Two-photon absorption of photochromic diarylethene and its application to rewritable holographic recording

A new photochromic diarylethene (1a) has been prepared. Both its photochromic behavior and nonlinear optical properties are investigated. 1a shows excellent ring-opening (λ_max = 386 nm) and ring-closing (λ_max = 652 nm) photoisomerization with UV-Vis light irradiation. With 800 nm femtosecond pulsed laser irradiation, 1a shows two-photon-induced photoisomerization, and a two-photon absorption cross-section (σ = 423×10⁵⁰ cm⁴·s per photon) is obtained by using two-photon induced fluorescence method. The applications of two-photon absorption of 1a to holographic recording has been also investigated. A two-photon induced micro-pattern is recorded on the diarylethene 1a-PMMA film with an femtosecond laser of 800 nm, 100 fs, 1 kHz, 50 mW.     

Blue two-photon excited fluorescence of several D-π-D, A-π-A, and D-π-A compounds featuring dimesitylboryl acceptor

Several donor-π-donor (D-π-D), acceptor-π-acceptor (A-π-A), and donor-π-acceptor (D-π-A) types of organic compounds with fluorene as π bridge and dimesitylboryl group as electron acceptor, which show strong two-photon excited blue fluorescence, have been synthesized and structurally investigated. The symmetric A-π-A type of compound exhibits the shortest wavelength of two-photon excited fluorescence (TPEF) at λ_em=405 nm under the excitation of λ_ex=730 nm; the unsymmetric D-π-A type of compound with diphenylamino as donor exhibits the most intense TPEF at blue region (λ_em=484 nm) with a two-photon absorption cross-section of 425 GM under λ_ex=800 nm.     

One- and two-photon laser photolysis of dicyanoacetylene

The time-resolved electronic absorption spectra of CN radical, resulting from the laser photolysis of dicyanoacetylene (DCA) at 193 and 248 nm, were analyzed. Detection of the other probable photodissociation product—C₃N radical—has not been possible. Photolysis at 193 nm produces CN both in the X²∑⁺ and in the A²Π_i manifolds, the latter—probably populated via a two-photon process—being revealed by a delayed (collision-induced) transfer to the higher vibrational levels of the ground electronic state. Photolysis at 248 nm is an efficient two-photon process. A simplified kinetic model for the decay of CN radical has been proposed and the rate constant for the CN + DCA reaction was derived. Semiempirical INDO/S CI-1 calculations of the DCA valence shell electronic transitions were performed.     

对位氨基能显著提高D-A-D型化合物的双光子吸收性能

报道了具有典型D-A-D型共轭结构的反式2,5-二氰基-1,4-二(4'-甲氧基苯乙烯基)苯(MOS-CN), 2,5-二氰基-1,4-二(4'-二甲胺基苯乙烯基)苯(MAS-CN)和1,4-二(4'-甲氧基苯乙烯基)苯(MOS)的合成. 用核磁、红外和元素分析进行了表征. 测试了紫外吸收光谱、单光子荧光光谱、双光子荧光光谱、双光子吸收系数及双光子吸收截面. 在800 nm的飞秒脉冲激光激发下, 化合物MOS-CN, MAS-CN和MOS分别发出很强的绿色、黄色和蓝色上转换荧光. 化合物MOS-CN, MAS-CN和MOS的最大吸收波长、单光子发射波长、双光子诱导荧光波长、荧光量子产率、双光子吸收系数、双光子吸收截面及双光子荧光寿命各分别是393, 473, 367 nm; 470, 569, 434 nm; 475, 574, 438 nm; 0.12, 0.72, 0.21; 0.8, 5.3, 0.3 cm/GW; 270, 1790, 101 GM; 140 ps, 1.32 ns, 54 ps. MAS-CN的双光子吸收截面是MOS-CN的6.63倍, MOS-CN的双光子吸收截面是MOS的2.67倍, 表明对位氨基显著地提高了化合物的双光子吸收性能, 氰基也较大地提高了双光子吸收截面.     

Competition between photodissociation and photoionization in methyl iodide

CH₃I has a number of 5pπ → 6p Rydberg states which can be excited by two-photon absorption from the ground state. These two-photon absorptions have been previously detected by molecular ionization with a third photon. They can also be studied by observation of the VUV fluorescence from neutral photofragments (excited iodine atoms) following absorption of a third photon. Similarly it is found that absorption of one photon at 118 nm (10.5 eV) is followed by dissociation into excited iodine atoms as well as ionization. Iodine atom VUV emission is 2–10 times stronger from CD₃I than from CH₃I.     

