Low Temperature Phosphorescence Spectra Research Articles

Two distinct mechanisms of flavoprotein spectral tuning revealed by low-temperature and time-dependent spectroscopy.

Flavins such as flavin mononucleotide or flavin adenine dinucleotide are bound by diverse proteins, yet have very similar spectra when in the oxidized state. Recently, we developed new variants of flavin-binding protein CagFbFP exhibiting notable blue (Q148V) or red (I52V A85Q) shifts of fluorescence emission maxima. Here, we use time-resolved and low-temperature spectroscopy to show that whereas the chromophore environment is static in Q148V, an additional protein-flavin hydrogen bond is formed upon photoexcitation in the I52V A85Q variant. Consequently, in Q148V, excitation, emission, and phosphorescence spectra are shifted, whereas in I52V A85Q, excitation and low-temperature phosphorescence spectra are relatively unchanged, while emission spectrum is altered. We also determine the x-ray structures of the two variants to reveal the flavin environment and complement the spectroscopy data. Our findings illustrate two distinct color-tuning mechanisms of flavin-binding proteins and could be helpful for the engineering of new variants with improved optical properties.

Triplet state spectroscopy reveals involvement of the buried tryptophan residue 310 in Glyceraldehyde-3-phosphate dehydrogenase (GAPD) in the interaction with acrylamide

Using conventional steady state and time resolved fluorescence study of the interaction between a multi-tryptophan protein and a quencher, it is difficult, if not impossible to identify the particular tryptophan residue/residues involved in the interaction. In this work we have exemplified the above contention using a multi-tryptophan protein, Glyceraldehyde-3-phosphate dehydrogenase (GAPD) from rabbit muscle having three tryptophan (Trp) residues at positions 84, 193 and 310 and a neutral quencher acrylamide in Tris buffer of pH 7.5. From the steady state and time resolved fluorescence quenching (at 298 K) with acrylamide Ksv, K and kq for the system have been calculated. Low temperature phosphorescence (LTP) spectra at 77 K of GAPD in suitable cryosolvent is known to exhibit two (0,0) bands corresponding to two tryptophan residues 193 and 310. Using the LTP study of free GAPD and GAPD – acrylamide it is possible to identify that the buried Trp 310 residue is specifically involved in the interaction with acrylamide. This is possible without doing any site-directed mutagenesis of GAPD which contains Trp residues at 84, 193 and 310. Tyrosine 320 is also specifically quenched. The results have been corroborated using the molecular docking studies. Molecular Dynamics simulation supports our contention of the involvement of Trp 310 and also shows that the other nearest residues of acrylamide are Val175 and Val232.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

High Power Efficiency Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Using Exciplex-Type Host with a Turn-on Voltage Approaching the Theoretical Limit.

Three solution-processable exciplex-type host materials were successfully designed and characterized by equal molar blending hole transporting molecules with a newly synthesized electron transporting material, which possesses high thermal stability and good film-forming ability through a spin-coating technique. The excited-state dynamics and the structure-property relationships were systematically investigated. By gradually deepening the highest occupied molecular orbital (HOMO) level of electron-donating components, the triplet energy of exciplex hosts were increased from 2.64 to 3.10 eV. Low temperature phosphorescence spectra demonstrated that the excessively high triplet energy of exciplex would induce a serious energy leakage from the complex state to the constituting molecule. Furthermore, the low energy electromer state, which only exists under the electroexcitation, was found as another possible channel for energy loss in exciplex-based phosphorescent organic light-emitting diodes (OLEDs). In particular, as quenching of the exciplex-state and the triplet exciton were largely eliminated, solution-processed blue phosphorescence OLEDs using the exciplex-type host achieved an extremely low turn-on voltage of 2.7 eV and record-high power efficiency of 22.5 lm W(-1), which were among the highest values in the devices with identical structure.

Derivatives of 1,4‐naphthoquinone as visible‐light‐absorbing one‐component photoinitiators for radical polymerisation

Bifunctional visible‐light photoinitiators for free radical polymerisation based on a 1,4‐naphthoquinone skeleton were synthesised and characterised using proton nuclear magnetic resonance spectroscopy and chemical ionisation mass spectrometry. Their basic spectroscopic properties, such as absorption and low‐temperature phosphorescence spectra, and their red‐ox properties were also measured. These initiators contain an appropriate electron/hydrogen donor group in the skeleton and do not require an additional coinitiator for initiation. During irradiation they act both as a triplet photosensitiser and as a hydrogen/electron donor. The ability of each naphthoquinone to act as a photoinitiator strongly depends on its chemical structure. These studies suggest that initiator radicals are generated from the triplet state in an intermolecular electron/hydrogen transfer reaction.

