Early events in the photochemistry of 2-naphthyl azide from femtosecond UV/Vis spectroscopy and quantum chemical calculations: direct observation of a very short-lived singlet nitrene

Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.     

Excited-state processes in the carotenoid zeaxanthin after excess energy excitation

Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S2 state at 485 nm, into the 0-3 vibrational band of the S2 state at 400 nm, and into the 2B(u)+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S2 --> hot S1 --> S1 --> S0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the population toward the S state. The S2-S1 internal conversion time was shortened from 135 to 70 fs when going from 485 to 400 nm excitation, as a result of competition between the S2-S1 internal conversion from the vibrationally hot S2 state and S2 vibrational relaxation. The S1 lifetime of zeaxanthin was within experimental error the same for all excitation wavelengths, yielding approximately 9 ps. No long-lived species have been observed after excitation by femtosecond pulses regardless of the excitation wavelength, but excitation by nanosecond pulses at 266 nm generated both zeaxanthin triplet state and cation radical.     

A Two‐Dimensional Ruddlesden–Popper Perovskite Nanowire Laser Array based on Ultrafast Light‐Harvesting Quantum Wells

Miniaturized nanowire nanolasers of 3D perovskites feature a high gain coefficient; however, room-temperature optical gain and nanowire lasers from 2D layered perovskites have not been reported to date. A biomimetic approach is presented to construct an artificial ligh-harvesting system in mixed multiple quantum wells (QWs) of 2D-RPPs of (BA)₂(FA)_n−1Pb_nBr_3n+1, achieving room-temperature ASE and nanowire (NW) lasing. Owing to the improvement of flexible and deformable characteristics provided by organic BA cation layers, high-density large-area NW laser arrays were fabricated with high photostability. Well-controlled dimensions and uniform geometries enabled 2D-RPPs NWs functioning as high-quality Fabry–Perot (FP) lasers with almost identical optical modes, high quality (Q) factor (ca. 1800), and similarly low lasing thresholds.     

Narrowing of spontaneous emission and lasing in lyotropic and thermotropic liquid crystals

The narrowing of spontaneous emission and lasing are reported for the first time for a dye-doped lyotropic liquid crystal consisting of a methylbenzylamine solution of polybenzylglutamate (PBLG). Lasing was also studied in twisted nematics based on cholesterol derivatives. PBLG produces a cholesteric liquid crystal (CLC) with selective reflection in the visible region at PBLG concentrations above 55%. A comparison is made of the narrowing of spontaneous emission and lasing in lyotropic vs. thermotropic liquid crystals. In both cases lasing occurs where the selective reflection band overlaps the dye emission band. Thermotropic liquid crystals show a much lower lasing threshold than lyotropic systems. The lasing mechanism and the role of disorder in both systems are discussed.     

Narrowing of spontaneous emission and lasing in lyotropic and thermotropic liquid crystals

The narrowing of spontaneous emission and lasing are reported for the first time for a dye-doped lyotropic liquid crystal consisting of a methylbenzylamine solution of polybenzylglutamate (PBLG). Lasing was also studied in twisted nematics based on cholesterol derivatives. PBLG produces a cholesteric liquid crystal (CLC) with selective reflection in the visible region at PBLG concentrations above 55%. A comparison is made of the narrowing of spontaneous emission and lasing in lyotropic vs. thermotropic liquid crystals. In both cases lasing occurs where the selective reflection band overlaps the dye emission band. Thermotropic liquid crystals show a much lower lasing threshold than lyotropic systems. The lasing mechanism and the role of disorder in both systems are discussed.     

Optical gain from the open form of a photochromic molecule in the solid state

This work presents evidence for line-narrowing from the UV photoexcited open form of the photochromic molecule, indolinospiropyran (1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2,2'-(2H)-indole]) in the solid state. The line-narrowing is attributable to amplified spontaneous emission induced by optical gain and assisted by the waveguiding within the organic film. Optical gain throughout a band as large as 28 nm, with a maximum gain coefficient of 5.6 cm(-1), is observed in the merocyanine emission region (660-730 nm). These results open the way to the realization of hybrid devices based on the coupling between photochromic behavior and stimulated emission from conjugated molecules, such as lasing optical memories, and lasers gated by optical molecular switches.     

Lasing from chiral photonic band gap materials based on cholesteric glasses

Glass-forming cholesteric liquid crystals were studied as promising dye-doped lasing materials at different pumping energies and temperatures. Cholesteric samples doped with laser dye pyrromethane 597 were pumped by Nd:YAG laser. Lasing was found to depend strongly on the vitrification rate of cholesteric samples, their temperature and multidomain structure. The lasing threshold and intensity as a function of thickness of cholesteric resonator are determined by two competing factors: narrowing of the band edge modes and increasing disorder.     

