Ground State Absorption Research Articles

Absorption Spectra of Protonated Corroles: Two Distinct Patterns Due to Peripheral Substitution Architecture

The origin of individual features in the ground state absorption spectra of two protonated corroles differing in the architecture of peripheral substitution (either Cm-aryl or Cb-alkyl) have been studied in detail with the ground state absorption spectroscopy and density functional theory calculations. The geometry optimization, molecular orbitals and absorption spectra calculation have been carried out. It was found that protonation leads to the saddle type macrocycle conformation in contrast with the wave type conformation known for the parent-free base corroles. The mean plane deviation parameter Δ23 for the macrocycle, pyrrole tilting angles and the degree of pyramidalization λ2 of all four pyrrole nitrogens was found to depend on the peripheral substitution architecture. Macrocycle conformation of the protonated forms has distinct asymmetrical features which are reflected by the sets of values of the tilting angles and values of pyramidalization degree. The pair of pyrroles B and C has smaller tilting angles and higher pyramidalization degree values, whereas the opposite trend was found for the pair of pyrroles A and D. Electronic effects and structural differences induced by substitution lead to the pronounced shifts of the molecular orbitals. In the Cb-alkylated corrole, almost-degenerated HOMO and HOMO-1 molecular orbitals lead to enhancement of the configuration interaction. As a result, the Qx transition oscillator strength goes down, becoming comparable to that of the Qy one. A large HOMO-HUMO-1gap in the Cm-aryl corrole minimizes the configuration interaction, giving rise to Qx band domination in the visible range spectrum.

Nanostructured Coordination Polymers for Solid State Dye-Sensitized Solar Cells

Dye-sensitized solar cells have the potential to provide a viable source of renewable energy to help decarbonize our economy and power wearable devices. With their exceptional performance in diffused light under indoor conditions, DSCs remain competitive for powering the next digital revolution forming the internet of things.[1] Conventional DSCs use a liquid electrolyte; however, a lot of research is going into replacing it with a solid hole transporter material to make the technology better suited for scale-up and commercialization. We work with an emerging class of materials called nanostructured coordination polymers (CP) and explore their application as a hole transport material aimed at creating high performing monolithic solid-state DSCs.Nanostructured CPs possess the highly ordered structure of inorganic materials combined with the chemically tailorable properties and low cost of organics.[2] Thanks to their high external surface area, aspect ratios, nanoscopic dimensions and novel electronic and optical properties, CPs have been found to outperform their precursors in a wide range of applications and are ideal for evaluation as solid-state hole transport materials in DSCs. CPs facilitate the formation of extended polymeric structure of metal ions and the coordinating atoms of the organic ligands. The systems are designed by taking the following conditions into consideration- the energy overlap of the orbitals of the metal ions and the coordinating atoms and their relative electronegativities. Incorporating redox-active metal centres into conducting polymer substrates thus creates highly efficient redox conductivity. Metal centres can provide efficient sites for redox conductivity but can also act as thermodynamic sinks that trap/localize charges due to their low-lying energetic states.[3] Based on these design criteriae, we explore Copper benzenetetrathiol (Cu-BTT) as an ideal candidate for hole transporting roles in solid state dye sensitized solar cells.We show that Cu-BTT is formed by alternating Cu2+ and C6H2S4 2- units with pairs of chelating S atoms from the ligand coordinating around the metal centre. Formation of 1D chain-type nanowires is a good strategy for through bond charge transport. The as-prepared pristine Cu-BTT coordination polymers were found to exhibit a conductivity of the order 10-6 S cm-1. Upon altering the technique used for thin film preparation, the conductivity was found to increase by up to an order of a magnitude. Layer by layer assembly of the coordination polymer was found to be the best performing, giving a fairly uniform epitaxial growth of Cu-BTT over large areas and showing conductivities of the order 10-3 S cm-1 without additives. Using AFM imaging studies, we show that this morphology creates the formation of highly interconnected networks of CuBTT nanowires which improves the conductivity. We also demonstrate that epitaxially grown Cu-BTT also shows better performance as a hole transport material in the DSSC devices as compared to Cu-BTT films prepared using drop casting or spin-coating technique. This is because during epitaxial growth the precursors get a better chance to infiltrate the pores in the mesoporous layer and the polymeric HTM forms direct contact with the dye molecules. This is corroborated by photoinduced absorption spectroscopy (PIA) measurements where we observe that the ground state bleach and absorption peaks of the dye (Y123) disappear upon introduction of the epitaxially grown CuBTT HTM. This demonstrates an efficient and complete reduction of the oxidized dye molecules therefore proving that CuBTT nanowire type 1D coordination polymers can be utilized successfully as hole transport layers in solid state dye sensitised solar cells. This work shows that 1D coordination polymers hold significant potential as solid-state hole transport materials to create monolithic solid state DSSCs with improved performance. REFERENCES [1] M. Freitag et al., “Dye-sensitized solar cells for efficient power generation under ambient lighting,” Nat. Photonics, vol. 11, no. 6, pp. 372–378, 2017, doi: 10.1038/nphoton.2017.60.[2] K. Sasitharan et al., “Metal‐Organic Framework Nanosheets as Templates to Enhance Performance in Semi‐Crystalline Organic Photovoltaic Cells,” Adv. Sci., vol. 2200366, p. 2200366, 2022, doi: 10.1002/advs.202200366.[3] A. J. Clough et al., “Room Temperature Metallic Conductivity in a Metal − Organic Framework Induced by Oxidation,” 2019, doi: 10.1021/jacs.9b06898. Figure 1