A Comparative Study of Interaction of Tetracycline with Several Proteins Using Time Resolved Anisotropy, Phosphorescence, Docking and FRET

A comparative study of the interaction of an antibiotic Tetracycline hydrochloride (TC) with two albumins, Human serum albumin (HSA) and Bovine serum albumin (BSA) along with Escherichia Coli Alkaline Phosphatase (AP) has been presented exploiting the enhanced emission and anisotropy of the bound drug. The association constant at 298 K is found to be two orders of magnitude lower in BSA/HSA compared to that in AP with number of binding site being one in each case. Fluorescence resonance energy transfer (FRET) and molecular docking studies have been employed for the systems containing HSA and BSA to find out the particular tryptophan (Trp) residue and the other residues in the proteins involved in the binding process. Rotational correlation time (θc) of the bound TC obtained from time resolved anisotropy of TC in all the protein-TC complexes has been compared to understand the binding mechanism. Low temperature (77 K) phosphorescence (LTP) spectra of Trp residues in the free proteins (HSA/BSA) and in the complexes of HSA/BSA have been used to specify the role of Trp residues in FRET and in the binding process. The results have been compared with those obtained for the complex of AP with TC. The photophysical behaviour (viz., emission maximum, quantum yield, lifetime and θc) of TC in various protic and aprotic polar solvents has been determined to address the nature of the microenvironment of TC in the protein-drug complexes.

Structure and phosphorescence of meta-bromobenzophenone crystal

Solid state properties of meta-bromobenzophenone crystals were studied, including X-ray diffraction, integrated phosphorescence spectrometry, and ab initio calculations. The structure of meta-bromobenzophenone crystals has been determined by powder X-ray diffraction to be orthorhombic (space group Pbca) with a=11.70461(15)Å, b=7.70025(9)Å, c=24.0055(2)Å, Z=8. This structure includes a supramolecular network formed by weak hydrogen bonds of CH⋯O and CH⋯π short contacts. A tight π–π short contact between virtually parallel substituted and unsubstituted phenyl rings of neighbor molecules is another distinguishing feature of the crystal structure, which might be the cause of the unusual very broad lonely band in the room-temperature phosphorescence spectrum. The low-temperature (1.6K) phosphorescence spectra are of monomeric character, revealing sharp lines superimposed on broad 0→ν bands, which is a signature of emission from some traps.

Oxygen sensing nanofibers doped with red-emitting Eu(III) complex: Synthesis, characterization, mechanism, and sensing performance

In this paper, we synthesize a novel diamine ligand of PIP and its corresponding Eu(III) complex of Eu(DBM) 3PIP, where PIP = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, and DBM = 1,3-diphenyl-propane-1,3-dione. The UV–Vis absorption spectra, low temperature phosphorescence spectra, energy transfer mechanism, excited state lifetime of Eu(DBM) 3PIP, as well as its photoluminescence spectra, are investigated in detail. Data suggest that the emission of Eu(DBM) 3PIP is quenchable by molecular oxygen due to the back-energy transfer process. By doping Eu(DBM) 3PIP into a polymer matrix of poly(vinylpyrrolidone) (PVP), the oxygen sensing performance of the resulted nanofibrous membrane is investigated. Finally, the 0.6 wt% doped sample exhibits a linear response towards molecular oxygen, with a sensitivity of 3.5 and response/recovery time of 15 s/24 s.

Synthesis, photophysical properties and thermal relaxation processes of carbazolyl-labeled polysiloxanes

The photophysical properties of a fluorescent silicone prepared by a hydrosilylation reaction from poly(dimethylsiloxane-co-methylhydrogensiloxane) terminated by dimethylhydrogensilyloxy groups, poly(dimethylsiloxane-co-methylvinylsiloxane) terminated by dimethylvinylsilyloxy groups and 9-vinylcarbazole, as the luminescent group, have been studied under steady-state fluorescence and time-resolved conditions. At low temperature both steady-state fluorescence and phosphorescence spectra of isolated species are observed. Phosphorescence is quenched at temperatures higher than 150K indicating that the environments where the carbazolyl moieties are located are very flexible, as expected for silicones. Fluorescence decays and phosphorescence (at 77K) showed typical carbazolyl lifetimes and no rise-time was observed, which is consistent with the absence of excimer emission. The temperature dependence of the emission can be used to determine the polymer relaxation processes and transitions (Tγ=80–100K, Tg=130–150K and Tm=220K), these last two comparable with those observed by differential scanning calorimetry.

