One- and two-photon absorptions in asymmetrically substituted free-base porphyrins: a density functional theory study

Electronic spectra and structures of a new family of free-base porphyrin (H(2)P) derivatives with 4-(diphenylamino)stilbene (DPAS) or 4,4'-bis-(diphenylamino)stilbene (BDPAS) asymmetric substituents, recently synthesized and studied by Drobizhev et al. [J. Phys. Chem. B 110, 9802 (2006)] are investigated by density functional theory (DFT) using modern density functionals and the 6-31G* basis set. The time-dependent DFT technique is applied for calculations of one- and two-photon absorption spectra, electric and magnetic dipole moments, and for prediction of electronic circular dichroism for these chiral molecules. The four-band absorption spectrum of the H(2)P molecule (Q(x), Q(y), 0-0 and 1-0 bands) is enhanced in single-bond-linked DPAS. This enhancement is explained by hyperconjugation of the almost orthogonal pi systems and by small charge-transfer admixtures. The effect is much stronger for the double-bond- and triple-bond-linked DPAS and BDPAS substituents where absorption in the Q region transforms into a two-band spectrum. These molecules with ethenyl and ethynyl bonding of the porphyrin and donor substituent show very strong two-photon absorption in the near-infrared region. DFT calculations explain this by more efficient conjugation between the H(2)P and DPAS (BDPAS) chromophores, since they are almost coplanar: "Gerade" states of the H(2)P molecule occur in the Soret region and transform into charge-transfer states with nonzero transition moments. They are responsible for the strong two-photon absorption effects. Mixing of excitations in both chromophores explains the broadening of the Soret band. Though the calculated two-photon absorption cross sections are overestimated, the qualitative trends are reproduced and help understanding the whole genesis of spectra of these asymmetrically substituted H(2)P derivatives.     

Theoretical study of the two-photon absorption properties of several asymmetrically substituted stilbenoid molecules

Two-photon absorption (TPA) properties of noncentrosymmetric pi-conjugated stilbenoid molecules with D-pi-A structures, TPA spectra of which have been reported [L. Antonov et al., Phys. Chem. Chem. Phys. 5, 1193 (2003)], have been investigated theoretically by ab initio molecular orbital methods. The difference in the observed one-photon absorption and TPA spectra among compounds with the same donor (D) and acceptor (A) units is well reproduced by the present calculations, although the calculated excitation energies are overestimated by the configuration interaction with single excitation method used. It was found that the spectral differences among the compounds were mainly due to the deviation from the planar structure by intramolecular rotation around the N[Single Bond]C (phenyl) bond of the N-benzilideneanilines having the C[Double Bond]N linkage as the central pi bridge. Substitution of the end donor or acceptor groups with weaker ones leads to a decrease in the TPA intensity of the lowest pi-pi(*) TPA states, resulting mainly from the decrease in the dipole moment of the excited states. The total TPA cross section spectra have been separated into contributions of the dipolar term, which appear only in noncentrosymmetric systems, and the three-state term, which appear in any systems irrespective of symmetry. The dipolar term predominates only for the lowest pi-pi(*) state, while for the higher excited states the three-state terms become predominant. An analysis employing the index R(f) defined with the transition polarizability shows that the TPA properties of the higher excited states are well described by the three-state approximation mediated by the lowest pi-pi(*) state. The differences found between the centrosymmetric and dipolar molecules in the enhancement mechanism of the TPA intensity by substituting the end groups with strong donors are discussed by comparison with the TPA properties of azobenzenes symmetrically substituted with the same donors.     

具有不同长度碳链的不对称四苯基卟啉及其金属配合物的合成

叶绿素、血红素、维生素B_(12)等都是不对称的金属卟啉类化合物叫,1975年Anton等首次合成了几种不对称取代四苯基卟啉及其金属配合物.其后,Leznoff,Molinaro等也发表了有关工作.我们模拟叶绿素的结构合成了三个系列13个具有不同长度碳链的单取代四苯基卟啉及其铜、锌、钴、镍等金属配合物(图1).用元素分析、红外光谱、可见吸收光谱和核磁共振谱等表征了它们的可能结构.     

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.     

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.     

Vibronic induced one- and two-photon absorption in a charge-transfer stilbene derivate

Both the electronic and the vibronic contributions to one- and two-photon absorption of a D-pi-D charge-transfer molecule (4-dimethylamino-4'-methyl-trans stilbene) are studied by means of density functional response theory combined with a linear coupling model. Vibronic profiles of the first four excited states are fully explored. The dominating vibrational modes for both Franck-Condon and Herzberg-Teller contributions are identified. The Franck-Condon contribution dominates the spectra of first, second, and fourth excited states. The Herzberg-Teller contribution is on the other hand of comparable size for the third excited state, where its inclusion leads to a blueshift with respect to the vertical transition. A similar vibronic coupling behavior is found for both one- and two-photon absorptions.     

Resonant enhancement of two-photon absorption in substituted fluorene molecules

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.     

