Strong two-photon absorption of self-assembled butadiyne-linked bisporphyrin

Two-photon absorption (2PA) properties of self-assembled porphyrins were investigated. The butadiyne-linked porphyrin array exhibited a 20 times larger 2PA cross section than the meso-meso-linked self-assembled array due to the expansion of pi-conjugation. Higher-order nonlinear absorption was also observed in the former porphyrin.     

Nonlinear absorption and nonlinear refraction of self-assembled porphyrins

Nonlinear refraction and nonlinear absorption of self-assembled porphyrins in the nanosecond and picosecond regimes were studied at 532 nm by the Z-scan technique. First, a marked difference in nonlinear refraction was observed between self-assembled zinc porphyrins and free base porphyrins; however, the effects of self-assembly and metallization on nonlinear absorption are small. Second, an enhancement of nonlinear absorption was observed for the monomeric components of self-assembled structures by adding pyridine, while their nonlinear refractions remained almost unchanged as pyridine was added. It is expected that the metallization and addition of ligand can provide more convenient routes to alter the optical nonlinearities of porphyrins than the modifications of molecular structures of traditional covalent-bond organic materials.     

Charge transfer enhances two-photon absorption in transition metal porphyrins

Two-photon absorption processes were investigated in electropolymerized Fe(III), Mn(III), and Co(II) 5,10,15,20-tetrakis-(4-hydroxytetraphenyl)porphyrin films. Degenerate four wave mixing (DFWM) spectroscopy with 100 fs pulses in the near-IR spectral region was used. Metalloporphyrins with strong charge transfer (CT) transitions in the linear absorption spectra also show enhanced two-photon absorption. (Metalloporphyrin two-photon absorption cross section, delta, increases >10 times over that for the metal free porphyrin.) This effect was attributed to a two-photon induced charge transfer between the metal ion's d orbitals and the pi-system of the porphyrin. Correlation of one- and two-photon absorption properties of transition metal porphyrins suggests a new and simple approach to improve organic materials for photonic applications.     

Photon energy upconversion in porphyrins: one-photon hot-band absorption versus two-photon absorption

We study the porphyrin S₁→S₀ fluorescence and the photosensitized singlet oxygen ¹Δ_g→³Σ_g phosphorescence, both originating from absorption of photons with energy less than the porphyrin S₀→S₁ transition energy. By measuring the excitation intensity dependence of fluorescence at lowered sample temperatures, we are able to discriminate between two parallel processes of one-photon hot-band absorption (HBA) and simultaneous two-photon absorption (TPA). When the HBA and TPA contributions are comparable in magnitude, we use this new method to determine absolute TPA cross-section. We also demonstrate for the first time a singlet oxygen photosensitization via HBA in porphyrin.     

An insight into a novel class of self-assembled porphyrins: geometric structure, electronic structure, one- and two-photon absorption properties

We have theoretically investigated a series of butadiyne-linked porphyrin derivatives that exhibit large two-photon absorption (TPA) cross sections in the visible-IR range. The electronic structure, one-photon absorption (OPA), and TPA properties have been studied in detail. We found that the introduction of a butadiyne linkage and the increase of the molecular dimensionality from monomer to dimer determine the OPA intensities of Q band and Soret band, respectively. A most important role for the enhancement of the TPA cross section is played by introducing a butadiyne bridge. The complementary coordination and the combination of the terminal free base and the core zinc porphyrin are also two effective factors for the enhancement of the TPA efficiency. The dimer with two porphyrins linked at meso-positions by a butadiyne linkage results in a maximum TPA cross section (79.35 x 10(-48) cm4 s per photon). Our theoretical findings are consistent with the recent experimental observations. This series of porphyrin derivatives as promising TPA materials are the subject of further investigation.     