Lasing from chiral photonic band gap materials based on cholesteric glasses

Glass-forming cholesteric liquid crystals were studied as promising dye-doped lasing materials at different pumping energies and temperatures. Cholesteric samples doped with laser dye pyrromethane 597 were pumped by Nd:YAG laser. Lasing was found to depend strongly on the vitrification rate of cholesteric samples, their temperature and multidomain structure. The lasing threshold and intensity as a function of thickness of cholesteric resonator are determined by two competing factors: narrowing of the band edge modes and increasing disorder.     

Ultrafast processes in OClO molecules excited by femtosecond laser pulses at 386-409 nm

Ultrafast dissociation dynamics in OClO molecules is studied, induced by femtosecond laser pulses in the wavelength region from 386 to 409 nm, i.e., within the wide absorption band to the (approximately)A (2)A(2) electronic state. The decay of the initially excited state due to nonadiabatic coupling to the close lying (2)A(1) and (2)B(2) electronic states proceeds with a time constant increasing from 4.6 ps at 386 nm to 30 ps at 408.5 nm. Dissociation of the OClO molecule occurs after internal conversion within about 250 fs. In addition, a minor channel of direct excitation of the (2)A(1) electronic state has been identified, the lifetime of which increases from a few 100 fs at 386 nm to 2.2 ps at 408.5 nm. Simultaneous excitation of two neighboring vibrational bands in the (approximately)A (2)A(2) state leads to a coherent oscillation of the parent ion signal with the frequency difference of both modes.     

Ultrafast and large third-order nonlinear optical properties of CdS nanocrystals in polymeric film

We report the ultrafast and large third-order nonlinear optical properties of CdS nanocrystals (NCs) embedded in a polymeric film. The CdS NCs of 2 nm radius are synthesized by an ion-exchange method and highly concentrated in the two layers near the surfaces of the polymeric film. The two-photon absorption coefficient and the optical Kerr coefficient are measured with laser pulses of 250 fs duration at 800 nm wavelength. The one-photon and two-photon figures of merit are determined to be 3.1 and 1.3, respectively, at irradiance of 2 GW/cm(2). The observed nonlinearities have a recovery time of approximately 1 ps. The two-photon-generated free carrier effects have also been observed and discussed. These results demonstrate that CdS NCs embedded in polymeric film are a promising candidate for optical switching applications.     

Temperature dependent spectral properties of type-I and quasi type-II CdSe/CdS dot-in-rod nanocrystals

We investigated systematically the temperature dependence of the spectral properties such as the band gap, bandwidth and fluorescence intensity of CdSe/CdS dot-in-rod nanocrystals. These asymmetry nanoparticles were synthesized by seeded growth techniques with band alignment of the type-I and quasi type-II with initial core sizes of 3.3 and 2.3 nm, respectively. With increasing temperature the band gap decreases and bandwidth increases, largely due to exciton-phonon scattering. Anomalous variations of the band gap and bandwidth were observed at 200-240 K, and the variations are attributed to the anisotropic strain in the CdSe/CdS interface due to temperature dependent lattice mismatch. The integrated intensity of fluorescence shows two variation regimes. In the low temperature regime, the intensity remained roughly constant due to the temperature dependent carrier mobility and trapping by the defect states in the CdS shell. However, in the higher temperature regime, the intensity decreased quickly due to thermal/phonon assisted escape from the CdSe dot. The barrier depths are estimated to be about 557 and 285 meV for type-I and quasi type-II samples, respectively.     

Synthesis and excited-state photodynamics of a chlorin-bacteriochlorin dyad--through-space versus through-bond energy transfer in tetrapyrrole arrays.

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne-linked chlorin-bacteriochlorin dyad (FbC-pe-FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC-pe-FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of approximately (5 ps)(-1) and efficiency of >99%. The excited bacteriochlorin resulting from the energy-transfer process in FbC-pe-FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Phif=0.19). F?rster calculations are consistent with energy transfer in FbC-pe-FbB occurring predominantly by a through-space mechanism. The energy-transfer characteristics of FbC-pe-FbB are compared with those previously obtained for analogous phenylethyne-linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular-orbital characteristics of the energy donor and acceptor constituents. The electron-density distributions in the frontier molecular orbitals provide insights into the through-bond electronic interactions that can also contribute to the energy-transfer process in the different types of dyads.     

New laser dye based on the 3-styryl analog of the BODIPY dye PM567

We report the synthesis, photophysical properties and evaluation of laser dye of a new BODIPY dye with a 3-styryl substituent, PMS, and with the rest of the substituents as in the commercial dye PM567. PMS shows an emission band at 584 nm in methanol, i.e. displaced ca. 50 nm to longer wavelengths with regard to the green-emission band of PM567, as well as a high-fluorescence quantum yield (0.82) and also a high-molar absorption coefficient (10⁵ M⁻¹ cm⁻¹) in the same solvent. The laser action of the new dye has been analyzed under transversal pumping at 532 nm, 5.5 mJ pulse⁻¹ and up to 10 Hz repetition rate, in both liquid phase and incorporated into solid polymeric matrices of methyl methacrylate copolymerized with crosslinking or fluorinated monomers. Lasing emission at 602–610 nm, with maximum efficiencies of 18%, were reached in these media. In solid-fluorinated matrices, good lasing photostabilities were established, with 30% of the initial laser output remaining after 100,000 pump pulses at 10 Hz.     