Two-Photon Absorption Response of Functionalized BODIPY Dyes in Near-IR Region by Tuning Conjugation Length and Meso-Substituents.

BODIPY dyes substituted by phenol or -COOMe units at the meso-position (C8) with and without a distyryl group including a methoxy moiety at the -C3 and -C5 positions of the BODIPY have been synthesized to analyze the photophysical properties. To clarify the ground-state interaction, absorption and emission features were investigated in the THF environment. Extending the π-conjugation with the methoxy moiety at -C3 and -C5 positions of BODIPY leads to a spectral shifting of the absorption maxima toward red by 120 nm. In addition, attaching the -COOMe unit at the meso-position of the BODIPY structure increases nonradiative molecular relaxation as compared to compounds possessing phenol substituents at the same position. We have investigated the effect of phenol and a -COOMe group and π-extended conjugation length with a methoxy moiety on the properties of two-photon absorption (TPA) and electron transfer dynamics by performing open-aperture (OA) Z-scan and femtosecond transient absorption spectroscopy measurements, respectively. The synthesized BODIPY compounds with the distyryl group including the methoxy unit show TPA character due to the longer conjugation length and therefore intramolecular charge transfer ability. Based on the OA Z-scan experiments upon photoexcitation with 800 nm pulsed laser light, TPA cross-section values were obtained as 74 and 81 GM for the compounds possessing phenol and -COOMe units at the meso-position of BODIPY treated by distyryl group with methoxy moieties, respectively. Additionally, optical and electronic properties were calculated theoretically by using the DFT method.

Triplet-Triplet Energy Transfer in Bis-Cyclometalated Iridium Complexes with Pyrene-Substituted Isocyanides.