1,3,4-噁二唑衍生物的双光子吸收和双光子泵浦荧光

依据“推电子基-共轭中心-拉电子基-共轭中心-推电子基”的模型将电荷传输型1,3,4-噁二唑环嵌入芳香共轭体中, 通过Wittig-Horner反应合成了2种对称型强双光子吸收和双光子诱导荧光分子2,5-二[4-(2-芳基乙烯基)苯基]-1,3,4-噁二唑. 它们的氯仿溶液在锁模Nd: YAG激光器800 nm激光照射下, 发射出很强的双光子上转换荧光, 其最强荧光分别在波长507和475 nm. 采用非线性透过率法测得其双光子吸收截面分别为1.07×10^－46和6.6×10^－47 cm⁴•s•photon^－1. 这2个对称型D-π-A-π-D生色分子从激发端基到π共轭桥的有效能量传输, 对双光子吸收和双光子荧光发射能力贡献较大.     

Two-photon absorption study of Zn(II)tetraphenylporphin: Role of a higher excited singlet state of gerade symmetry

The S₂ state fluorescence of Zn(II)tetraphenylporphin has been studied by using two-photon absorption and optical—optical double-resonance techniques. The main process to populate the S₂ state was found to be a stepwise two-photon absorption to the S_nstate through the S₁ state. The large absorption cross section of the S_n ← S₁ transition (6.8 × 10^?16 cm² molecule^?1) at 540 nm suggests that there exists a higher excited singlet state of gerade parity.     

193 nm photolysis of CHCl3: Probe of the CH product by CRDS

The CH radical production induced by 193 nm two-photon photolysis of CHCl₃ has been measured for the first time via the cavity ring-down absorption spectroscopy of its A–X bands, using a commercial nanosecond pulsed dye laser. The range of pressure and laser intensity, as well as the time window detection, have been carefully chosen to ensure a constant CH number density during the measurement and to avoid post-photolysis reactivity. Internal energy distribution of the CH(X²II) fragment has been derived from population distribution simulations, leading to an average vibrational temperature T_vib = 1900 ± 50 K and rotational temperature T_rot = 300 ± 20 K. Two competing mechanisms can be invoked for the CH production channel: either two-photon absorption via resonant excited states of CHCl₃ leading to dissociation of excited CHCl₃, or two-photon sequential dissociation via the formation of the vibrationally excited CHCl₂ fragment. The latter mechanism is proposed to be the prominent process for CH formation.     

Quantum-sized gold clusters as efficient two-photon absorbers

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.     

Two-photon and multi-photon excitation of naphthalene in the vapor phase,studied by excitation and fluorescence spectra

Naphthalene vapor is irradiated by μsec dye laser pulses of 150 kW peak power and a spectral bandwidth of 0.3 nm. A two-photon excitation spectrum is detected by monitoring the near UV fluorescence as a function of laser wavelength which is tuned between 570 and 610 nm. The fluorescence obtained by irradiation into the strongest band of the two-photon spectrum could be spectroscopically resolved using a bandwidth of 80 cm^?1. The spectrum exhibits vibrational structure which lies on a strong non-resolved background. From information in both spectra it can be definitely concluded that vibronic levels of B_3u × b_3u species in the lowest singlet state are predominantly excited in a two-photon process. The non-resolved background in the fluorescence spectrum is attributed to subsequent excitation of the two-photon state by a third photon. Further stepwise excitation in the strong radiation field of the laser is also taken into account.     

两个具有强双光子荧光的有机硼化合物

以二米基硼为电子受体, 苯乙烯基噻吩为共轭桥, 合成了两个新的稳定的有机硼化合物: 反式,反式-2-二米基 硼-5-{2-[4-(2-噻吩乙烯基)苯基]乙烯基}噻吩(1)和反式,反式-1,4-二-[2-(5-二米基硼噻吩)乙烯基]苯(2). 前者为不对称结构的偶极分子, 后者为对称的A-π-A型四极分子. 对称性不同的化合物表现出不同的双光子吸收性质. 对于偶极分子1, 单双光子吸收达到的激发态能级接近, 而对于四极分子2, 双光子吸收达到的激发态则比单光子吸收所达到的激发态高出0.35 eV. 在波长为710到 900 nm范围的飞秒脉冲激光激发下, 化合物1和2在THF溶液中都可以发出很强的绿色上转换荧光 (1, λ_max＝505 nm; 2, λ_max＝513 nm). 用双光子荧光法测得A-π-A型化合物2在775 nm处的双光子吸收截面达1340 GM.     

Resolved single vibronic level fluorescence after two-photon excitation of benzene vapor: a direct confirmation of the inducing mode

Flourescence spectra have been observed from 1 and 5 torr of benzene after 504.2 nm excitation of the strongest two-photon band (linear polarization, ω₁ = ω₂) in the forbidden 260 nm ¹B_2u ← ¹A_1g absorption. The flourescence structure at 0.2 nm resolution is consistent with that expected from the fundamental ν′₁₄ in the excited state. This confirms directly the previous assignments of the dominant two-photon absorption progression as 1ⁿ_o14^L_o as well as the identity of the principal inducing mode, ν′₁₄ (b_2u). Vibrational relaxation from the level 14¹ appears similar to that observed from the one-photon levels.     