Formation of photochemical π-cation radicals

We have monitored the spectra and kinetics of μ-oxo iron (III) tetraphenyl porphyrin. The spectral changes between 460 nm and 770 nm were observed after excitation with 25 ps fwhm, 355 nm pulse. Kinetic studies from — 100 ps to 4 ns suggest that after the immediate formation of the excited states a transient species is formed which we assign to the π-caion radical-ferrous porphyrin pair. This pair decays with a time constant of 600 ps ± 100 ps leaving a small amount of disproportionation reaction photoproducts. The spectra and kinetics of the transients, were not altered by concentration (0.15 OD – 1.0 OD at 571 nm), solvent or addition of oxygen.     

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).     

多路发光CdxZn1-xSe量子点的制备及防伪功能设计

采用化学输运法制备了一系列CdxZn1-xSe合金量子点。通过对量子点的调控,改变量子点中Cd与Zn的组分,可有效地调控合金量子点的吸收带边,同时可以在524―627nm之间实现对CdxZn1-xSe合金量子点发射峰位的连续调控。并且利用量子点的可调色性,讨论了基于基于CdxZn1-xSe量子点多路发光的安全识别设计原理,认为这种新型的隐形防伪技术将有望应用于超市标签,安全系统和护照等领域。     

A study of electron transfer in Ru(dcbpy)2(NCS)2 sensitized nanocrystalline TiO2 and SnO2 films induced by red-wing excitation

Excited state dynamics and electron transfer from the Ru(dcbpy)2(NCS)2 (RuN3) sensitizer to semiconductor nanoparticles were studied using time-resolved femtosecond absorption spectroscopy. We found that excitation of the red wing of the absorption spectrum of the sensitizer populates the (3)MLCT state directly, both in solution and attached on semiconductor nanoparticle films. Electron injection is slowed down and becomes gradually less efficient as excitation moves towards red from the absorption maximum at 535 nm. At 675 nm the injection is non-exponential and characterized by 5, 30 and 180 ps time constants. The non-exponential electron injection observed is assigned to injection from a distribution of triplet states with energies below the semiconductor conduction band edge.     

The Origin and Dynamics of Soft X‐Ray‐Excited Optical Luminescence of ZnO

The distinct optical emission from ZnO materials, nanoneedles and microcrystallites synthesized with different sizes and morphologies by a flow deposition technique, is investigated with X‐ray excited optical luminescence (XEOL) and time‐resolved X‐ray excited optical luminescence (TR‐XEOL) from a synchrotron light source at the O K and Zn L_3,2 edges. The innovative use of XEOL, allowing site‐specific chemical information and luminescence information at the same time, is fundamental to provide direct evidence for the different behaviour and the crucial role of bulk and surface defects in the origin of ZnO optical emission, including dynamics. XEOL from highly crystalline ZnO nanoneedles is characterized by a sharp band‐gap emission (～380 nm) and a broad red luminescence (～680 nm) related to surface defects. Luminescence from ZnO microcrystallites is mostly dominated by green emission (～510 nm) associated with defects in the core. TR‐XEOL experiments show considerably faster decay dynamics in nanoneedles compared to microcrystallites for both band‐gap emission and visible luminescence. Herein we make a fundamental step forward correlating for the first time the interplay of size, crystallinity, morphology and excitation energy with luminescence from ZnO materials.     

Z-scan analysis and ab initio studies of β-BaTeMo2O9 single crystal

The Z-scan measurements for the non-centrosymmetric optical crystals β-BaTeMo₂O₉ (BTMO) were performed. The corresponding experiments were carried out using the 5 ns pulses of the second harmonic of a nanosecond Nd:YAG laser at the 532 nm wavelength. It was shown that the studied crystals possess promising third-order optical susceptibilities, which allow to use the crystal as optical limiters. The comparison with other oxide materials is presented. To clarify the origin of the observed effect, the electronic and optical properties of BTMO were calculated using the density functional theory (DFT)-based method. The performed calculations of the electronic and optical properties revealed certain peculiar features that can be suitable for the nonlinear optical applications. The relation between the observed nonlinear optical features and the calculated band structure is emphasized. The values of the calculated band gap and refractive index for β-BaTeMo₂O₉ are also reported.     

Picosecond time-resolved kinetic studies on the formation of short-lived pseudoisocyanine iodide photoisomers in methanol and ethylene glycol

Upon exciting pseudoisocyanine iodide with 20 ps pulses at wavelengths between 490 and 530 nm within the absorption band, formation of a new transient absorption with a maximum at 545 nm was observed. It is attributed to an unstable photoisomer of pseudoisocyanine iodide. The quantum yield of the photoisomer is 31% in methanol and 6% in ethylene glycol