Nonlinear optical (NLO) materials are able to modulate responses of electromagnetic radiation, leading to phenomena critical to modern telecommunications technologies. The last two decades have seen significant advances in the area of molecular nonlinear chromophores, particularly with respect to reverse-saturable absorption (RSA). Here, we introduce a strategy for intense excited-state absorption (ESA) that involves bis-cyclometalated iridium complexes with isocyanide ancillary ligands decorated with pyrene triplet acceptors. Upon excitation, the complexes undergo rapid triplet-triplet energy transfer (TTET) to the acceptor excited states. This report describes five bis-cyclometalated iridium complexes using two different pyrene-substituted isocyanides with the general formula [Ir(C^N)2(CNAr)2]PF6 (C^N = cyclometalating ligand, CNAr = isocyanide ancillary ligand: CNArpyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide, CNpyr = 1-pyrenyl isocyanide). The synthesized complexes were thoroughly characterized via 1H and 13C{1H} NMR spectroscopy, Fourier-transform Infrared spectroscopy, and electrospray ionization mass spectrometry. The excited states were evaluated with UV-vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy. Phosphorescence is completely quenched at room temperature, but in the solvent glass matrix at 77 K, there is luminescence originating from a π → π* triplet state on the pyrene moiety, abbreviated herein as 3pyrene. All five complexes display intense and long-lived ESA originating from the 3pyrene state. The localization of the ground-state absorption on the cyclometalating ligands and the excited-state absorption on the pyrene moiety allows for independent tuning of ground-state absorption (GSA) and ESA to optimize RSA and other NLO attributes.

976 nm and 808 nm dual-GSA-wavelength pumped 3 µm erbium-doped solid-state laser.

A dual-wavelength pumping scheme at 976 nm and 808 nm is proposed to improve the performance of 3 µm Er:YAP laser. 976 nm and 808 nm correspond to the ground state absorption processes of 4I15/2→I11/2 and 4I15/2→I9/2, respectively. The experimental results indicate that the introduction of 808 nm pumping not only increases the total inversion population, but also can adjust the population distribution among the sublevels in the upper and lower manifold, thus supporting higher output power and multiple wavelengths emissions. Under the single-wavelength pumping, the maximum output powers of 1.192 W and 0.223 W are obtained for 976 nm and 808 nm pumping, respectively. With regard to the 976/808 nm dual-wavelength pumping, the achievable maximum output power is 1.398 W, increased by 17.3% compared to the case of single-wavelength pumping at 976 nm. The dual-wavelength pumped Er:YAP laser can also operate in a state of multi-wavelength emissions at 2.79 µm, 2.82 µm and 2.92 µm with different dual-wavelength pump power combinations. Considering the broadband absorption characteristics of ground state absorption and the convenience of obtaining near-infrared laser diodes pumping sources, the proposed dual-wavelength pump scheme shows great potential to realize high-power, high-efficiency 3 µm erbium-doped solid-state lasers with better cost-effectiveness and more compact structure.

Nonlinear optical studies of phenothiazine Schiff bases for optical limiting application

Synthesized Schiff-based organic molecules consisting of phenothiazine backbone (Phe-DNP, Phe-Ma, Phe-3) to investigate its optical limiting applications. The structural confirmation of compounds is carried out by FTIR and NMR spectroscopy. The quantum mechanical calculations of compounds are measured using Density functional theory (DFT) to investigate the electronic orientation in molecular orbitals and the theoretical bandgap. All the molecules show a broad absorption in the visible domain and the conjugated molecule absorption shifts towards the higher wavelength region. The effective intramolecular charge transfer of molecules in the order Phe-DNP> Phe-Ma>Phe-3 is understood from the ground state absorption spectra results. All the organic moieties exhibited sequential two-photon absorption upon studying the open aperture Z-scan technique. Phe-Ma molecule shows the highest nonlinear absorption (1.212× 10−10 m/W) and lowest optical limiting threshold (3.19 × 1012 W/m2) compared to other molecules suggesting that Phe-Ma could be a potential nonlinear optical limiter.

A turn-on fluorescence sensor for detection of heparinase with heparin templated aggregation of tetracationic porphyrin derivative