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.     

Photo-CIDNP evidence for charge-transfer interaction between synthetic water-soluble porphyrins and guanine

Abstract Photoreactions of aqueous solutions of synthetic water-soluble porphyrins were studied by the ¹H and ¹³C CIDNP technique. Strong polarizations, which were very sensitive to the presence of added acid, were observed on the cationic porphyrins (TMePyPH₂-TAPPH₂) when irradiated through continuous UV-visible light. They resulted from the reverse electron transfer between the semi-oxidized and the semi-reduced species of the derivative. When the experiments were carried out in the presence of nucleobases, guanine (and its derivatives) was the only residue that was polarized. This is thoroughly interpreted in terms of a reversible electron transfer reaction leading to guanine photooxidation by the porphyrin excited triplet state. It was shown to be drastically pH-dependent and was correlated to the redox potential of the porphyrin. It was not affected by the incident wavelength. The reaction proceeded through the intermediate formation of the correlated radical-ion pair: porphyrin radical anion-guanine radical cation. This study suggested that a Type I (free radical) reaction could be one of the primary processes in DNA photosensitization by porphyrins.     

Strong, low-energy two-photon absorption in extended amine-terminated cyano-substituted phenylenevinylene oligomers

Three quadrupolar oligophenylenevinylenes with five rings in the conjugated backbone, terminal donor groups, and various acceptors and/or donors along the backbone were synthesized and their two-photon spectroscopic properties investigated. These chromophores exhibit large two-photon absorption cross sections over a wide wavelength range and two distinct peaks, the strongest of which (deltamax > 3600 GM) is observed at 960-970 nm, a wavelength close to twice the value of the linear absorption maximum (2lambda(1)max). The findings on these chromophores are compared with those for analogous molecules with shorter conjugation length, for which the main two-photon band is at significantly shorter wavelength than 2lambda(1)max.     

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.     

Strong cooperative enhancement of two-photon absorption in double-strand conjugated porphyrin ladder arrays

We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin oligomers containing N = 2, 4, 8, and ca. 13 monomer units, meso-meso connected with butadiyne linkers. We demonstrate that, in the coplanar double-strand arrays, self-assembled upon addition of 4,4'-bipyridyl, the conjugation length increases dramatically, leading to very strong cooperative enhancement of 2PA. We analyze the scaling of 2PA in both the double-strand and rotationally free single-strand arrays and show how the effective conjugation length in both cases is linked to the observed 2PA properties. By introducing a "conjugation signature" for the 2PA strength, we show that, in double-strand arrangement, the conjugation embraces the whole molecule up to the tetramer level, whereas in single-strand arrangement, it is always less than N, except for N = 2, but keeps increasing until N = 8. Our finding of extremely strong 2PA cross section, sigma2 approximately 105 GM, in double-strand oligomers peaking at 1.3 mum can find use for signal processing in fiber-optic devices.     

Two-photon absorption strength: a new tool for the quantification of two-photon absorption

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDOS and HF+CIS3-21G) two-photon absorption properties of the 2,2'-bi(9,9-dihexylfluorene) molecule are compared.     

Metal-ion sensing fluorophores with large two-photon absorption cross sections: aza-crown ether substituted donor-acceptor-donor distyrylbenzenes

Chromophores based on a donor-acceptor-donor structure possessing a large two-photon absorption cross section and one or two mono-aza-15-crown-5 ether moieties, which can bind metal cations, have been synthesized. The influence of Mg(2+) binding on their one- and two-photon spectroscopic properties has been investigated. Upon binding, the two-photon action cross sections at 810 nm decrease by a factor of up to 50 at high Mg(2+) concentrations and this results in a large contrast in the two-photon excited fluorescence signal between the bound and unbound forms, for excitation in the range of 730 to 860 nm. Experimental and computational results indicate that there is a significant reduction of the electron donating strength of the aza-crown nitrogen atom(s) upon metal ion binding and that this leads to a blue shift in the position as well as a reduction in the strength of the lowest-energy two-photon absorption band. The molecules reported here can serve as models for the design of improved two-photon excitable metal-ion sensing fluorophores.     

