.163 kJ/mol) of C1 and C2 and positive ΔE^* (0.599 and 1.029 kJ/mol) of C3 and C4 indicated that withdrawing substituent groups were favorable for the occurrence of ESIPT. The reaction rate constants of proton transfer of these compounds were calculated in the S₀ and S₁ states respectively, and the high rate constants of these compounds were observed at S₁ state. C1 even reached at 1.45×10¹⁵ s^?1 in the excited state, which is much closed to 2.05×10¹⁵ s^?1 of the parent moiety (salicylidene methylamine). Electron‐donating and electron‐withdrawing substituent groups had different effects on the electron density distribution of frontier orbits and Mulliken charges of the atoms, resulting in different dipole moment changes in enol*→keto* process. These differences in turn suggested that C1 and C2 had more ability to undergo ESIPT than C3 and C4 . The ultraviolet/visible absorption spectra, normal fluorescence emission spectra and ESIPT fluorescence emission spectra of these compounds were predicted in theory.