Heparinase is the only mammalian endoglycosidase that breaks down the commonly used blood-anticoagulant heparin into therapeutically relevant low-molecular-weight-heparin. Importantly, heparinase has been considered a malignant disease diagnostic marker. Thus, it is essential to develop detection scheme for heparinase. However, optical methods for heparinase determination are limited. In the present work, we report a turn-on fluorescence sensor for detection of heparinase that utilizes heparin-templated aggregation of a tetra-cationic porphyrin derivative, TMPyP4+, as a sensing framework. Heparinase cleaves the glycosidic linkage between hexosamine and uronic acid in the structure of heparin to destroy its polyelectrolytic nature that originally causes the aggregation of TMPyP4+. Thus, heparinase leads to dissociation of TMPyP4+ aggregates and generates an optical signal. This system leads to a sensitive and selective response towards heparinase with a Limit of Detection (LOD) of 0.3 pmol/L. Further, the same system is demonstrated to sense a trace amount of Oversulfated Chondrootin Sulphate (OSCS) in heparin, which is a heparin adulterant, by utilizing the fact that OSCS serves as an inhibitor for heparinase activity, which leads to reverse modulation in the photo-physical features of the monomer/aggregate equilibrium of the TMPyP4+-heparin-heparinase system. The sensing mechanism has been thoroughly demonstrated by ground-state absorption, steady-state emission, and time-resolved emission measurements. The selectivity of the sensor was tested using lysozyme, α-amylase, pepsin, trypsin, lipase, and glucose oxidase in the heparinase selectivity study and the method is also validated using another method reported in the literature. The study provides a new approach for the development of optical methods for the detection of heparinase and oversulfated chondroitin sulfate, which is currently limited.

Nonlinear Absorption Features of Acid Blue 129 Dye in Polar Solvents: Role of Solvents on Solute Molecule.

This paper focuses the study of nonlinear absorption (NLA) features of anthraquinone dye namely, acid blue 129 with different polar solvents including ethanol, methanol, acetone, 1-proponal and DMSO. The acid blue 129 dye in polar solvents is characterized by UV-Visible absorption and fluorescence study. The NLA measurements of acid blue 129 dye is examined via open aperture (OA) Z-scan method employing at 650nm wavelength. The OA Z-scan curve of the experimental dye exhibits both saturable absorption (SA) and reveres saturable absorption (RSA) signature, which results from negative and positive NLA coefficient (β). The order of ground and excited state absorption cross-section is measured to be 10-22 m2 and 10-18 m2, respectively and proved the existence of RSA in acid blue 129 dye. The order of NLA coefficient and imaginary component of the third-order nonlinear susceptibility of acid blue 129 dye in polar solvents is calculated to be 10-3cm/W and 10-5 esu, respectively. The multi-parameter scale viz., Kamlet-Abboud-Taft is used to measure the influence of solvent parameters on solute molecule. The results from the experiments suggest that the acid blue 129 is a suitable candidate for nonlinear optical (NLO) applications.

Ab Initio Investigation of Intramolecular Charge Transfer States in DMABN by Calculation of Excited State X-ray Absorption Spectra

Dual fluorescence in 4-(dimethylamino)benzonitrile (DMABN) and its derivatives in polar solvents has been studied extensively for the past several decades. An intramolecular charge transfer (ICT) minimum on the excited state potential energy surface, in addition to the localized low-energy (LE) minimum, has been proposed as a mechanism for this dual fluorescence, with large geometric relaxation and molecular orbital reorganization a key feature of the ICT pathway. Herein, we have used both equation-of-motion coupled-cluster with single and double excitations (EOM-CCSD) and time-dependent density functional (TDDFT) methods to investigate the landscape of excited state potential energy surfaces across a number of geometric conformations proposed as ICT structures. In order to correlate these geometries and valence excited states in terms of potential experimental observables, we have calculated the nitrogen K-edge ground and excited state absorption spectra for each of the predicted "signpost" structures and identified several key spectral features that could be used to interpret a future time-resolved X-ray absorption experiment.

Broadband Tunable Optical Gain from Ecofriendly Semiconductor Quantum Dots with Near-Half-Exciton Threshold.