Theoretical investigation of one-photon and two-photon absorption properties for multiply N-confused porphyrins

We have theoretically investigated a series of multiply N-confused porphyrins and their Zn or Cu complexes for the first time by using DFT(B3LYP/6-31G*) and ZINDO/SOS methods. The electronic structure, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that the OPA spectra of multiply N-confused porphyrins are red-shifted and the OPA intensities decrease compared to normal porphyrin. The maximum two photon absorption wavelengths lambda(max) are blue-shifted and the TPA cross sections delta(max) are increased 22.7-112.1 GM when the N atoms one by one are inverted from core to beta position to form multiply N-confused porphyrins. Especially delta(max) of N3CP get to 164.7 GM. The electron donors -C6F5s at meso-position can make the TPA cross section delta(max) increase. After forming metal complexes with Cu or Zn, the TPA properties of multiply N-confused porphyrins are further increased except for N3CP, N4CP. Our theoretical findings demonstrate that the multiply N-confused prophyrins as well as their metal complexes and derivatives are promising molecules that can be assembled series of materials with large TPA cross section, and are sure to be the subject of further investigation.     

Influence of the central metal ion on nonlinear optical and two-photon absorption properties of push-pull transition metal porphyrins

We present a quantum-chemical analysis of the central metal ion's effect on first hyperpolarizabilities and two-photon absorption (TPA) cross sections at the infrared region of a series of push-pull porphyrins whose synthesis and NLO properties have been reported earlier (J. Am. Chem. Soc. 2005, 127, 9710). The molecular geometries are obtained via the B3LYP/6-31G(d,p) level optimization including SCRF/PCM approach, and the NLO and TPA properties are calculated with the ZINDO/CV method including solvent effects. It is found that the CT transition between the metal ion's d orbital and the macrocycle pi orbitals plays an important role on NLO and TPA properties of metal porphyrins. Our data suggest a new approach to enhance TPA properties of porphyrin materials. We also present a quantum-chemical analysis on porphyrin dimers and trimers to understand the relationship between structural and collective NLO properties. It has been observed that beta values can be improved about an order of magnitude and TPA properties can be enhanced by 2 orders of magnitude by the formation of a trimer. The importance of our results with respect to the design of photonic and photodynamic therapy materials have been discussed.     

Enhancement of two-photon absorption cross-section and singlet-oxygen generation in porphyrins upon beta-functionalization with donor-acceptor substituents

The microwave-enhanced synthesis, comparative singlet oxygen sensitization efficiency, and nonlinear optical characterization of a new beta-functionalized porphyrin and its copper complex are described. We show that the introduction of a donor-acceptor push-pull conjugated fragment in the beta position strongly perturbs the porphyrin electronic structure leading to a remarkable one- and two-photon NIR absorption enhancement.     

The effect of self-assembled monolayers on polarization-dependent two-photon photoemission and on the angular distribution of the photoelectrons

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p-polarized versus s-polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.     

Strong two-photon absorption in new asymmetrically substituted porphyrins: interference between charge-transfer and intermediate-resonance pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.     

Theory of two-photon absorption and emission second-order saturation effect

A simplified theory of small-signal two-photon absorption is developed, based on the solution of rate equations involving an infinitely short-living intimediate state. It is shown that in a first approximation this theory leads to results identical to those obtained using second-order perturbation techniques. Moreover, the present approach leads to a built-in saturation effect at high intensities depending on the parameters of the laser and of the absorbing medium. An analysis expression is given for the intensity dependent saturation.     

Solvent effects on the two-photon absorption of distyrylbenzene chromophores

A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modulate intramolecular charge transfer (ICT). These molecules allow an examination of solvent effects on the TPA cross section (delta) and on the TPA action cross section. In toluene, the delta values follow the order of ICT strength. The effect of solvent on delta is nonmonotonic: maximum delta was measured in an intermediate polarity solvent (THF) and was lowest in water. We failed to find a correlation between the observed solvent effect and previous theoretical predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are not included in calculational analysis, may be responsible for the discrepancies between experimental results and theory.     

Stepwise two-photon excitation of molecular iodine: the E state vindicated

The E ↔ B system of iodine has been examined by a stepwise two-photon process in which iodine molecules are excited first to individual vibrational—rotational levels of the B state, then to E state vibrational bands. In contrast to an earlier two-photon study by Danyluk and King, our data support the analysis of the emission spectra given by Wieland et al.     