Physical nature of the monomeric fluorescence of the ortho-bromobenzophenone crystal

The time-resolved low-temperature phosphorescence spectra of the ortho-bromobenzophenone (2-BrBP) crystal are investigated at a temperature of 52K. The spectra show that the emission of one series of vibrational bands of the phosphorescence decays in time, while the emission of another series of vibrational bands, which is identical but shifted to longer wavelengths, first rises and then decays. This behavior of the spectrum at the given temperature is explained by the overcoming of an energy barrier between two conformational states of the excited ortho-bromobenzophenone molecule. Besides the phosphorescence of the two series of vibrational bands, the time-resolved spectrum of the ortho-bromobenzophenone crystal at the given temperature exhibits excimer emission, which decays over a time ≈175μs.

Photophysics and Photochemistry of 1-Nitropyrene

1-Nitropyrene (1NPy) is the most abundant nitropolycyclic aromatic contaminant encountered in diesel exhausts. Understanding its photochemistry is important because of its carcinogenic and mutagenic properties, and potential phototransformations into biologically active products. We have studied the photophysics and photochemistry of 1NPy in solvents that could mimic the microenvironments in which it can be found in the atmospheric aerosol, using nanosecond laser flash photolysis, and conventional absorption and fluorescence techniques. Significant interactions between 1NPy and solvent molecules are demonstrated from the changes in the magnitude of the molar absorption coefficient, bandwidth at half-peak, oscillator strengths, absorption maxima, Stokes shifts, and fluorescence yield. The latter are very low (10 (-4)), increasing slightly with solvent polarity. Low temperature phosphorescence and room temperature transient absorption spectra demonstrate the presence of a low energy (3)(pi,pi*) triplet state, which decays with rate constants on the order of 10 (4)-10 (5) s (-1). This state is effectively quenched by known triplet quenchers at diffusion control rates. Intersystem crossing yields of 0.40-0.60 were determined. A long-lived absorption, which grows within the laser pulse, and simultaneously with the triplet state, presents a maximum absorption in the wavelength region of 420-440 nm. Its initial yield and lifetime depend on the solvent polarity. This species is assigned to the pyrenoxy radical that decays following a pseudo-first-order process by abstracting a hydrogen atom from the solvent to form one the major photoproducts, 1-hydroxypyrene. The (3)(pi,pi*) state reacts readily ( k approximately 10 (7)-10 (9) M (-1) s (-1)) with substances with hydrogen donor abilities encountered in the aerosol, forming a protonated radical that presents an absorption band with maximum at 420 nm.

Triplet Excited State in Platinum−Acetylide Oligomers: Triplet Localization and Effects of Conformation

An experimental and theoretical investigation was carried out on a series of platinum-acetylide oligomers of the general structure Ph-CC-[PtL2-CC-(1,4-Ph)-CC-]n-PtL2-CC-Ph (where n = 1, 2, 3, 4, 6; Ph = phenyl, 1,4-Ph = 1,4-phenylene; L = P(n-Bu)3, and the geometry at Pt = trans). The objective of this work is to understand the geometry and electronic structure of the ground and triplet excited states of Pt-acetylide oligomers. The experiments carried out include temperature-dependent absorption and photoluminescence spectroscopy (80-298 K range) and ambient temperature transient absorption spectroscopy. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were carried out on several of the oligomers using the hybrid Becke's three-parameter functional with the B3LYP correlation functional and the SDD basis set. The combined experimental and theoretical results provide very clear evidence that the triplet excited state is localized on a chromophore consisting approximately of a single -[PtL2-CC-(1,4-Ph)-CC-PtL2]- repeat unit. DFT calculations indicate that in the ground state conformers that differ in the (rotational) orientation of the 1,4-phenylenes with respect to the plane defined by the PtL2(C)2 units (twisted = t and planar = p) are very close in energy (difference of <1 kcal.mol-1). By contrast, in the triplet excited state, the p conformer is 3 kcal.mol-1 lower in energy than the t conformer. The torsional geometry change in the triplet state is reflected in the low-temperature phosphorescence spectra of the short oligomers, where separate emission bands are seen from the t and p conformers.