Optical gain in solution-processable quantum dots (QDs) has attracted intense interest toward next-generation optoelectronics; however, the development of optical gain in heavy-metal-free QDs remains challenging. Herein, we reveal that the ZnSe1-xTex-based QDs show excellent optical gain covering the violet to near-red regime. A new gain mechanism is established in the alloy QDs, which promotes a theoretically threshold-less optical gain thanks to the ultrafast carrier localization and suppression of ground-state absorption by the Te-derived isoelectronic state. Further, we disclose that the hot-carrier trapping represents the main culprit to exacerbate the gain performance. With the increase of Te-to-Se ratio, a sub-band-gap photoinduced absorption (PA) appears and extinguishes the optical gain. To overcome this issue, we modulate the inner ZnSe shell thickness, and the gain is recovered by reducing the overlap between the gain and PA regions in the Te-rich QDs. Our finding represents a significant step toward sustainable QD-based optoelectronics.

New Hybrid Compound Candidate as Photothermal Agent Based on DPP Derivatives and Toluidine Blue: A Theoretical Perspective.

In this article, the synthesis of a new hybrid compound, candidate as photothermal agent, is proposed, based on TDPP (3,6-di(thiophene-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) and toluidine blue. Electronic structure calculations at the DFT, TD-DFT and CCSD level of theories were performed to obtain ground and excited states molecular structures, photophysical properties and absorption spectrum of the hybrid and the starting compounds. Additionally, ADMET calculations were performed to predict the pharmacokinetic, metabolic and toxicity properties of the proposed compound. The results showed that the proposed compound is a strong candidate for photothermal agent since (1) it absorbs close to the near-infrared region, (2) it has low fluorescence and intersystem crossing rate constants, (3) it has accessible conical intersection with low energy barrier, (4) the compound shows lower toxicity than the well know compound toluidine blue, which is used in photodynamic therapy, (5) the compound does not show carcinogenic potential, and (6) it obeys the Lipinski's rule of five, used as a reference for the design of new pharmaceuticals.

Eu3+‐Activated Single‐Band Ratiometric Nanothermometry by Lattice Negative Thermal Expansion

AbstractSingle‐band ratiometric (SBR) thermometry has recently emerged as a powerful alternative to its dual‐emission counterparts because it can avoid the large uncertainties related to the emission spectral overlap or light absorption/scattering by the medium. Herein, a novel SBR thermometric scheme in Sc2Mo3O12:Eu3+ nanosheet that depends on thermal enhancement of charge‐transfer state absorption between O2− and Eu3+ is reported. Mechanistic investigation reveals the vital role of the lattice negative thermal expansion (NTE) for thermally enhanced Eu3+ emissions according to the configuration coordinate model. In contrast, serious thermal quenching of Eu3+ emissions is detected under the excitation wavelength corresponding to ground state absorption. Such excitation wavelength‐dependent thermal behavior of luminescence enabled SBR thermometry with high sensitivity and resolution (Sr = 2.0% K−1, δT = 0.121 K at 363 K). Finally, the applicability of the proposed SBR model to real‐world sensing scenarios is demonstrated using the as‐fabricated flexible thin‐film, offering accurate and real‐time temperature detection at the local hotspot in the electronic component.

Benchmarking DFT functionals for photophysics of pyranoflavylium cations

An intense absorption, phosphorescence, a long triplet excited state lifetime and singlet oxygen generation capabilities are characteristics of pyranoflavylium cations, analogues to pyranoanthocyanidins originated in the maturation process of red wine. Such properties make these compounds potential photosensitizers to be applied in photodynamic therapy. In this context, the photophysical processes underlying that treatment critically depend on the electronic structure of the pyranoflavylium molecules. When employing density functional theory to describe the electronic structure of molecules, the choice of the most suitable functional is not trivial, and benchmark studies are needed to orient practitioners in the field. In this work, a benchmark of seven of the most commonly used density functionals in addressing the photophysical properties of a set of eight pyranoflavylium cations is reported. Ground and excited state geometries, molecular orbitals, and absorption, fluorescence and phosphorescence transition energies were calculated using density functional theory approaches, and evaluated and compared to experimental data and monoreferential wave function-based methodologies. Statistical analysis of the results indicates that global-hybrid functionals allow an excellent description of absorption and emission energies, with errors around 0.05 eV, while range-separated variants led to somewhat larger errors in the range 0.1–0.2 eV. In contrast, range-separated functionals display excellent phosphorescence energies with errors close to 0.05 eV, in this case global-hybrids showing increased discrepancies around 0.5–0.1 eV.