Charge transfer induced enhancement of near-IR two-photon absorption of 5,15-bis(azulenylethynyl) zinc(II) porphyrins

Intramolecular charge transfer in 5,15-bis(azulenylethynyl) substituted zinc(ii) porphyrin leads to a significant enhancement of two-photon absorption at near-IR region, which has been investigated by femtosecond Z-scan method.     

Calculation of two-photon absorption spectra of monosubstituted benzenes with accounting for the Herzberg-Teller effect

A calculation was carried out of relative line intensities of two-photon absorption spectra of fluoro- and chlorobenzene in the¹A₁¹B₁ transition region. The separate aspects of the calculation procedure used are discussed, which is based on the concept of the matrix element of a transitional dipole moment in the Herzberg—Teller approximation and was previously successfully applied to electron-absorption and resonance Raman spectra. A satisfactory correspondence between the calculation results and the experimental data has been found.Saratov State Pedagogical Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 101–104, January–February, 1993.     

Stepwise synthesis of Gly-Gly-His on gold surfaces modified with mixed self-assembled monolayers

Peptide-modified electrode surfaces have been shown to have excellent recognition properties for metal ions. An efficient method of screening a potential peptide for its selectivity for a given metal would involve the synthesis of the peptide directly on the electrode surface. This paper outlines a procedure in which the tripeptide Gly-Gly-His was synthesized one amino acid at a time on a gold surface modified with a self-assembled monolayer of the mixed alkanethiolates 3-mercaptopropionic acid (MPA) and 3-mercaptopropane (MP). Electrochemistry and high-resolution mass spectrometry were used to elucidate the structure of the adsorbed species and follow the synthesis. The amino acids can be attached only to MPA, but the presence of a diluting unreactive molecule of MP reduces steric crowding about the reaction center. The maximum coverage of synthesized tripeptide occurs at a ratio of MPA/MP of 1:1.     

Scanning tunneling microscopy observation of self-assembled monolayers of strapped porphyrins

In this paper, we reveal that the free-base and zinc strapped porphyrins possessing long alkyl chains, C 24OPP-HQ and Zn(C 24OPP-HQ), respectively, can be arranged on surfaces. We used scanning tunneling microscopy (STM) to observe alkyl-chain-assisted self-assembled monolayers (SAMs) of these strapped porphyrins at the solid-liquid interface. STM images revealed that the strapped benzene moiety was detectable on the porphyrin core: that is, the strapped porphyrins could be differentiated from nonstrapped analogues. We compared the population of the nonstrapped porphyrin (C 24OPP) and either of the strapped porphyrins C 24OPP-HQ or Zn(C 24OPP-HQ) in the mixed SAMs. We then confirmed that Zn(C 24OPP-HQ) is more favorably incorporated in the mixed SAMs than C 24OPP-HQ. From (1)H NMR spectroscopic and X-ray crystallographic analyses, we concluded that the factors increasing the population of Zn(C 24OPP-HQ) in the mixed SAMs are the enhanced rigidity of the porphyrin core by the zinc coordination and the flat structure of the porphyrin moiety in the saddle conformation. This study demonstrates that strapped porphyrins possessing long alkyl chains are available to arrange the functional modules on the surface via chemical modification on the strapped moiety.     

Stepwise assembly of unsymmetrical supramolecular arrays containing porphyrins and coordination compounds