Photophysical properties of novel lanthanide (Tb 3+, Dy 3+, Eu 3+) complexes with long chain para-carboxyphenol ester p-L-benzoate (L = dodecanoyloxy, myristoyloxy, palmitoyloxy and stearoyloxy)

In this paper, a series of 12 binary luminescent lanthanide coordination compounds with long chain p-carboxyphenol ester were assembled. Both elemental analysis and infrared spectroscopy allowed to determine the complexes formula: LnL 3, where Ln = Tb, Dy, Eu; L = p-dodecanoyloxybenzoate (12-OBA), p-myristoyloxybenzoate (14-OBA), p-palmitoyloxybenzoate (16-OBA) and p-stearoyloxybenzoate (18-OBA), respectively. The photophysical properties of these complexes were studied in detail with various of spectroscopies such as ultraviolet–visible absorption spectra, low temperature phosphorescence spectra and fluorescent spectra. The ultraviolet–visible absorption spectra showed that some bands shift with the different chain length of p-carboxyphenol ester. From the low temperature phosphorescent emission, the triplet state energies for these four ligands were determined to be around 24,242 cm −1 (12-OBA), 24,612 cm −1 (14-OBA), 24,084 cm −1 (16-OBA) and 24,125 cm −1 (18-OBA), respectively, suggesting they are suitable for the sensitization of the above lanthanide ions, especially for Tb 3+ and Dy 3+. The fluorescence excitation and emission spectra for these lanthanide complexes of the four ligands take agreement with the above predict from energy match.

Spectroscopic Study on the Photophysical Properties of Lanthanide Complexes with Long Chain Mono-Docosyl Phthalate

Ortho phthalic anhydride was modified with long chain alcohol (1-docosanol) to its corresponding monodocosyl phthalate (22-Phth). Subsequently, three novel lanthanide (Eu(3+), Tb(3+), and Dy(3+)) complexes with the long chain monodocosyl phthalate were synthesized and characterized by elemental analysis and Infrared spectra. The photophysical properties of these complexes were studied in detail with ultraviolet-visible absorption spectra, low temperature phosphorescence spectra and fluorescent spectra. The triplet state energy of 22-Phth was determined to be around 25,000 cm(-1) from the maximum phosphorescent peak at 400 nm, suggesting 22-Phth is suitable for the sensitization of the luminescence of Eu(3+), Tb(3+), and Dy(3+). The fluorescence excitation and emission spectra for these lanthanide complexes of the three ligands take agreement with the above predict from energy match principle.

Spectroscopic study on the photophysical properties of novel lanthanide complexes with long chain mono-L phthalate (L = hexadecyl, octadecyl and eicosyl)

Ortho-phthalic anhydride was modified with long chain alcohol (1-hexadecanol, 1-octadecanol and 1-eicosanol) to their corresponding mono-L phthalate (L = hexadecyl, octadecyl and eicosyl), i.e. monohexadecyl phthalate (16-Phth), monooctadecyl phthalate (18-Phth), and monoeicosyl phthalate (20-Phth), respectively. Nine novel lanthanide (Eu 3+, Tb 3+ and Dy 3+) complexes with these three mono-L phthalate ligands were synthesized and characterized by elemental analysis and IR spectra. The photophysical properties of these complexes were studied in detail with various spectroscopes such as ultraviolet–visible absorption spectra, low temperature phosphorescence spectra and fluorescent spectra. The ultraviolet–visible absorption spectra show some band shifts with the different chain-length of phthalate monoester and homologous lanthanide complexes. From the low temperature phosphorescent emission, the triplet state energies for these three ligands were determined to be around 22,650 cm −1 (16-Phth), 23,095 cm −1 (18-Phth) and 22,400 cm −1 (20-Phth), respectively, suggesting they are suitable for the sensitization of the luminescence of Eu 3+, Tb 3+ and Dy 3+. The fluorescence excitation and emission spectra for these lanthanides complexes of the three ligands take agreement with the above predict from energy match.