Mixed substituted porphyrins: structural and electrochemical redox properties

To examine the influence of mixed substituents on the structural, electrochemical redox behavior of porphyrins, two new classes of beta-pyrrole mixed substituted free-base tetraphenylporphyrins H2(TPP(Ph)4X4) (X = CH3, H, Br, Cl, CN) and H2(TPP(CH3)4X4) (X = H, Ph, Br, CN) and their metal (M = Ni(II), Cu(II), and Zn(II)) complexes have been synthesized effectively using the modified Suzuki cross-coupling reactions. Optical absorption spectra of these porphyrins showed significant red-shift with the variation of X in H2(TPPR4X4), and they induce a 20-30 nm shift in the B band and a 25-100 nm shift in the longest wavelength band [Q(x)(0,0)] relative to the corresponding H2TPPR4 (R = CH3, Ph) derivatives. Crystal structure of a highly sterically crowded Cu(TPP(Ph)4(CH3)4).2CHCl3 complex shows a combination of ruffling and saddling of the porphyrin core while the Zn(TPP(Ph)4Br4(CH3OH)).CH3OH structure exhibits predominantly saddling of the macrocycle. Further, the six-coordinated Ni(TPP(Ph)4(CN)4(Py)2).2(Py) structure shows nearly planar geometry of the porphyrin ring with the expansion of the core. Electrochemical redox behavior of the MTPPR4X4 compounds exhibit dramatic cathodic shift in first ring oxidation potentials (300-500 mV) while the reduction potentials are marginally cathodic in contrast to their corresponding MTPPX4 (X = Br, CN) derivatives. The redox potentials were analyzed using Hammett plots, and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap decreases with an increase in the Hammett parameter of the substituents. Electronic absorption spectral bands of H2TPPR4X4 are unique that their energy lies intermediate to their corresponding data for the H2(TPPX8) (X = CH3, Ph, Br, Cl) derivatives. The dramatic variation in redox potentials and large red-shift in the absorption bands in mixed substituted porphyrins have been explained on the basis of the nonplanarity of the macrocycle and substituent effects.     

Strong two-photon absorption and its saturation of a self-organized dimer of an ethynylene-linked porphyrin tandem

The two-photon absorption properties of a self-organized dimer of a free-base and zinc(II) porphyrins tandem linked with an ethynylene group and terminated by imidazolyl and phenylethynyl groups were investigated. The self-organized dimer was found to exhibit strong two-photon absorption and furthermore the saturation of the two-photon absorption owing to the intense transition.     

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.     

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 two-photon absorption allowed excited states in symmetrically substituted diacetylenes by ab initio molecular-orbital method

Symmetrically substituted diacetylene compounds, which shows large two-photon absorption (TPA) cross sections, have been theoretically investigated by the ab initio molecular-orbital method employing several theoretical models including the configuration interaction with single excitation (CIS), random phase approximation (RPA), and time-dependent density-functional theory (TDDFT) methods. The calculated excited energies are overestimated by CIS or RPA, whereas underestimated by TDDFT with the B3LYP parametrization for both one-photon absorption (OPA) and TPA allowed states. The lowest OPA state is well described by the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition. On the other hand, lower TPA allowed states can be represented as the superposition of the HOMO-LUMO+1 and HOMO-1-LUMO transitions, giving rise to two TPA allowed states. The absorption intensity for the lower TPA state of the diacetylenes molecules is discussed in terms of the alternancy symmetry and its breaking. The symmetry property is differently manifested for neutral and dicationic diacetylenes. Introduction of charges breaks the alternancy symmetry, which gives rise to an increase in the TPA cross sections at the lower frequency. The upper TPA state is calculated to show huge TPA cross sections, which reproduces the enhancement of the TPA cross section experimentally observed for one of the diacetylenes at the higher-frequency region. The enhancement is discussed employing an index defined as the ratio of the transition polarizability and its static limit, which represents the degree of influence of one-photon resonance on the TPA intensity. The huge TPA cross sections are found to be due to a near-resonance effect. The present theoretical calculation approves the previously proposed assumption based on the four-state (dual three-state) model, which consists of the ground, one OPA allowed, and two TPA allowed states.     

Solvent induced channel interference in the two-photon absorption process--a theoretical study with a generalized few-state-model in three dimensions

For the first time, we report the effect of interference between different optical channels on the two-photon absorption (TPA) process in three dimensions. We have employed response theory as well as a sum-over-states (SOS) approach involving few intermediate states to calculate the TPA parameters like transition probabilities (δ(TP)) and TPA tensor elements. In order to use the limited SOS approach, we have derived a new formula for a generalized few-state-model (GFSM) in three dimensions. Due to the presence of additional terms related to the angle between different transition moment vectors, the channel interference associated with the TPA process in 3D is significantly different and much more complicated than that in 1D and 2D cases. The entire study has been carried out on the two simplest Reichardt's dyes, namely 2- and 4-(pyridinium-1-yl)-phenolate (ortho- and para-betain) in gas phase, THF, CH(3)CN and water solvents. We have meticulously inspected the effect of the additional angle related terms on the overall TPA transition probabilities of the two 3D isomeric molecules studied and found that the interfering terms involved in the δ(TP) expression contribute both constructively and destructively as well to the overall δ(TP) value. Moreover, the interfering term has a more conspicuous role in determining the net δ(TP) associated with charge transfer transition in comparison to that of π-π* transition of the studied systems. Interestingly, our model calculations suggest that, for o- and p-betain, the quenching of destructive interference associated with a particular two-photon process can be done with high polarity solvents while the enhancement of constructive interference will be achieved in solvents having relatively small polarity. All the one- and two-photon parameters are evaluated using a range separated CAMB3LYP functional.