The stepwise coordination of meso-4'-pyridyl/phenyl porphyrins (4'-PyPs) to different metal centers proved to be an efficient synthetic approach leading to unsymmetrical arrays containing porphyrins and coordination compounds. The first step of this process, treatment of 4'-PyPs with a less than stoichiometric amount of cis,fac-RuCl2(Me2-SO)3(CO) (1), leads to the selective coordination of [cis,cis,cis-RuCl2(Me2SO)2(CO)] fragments ([Ru]) to some of the peripheral 4'-N sites of the 4'-PyPs. Column separation afforded four partially ruthenated 4'-PyPs in pure form: 4'-cis-DPyP[Ru] (2), 4'-trans-DPyP[Ru] (3), (4'-TPyP)[Ru] (4), and (4'-TPyP)[Ru]3 (5). These compounds, which have residual unbound peripheral 4'-N(py) sites (either one or three), were allowed to react with other metal centers that may belong either to a metalloporphyrin or to a coordination compound. When building blocks 2-5 were treated with [Ru(TPP)(CO)(EtOH)] (TPP = meso-tetraphenylporphyrin) in chloroform at room temperature, axial coordination of Ru(TPP)(CO) units ((Ru)) to the available 4'-N(py) sites readily occurred, generating the following arrays containing both perpendicular porphyrins and coordination compounds: (Ru)-(mu-4'-cis-DPyP)[Ru], (Ru)(mu-4'-trans-DPyP)[Ru], (Ru)3(mu-4'-TPyP)[Ru], and (Ru)(mu-4'-TPyP)[Ru]3. Furthermore, building blocks 2, 3, and 5 were treated with a series of coordination compounds capable of binding two pyridylporphyrins either cis to each other (trans-RuCl2(Me2SO)4 and trans,cis,cis-RuCl2(Me2SO)2(CO)2) or trans to each other (trans-PdCl2(C6H5CN)2). Homo- (Ru) and heterobimetallic (Ru-Pd) arrays with as many as seven metal atoms (six Ru and one Pd) and two 4'-PyPs were obtained as follows: trans,cis,cis-RuCl2(Me2SO)2(4'-cis-DPyP[Ru])2, trans,cis,cis-RuCl2(Me2SO)2(4'-trans-DPyP[Ru])2, trans,cis,cis-RuCl2(CO)2(4'-cis-DPyP[Ru])2, and trans-PdCl2(4'-TPyP[Ru]3)2. All the products were thoroughly characterized by 1H NMR spectroscopy. Since the [Ru] fragment is chiral, diastereomers are formed when two or more [Ru] units are bound to a porphyrin. We found that when two 4'-cis-DPyP[Ru] (2) units are coordinated cis to each other on the same metal center, the mutual anisotropic effect of the cis porphyrins differentiates the sulfoxide methyl resonances for the two forms. These and other results indicate that the pyridyl units react independently of the presence or absence of a substituent on the other py rings. Thus, the synthetic strategy should be a general method for linking diverse metal centers through pyridylporphyrins.     

双光子吸收材料的分子设计研究

介绍了双光子吸收材料分子设计原理.为了设计有大的双光子吸收响应的材料,对多种分子进行了系统的理论研究.用量子化学密度泛函理论和AM1方法进行分子几何构型优化.在优化结构的基础上,用ZINDO和自编程序求得分子的单、双光子吸收性质.设计了一些未知化合物,以期为合成新的具有大的双光子吸收截面的材料提供理论根据.以双层二聚二甲苯邻甲酸衍生物、铂乙炔化物、卟啉衍生物、C60、C70、八极矩分子为例,报道了我们在这方面的研究结果.     

Amphiphilic meso-disubstituted porphyrins: synthesis and the effect of the hydrophilic group on absorption spectra at the air-water interface

Five amphiphilic meso-disubstituted porphyrins bearing one polar group were synthesized, and their monolayer films were prepared. Their limiting molecular areas obtained from pi-A isotherms were 0.5-0.6 nm2, which were smaller than those of the corresponding meso-tetrasubstituted porphyrins. At the air-water interface, the disubstituted porphyrins showed a broad band in visible absorption spectra compared with the solution state, and the red shift of the Soret band exhibited a significant dependence on the kind of hydrophilic groups at the meso positions. Interestingly, the monolayer of the disubstituted porphyrin bearing a carboxyphenyl group (1-CO2H) showed a blue shift of the Soret band by adding cadmium chloride to the subphase, and the spectra varied upon multilayer deposition. The observed phenomena were interpreted by using the exciton theory. The effect of hydrophilic substituents on the absorption spectra of disubstituted porphyrin monolayers is discussed.