New asymmetric AB3-type free-base and metalloporphyrin-cyclotriphosphazene conjugates: Synthesis and photophysical-photochemical properties

In this work, new asymmetric H2-(3), Zn(II)-(4) and Pd(II)-(5) porphyrin derivatives conjugated with a cyclotriphosphazene ring bearing 2‑naphtoxy groups in paraposition were synthesized and fully characterized. For investigation of photophysical and photochemical properties of these compounds; their ground state absorption, fluorescence emission, fluorescence lifetime, fluorescence quantum yield and singlet oxygen quantum yield measurements were studied in THF. The singlet oxygen quantum yields were determined as 0.62 for 3, 0.49 for 4 and 0.64 for 5 in THF, respectively. As a result, conjugation of the porphyrin macrocycle with the cyclotriphosphazene ring have improved the singlet oxygen generation except compound 4 when compared to tetraphenylporphyrin (TPP) (0.58) and ZnTPP (0.61) in THF. Especially, compound 5 has the highest singlet oxygen quantum yield (0.64) due to the heavy metal effect of Pd(II).

Pump Excited-State Absorption at 442 ± 1 nm for Efficient Visible Fluoride Fiber Lasers

In the well-known excited-state absorption (ESA) process, population inversion of the laser transition can be achieved by a strong pump ESA involving a lower laser level. The exploitation of pump ESA has expanded from the visible to the near-infrared spectral range, based on various laser sources. Extending the operating wavelength of the pump ESA is greatly significant for the development of rare-earth-doped fluoride fiber lasers. Herein, we report, presumably, the shortest operating wavelength of pump ESA at 442 ± 1 nm for an efficient continuous-wave visible fluoride fiber laser. A novel scheme for direct blue diode pumping was proposed and demonstrated in an Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber. Utilizing a 442 ± 1 nm blue laser diode, single-wavelength ground-state absorption (GSA) and ESA pumping mechanism were investigated not only for double populating of the upper laser level by GSA and ESA, but also to depopulate the lower laser level by ESA. We report a slope efficiency of 50.3% with respect to the launched pump power of 442 ± 1 nm and an output power of up to 327 mW for a 750 nm fiber laser. This result is an order of magnitude higher than that previously reported for a directly blue-diode-pumped deep-red Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber laser. Moreover, the numerical modeling results were in good agreement with the experimental results. Our approach represents a significant advancement for blue pump ESA in visible fluoride fiber lasers, which opens new capabilities for blue-diode pumping for a wide range of future applications.

Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2, cubic SrF2, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems.

Employing Long-Range Inductive Effects to Modulate Metal-to-Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes

Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1-Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH2Cl2, τ = 2.5-4.3 μs. This series of molecules also feature high PL quantum efficiencies (ΦPL = 5.3-12% in CH2Cl2). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a 1MLCT excited state lying slightly above the 3MLCT state, 1050-1490 cm-1. Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH3CN. Combined PL and electrochemical data provided evidence that Cu1-Cu4 are highly potent photoreductants (Eox* = -1.73 to -1.62 V vs Fc+/0 in CH3CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that long-range electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1-Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10-diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 108 M-1 s-1. The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.