Triplet state studies of β-carbolines

Low temperature phosphorescence spectra, quantum yields and lifetimes, and room temperature triplet–triplet absorption spectra and lifetimes have been obtained for the neutral and cationic forms of the three β-carbolines: harmine (Hi), harmane (Ha) and norharmane (NorH). Based on the phosphorescence spectra, the energies of the lowest triplet states are 25 250 (Hi), 24 750 (Ha) and 24 400 cm−1 (NorH) for the neutral forms and 22 900 (Hi), 21 750 (Ha) and 21 850 cm−1 (NorH) for the corresponding cations. Phosphorescence quantum yields are in the range 0.4 and 0.6 for the neutral forms, and are found to decrease on protonation, while phosphorescence lifetimes vary from 3.15 s for the neutral form of Hi to 6.89 s for the cationic form of NorH. In all the cases, the lowest excited triplet state is suggested to have 3(π, π∗) character. For several of the systems, the sum of emission yields is less than 1, indicating that internal conversion may also be an important deactivation route for the lowest excited singlet state. Microsecond flash photolysis studies show that the triplet states of all three carbolines undergo acid–base equilibria in water with pKa values in the region 4.4–5.0 at room temperature. In water and ethanolic solutions, the cationic forms show two absorption maxima at ca. 450 and 580 nm, while the neutral forms have only one absorption maximum in the visible spectral region at ca. 510 nm. In nitrogen purged solutions, triplet state lifetimes are 530 (Hi), 380 (Ha) and 600 μs (NorH), and the triplet states are quenched by β-carotene with rate constants k(av)=3.3(±0.9)×109M−1s−1 close to but slightly lower than those expected for a diffusion controlled process.

Characterization of the tryptophan residues of Escherechia coli alkaline phosphatase by phosphorescence and optically detected magnetic resonance spectroscopy.

The phosphorescence and zero field optically detected magnetic resonance (ODMR) of the tryptophan (Trp) residues of alkaline phosphatase from Escherechia coli are examined. Each Trp is resolved optically and identified with the aid of the W220Y mutant and the terbium complex of the apoenzyme. Trp(109), known from earlier work to be the source of room-temperature phosphorescence (RTP), emits a highly resolved low-temperature phosphorescence (LTP) spectrum and has the narrowest ODMR bands observed thus far from any protein site, revealing a uniquely homogeneous local environment. The decay kinetics of Trp(109) at 1.2 K reveals that the major triplet population (70%) undergoes inefficient crystallike spin-lattice relaxation by direct interaction with lattice phonons, the remainder being relaxed efficiently by local disorder modes. The latter population is smaller than is typical for protein sites, suggesting an unusual degree of local rigidity and order consistent with the long-lived RTP. Trp(220) emits a broader LTP spectrum originating to the blue of Trp(109). It has typically broad ODMR bands consistent with local heterogeneity. The LTP of Trp(268) has an ill-defined origin blue shifted relative to Trp(220) and ODMR frequencies consistent with a greater degree of solvent exposure. Trp(268) has noticeable dispersion of its decay kinetics, consistent with quenching at the triplet level by a nearby disulfide residue.

Spectroscopic Study of the Photophysical Properties of Rare Earth Ions Encapsulated: Water Soluble Functional Calixresorcin[4]Arene

A novel macrocyclic compound-water soluble functional calixresorcin[4]arenes—tetra para sulfo-phenylmethyl-calixresorcin[4]arenes was synthesized for the first time. the photophysical properties of terbium and europium ions encapsulated in the macrocyclic ligand were studied in detail. the triplet state energy of the calixresorcin[4]arene was determined to be 24400 cm−1 by the low temperature phosphorescence spectrum and it was found that it can sensitize both terbium ion and europium ion. the possible energy transfer process between the functional calixresorcin[4]arene and the encapsulated Tb3+ and Eu3+ was discussed. the luminescence quantum efficiency of Tb3+- calixresorcin[4]arene was calculated.

Studies of hydrogen-bonded complexes of 4-methoxybenzonitrile in ground and lowest triplet states by IR, phosphorescence, and 15N NMR spectra

The formation of hydrogen-bonded complexes between 4-methoxybenzonitrile and various OH proton donors was studied in solution in the ground electronic state by means of IR absorption spectra, and the ΔH values were measured. The upfield shift of purely electronic 0–0 transition is observed in low temperature phosphorescence spectra under the interaction, the values of this shift for different proton donor molecules being correlated with the chemical shifts of 15N NMR signals under the complex formation. The nature of observed anomalous N signal shift in NMR spectra was explained by a decrease of paramagnetic deshielding because of the mixing of ground and lowest triplet electronic states of the nitrile studied.

A study of thermally activated delayed fluorescence in C 60

The existence of thermally activated delayed fluorescence in C 60 is demonstrated by the study of the temperature dependence (291–353 K) of the fluorescence intensity of C 60 degassed solutions. The determined singlet-triplet energy gap, 35 ± 2 kJ/mol, agrees with the value calculated from previously reported fluorescence and low temperature phosphorescence spectra for this molecule (35 kJ/mol). The estimated quantum yield of triplet formation, 0.8 ± 0.1, agrees with previous determinations. The fluorescence lifetime of C 60, also measured, τ = 1.1 ns, does not change appreciably with excitation wavelength, degassing nor with temperature, in the range 243–343 K.