Fluorescence resonance energy transfer in a novel two-photon absorbing system

A novel fluorescence resonance energy transfer (FRET) system containing a two-photon absorbing dye and a nile red chromophore has been synthesized. Upon two-photon excitation by laser at 815 nm this molecule displays efficient energy transfer from the two-photon absorbing dye to the nile red moiety, with an 8-fold increase in emission compared to the model compound. Similarly, single-photon excitation of the two-photon absorbing moiety at 405 nm results in >99% energy-transfer efficiency, along with a 3.4-fold increase in nile red emission compared to direct excitation of the nile red chromophore at 540 nm. This system provides an effective way to use IR radiation to excite molecules that, by themselves, have little or no two-photon absorption.     

Laser two-photon ionization spectroscopy of molecules in liquids

A two-photon lonization technique has been developed and applied to determine the photoionization threshold of a molecule in liquid solution. The photoionization of pyrene in n-pentane was studied by monitoring the photocurrent of an≈ 10^?6 M solution as a function of the laser wavelength in the region 360–530 nm, corresponding to two-photon, transition in the region 180–265 nm. The photoionization threshold thus determined is 4.80 ± 0.02 eV. Resonances were observed in the two-photon ionization spectrum which are tentatively ascribed to preionization of one-photon forbidden transitions to states energetically degenerate with the continuum, reached via two-photon absorption.     

Sequence-specific fragmentation of benzyloxycarbonyl dipeptides by resonance-enhanced multiphoton ionization

The mechanism of fragment ion formation in resonance-enhanced multiphoton ionization (REMPI) of benzyloxycarbonyl (CBZ)-derivitized dipeptides is presented. At 266 nm, the entire multiphoton process can be thought of as a two-part scheme where ionization occurs by resonant two-photon ionization followed by photodissociation of the created ions. When the energy of two photons exceeds the molecular ionization energy by a significant amount, REMPI has the advantage of producing both parent ions and low appearance energy fragments in large amounts. For CBZ dipeptides, resonant two-photon ionization at 266 nm produces parent ions as well as A type sequence ions with high abundance. On the other hand, a three-photon process (resonant two-photon ionization followed by parent ion photodissociation) forms sequence-related ions which also involve complex fragmentations of the CBZ chromophore. These results are compared to mass spectra obtained by other ionization/dissociation methods and to REMPI mass spectra of related compounds. Factors related to molecular structure elucidation based upon REMPI mass spectra are discussed. Enhanced isomer distinction is demonstrated for CBZ-leu-ala-OCH₃ and CBZ-ile-ala-OCH₃ based upon REMPI fragmentation.     

Formation of free radicals in water under high-power laser uv irradiation

It has been shown that under high-power laser UV irradiation (I = 10⁸–10¹⁰ W/cm², λ = 266 nm, τ_p = 30 ps) water becomes ionized by a two-photon mechanism to form some free radicals including the hydrated electron e^?_aq.     

The two-photon spectrum of liquid pyridine by thermal lensing techniques

Thermal lensing is exploited to investigate the electronic properties of pyridine. The two-photon spectrum and polarization ratio of liquid pyridine has been measured from 2900 to 2000 Å. Absorptions corresponding to the two lowest ππ* states, of benzene parentage, and to the second excited nπ* state have been assigned. The latter, of A₂ symmetry, borrows two-photon intensity from A₁ states through vibronic coupling induced by a₂ vibrations. Calculations of allowed contributions to the two-photon cross sections for the lowest four excited levels support the assignment.     

Synthesis, Characterization and Non-linear Optical Properties of a New Organic Dye： DEAHAS

Two-photon absorption (TPA) is a process in which two photons are simultaneously absorbed to an excited state via a virtual state. The synthesis of organic optical materials with large TPA cross section has become a subject of great interest in recent years due to various application such as three-dimensional fluorescence imaging1, optical data storage2,3 and lithographic microfabrication4-6. Recently we have synthesized a new organic dye DEAHAS that is a symmetrically substituted stilbe…     

Two photon excitation spectrum of benzene and benzene-d6 in the gas phase: Assignment of inducing modes by hot band analysis

The two-photon absorption spectrum for benzene-h₆ and -d₆ is reported and assigned, leading to the frequency assignment for hitherto unknown vibrations in the electronically excited state. In addition absorption has been measured from hot bands in the ground state. This latter technique has allowed us to make an unequivocal assignment of the vibrational modes responsible for inducing the otherwise forbidden two-photon process. The result is in disagreement with current theory, for this prototype two-photon spectrum in the gas phase.     

The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm^?1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm^?1 (the limit reached in this study) is about 200 times greater than at 47 490 cm^?1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm^?1 is 280 cm^?1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators (xz, yz) or (zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is ¹B_1u ← ¹A_1g in which the vibronic intensity is derived from the ¹E_1u state in the one-photon and ¹E_1g in the two-photon spectrum.