Synergic effects of Sn4+ doping on the nonlinear optical limiting properties of SnxCd1-xMoO4 nanostructures for optoelectronic applications

There is a rising demand in the development of novel nonlinear optical materials comprised out of transition metal molybdates owing to their fascinating optical and luminescent properties, resulting in wide application in future optoelectronic and optical limiting devices. Herein, we report non linear optical properties of novel tin (Sn4+) doped Cadmium molybdate (CdMoO4) nanostructures synthesized via chemical precipitation technique. The X-ray diffraction (XRD) pattern confirms that pristine as well as tin (Sn4+) doped Cadmium molybdate (CdMoO4) are highly crystalline with a tetragonal structure. Raman spectroscopy further confirms the structure and phase formation of SnxCd1-xMoO4 nanostructures. From the UV–Visible absorption spectra, it is observed that the ground state absorption increases in the UV region after the addition of the Sn4+, and the bandgap is found to be decreased. The morphology and particle size was analysed using Transmission electron microscopy (TEM). The surface binding energy and oxidation state was analysed by X-ray photo electron spectroscopy. The non-linear optical properties was studied by z scan technique using Q switched pulsed Nd YAG laser. Experimental results suggests, that two photon absorption mechanism is responsible for the observed nonlinearity. It is found that two photon absorption (β) is increased from 2.51 to 955.5×10−11 and third order susceptibility (χ(3)) enhances from 10.36 to 1480×10−13esu on Sn doping. The optical limiting threshold of pure and SnxCd1-xMoO4 nanostructures was also studied and found to be in the range of 41.45 to 11.92 J/cm2. These results clearly show that SnxCd1-xMoO4 nanostructures as a potential candidate for photonic, optical limi,ting and optoelectronic devices.

Synthesis, Photophysics and Tunable Reverse Saturable Absorption of Bis-Tridentate Iridium(III) Complexes via Modification on Diimine Ligand.

Five novel bis-tridentate Ir(III) complexes (Ir-1−Ir-5) incorporating versatile N^N^C ligands and a N^C^N ligand (1,3-di(2-pyridyl)-4,6-dimethylbenzene) were synthesized. With the combination of experimental and theoretical methods, their steady and transient state characteristics were researched scientifically. The UV-visible absorption spectra show that the broadband charge transfer absorbance of those bis-tridentate Ir(III) complexes can reach 550 nm, all of these complexes reveal the long-lasting phosphorescent emission. Because the excited-state absorption is more powerful than the ground-state absorption, a sturdy reverse saturable absorption (RSA) process can ensue in the visible and near-infrared regions when the complexes are exposed to a 532 nm laser. Therefore, the optical power limiting (OPL) effect follows the trend: Ir-5 > Ir-4 ≈ Ir-3 > Ir-2 > Ir-1. Generally speaking, the expansion of π-conjugation and the introduction of electron donating/withdrawing groups on the N^N^C ligand could effectively elevate the OPL effect. Therefore, these octahedral bis-tridentate Ir(III) complexes might be exploited as potential OPL materials.

Photophysical Modulation of Rhodamine‐B via π‐π stacking with GQD and Its Further Tuning by Cucurbit[7]uril**

AbstractHerein, we report modulation of photophysical properties of a rhodamine B (RhB) upon interaction with a synthesized graphene quantum dot (GQD). The GQD nanomaterial was prepared through a facile top‐down liquid phase peeling technique, starting from synthesized graphene oxide (GO). The detailed characterization using FT‐IR, XPS, AFM and TEM revealed that the GQDs consisted of 2–3 graphene layers with about 5 nm size dots and major functional groups −COOH and −OH on the surface edges. The modulations in the absorption and emission spectra of RhB in ethanol and water with the addition of the prepared GQD were observed using UV‐vis absorption, steady‐state and time‐resolved fluorescence spectroscopy. Such photophysical modulations of RhB can be attributed to the synergistic effect of charge transfer and π‐π interactions between the electron rich GQDs and the cationic RhB molecules. Importantly, strong ability of the GQDs to quench the fluorescence of RhB, in addition to its modulated ground state absorption spectra, indicated the dynamic and static quenching mechanism. Further, the addition of the macrocyclic host cucurbit[7]uril (CB7) to the RhB@GQD composite in water caused anticipated recovery of fluorescence of the RhB dye, indicating a weak non‐covalent interaction between the GQDs and RhB. Hence, this study might provide a fresh outlook on the interactions of photoluminescent GQDs and organic dye molecules for making useful functional materials.