Strong enhancement of the two-photon absorption of tetrakis(4-sulfonatophenyl)porphyrin diacid in water upon aggregation

Molecular Insight into Dye-Surfactant Interaction at Premicellar Concentrations: A Combined Two-Photon Absorption and Molecular Dynamics Simulation Study

Both electrostatic and hydrophobic interactions play pivotal roles in ligand-surfactant binding interaction, especially for ionic surfactants. While much studies have been reported in the micellar region, less attention has been paid on such interactions at a low (premicellar) surfactant concentration. We here study the interaction between the cationic dye rhodamine 6G (R6G) with surfactants of different charge types: anionic SDS, cationic CTAB, and nonionic Tx 100 using absorption and emission spectroscopy. We identify that R6G forms dimeric aggregates at a premicellar concentration of SDS. Formation of aggregates is also confirmed from classical simulation measurements. CTAB and Tx 100 do not form any such aggregate, presumably owing to unfavorable electrostatic interactions. For a molecular-level understanding, we perform two-photon absorption (TPA) spectroscopy for the same systems. TPA allows us to calculate the two-photon absorption cross section and subsequently the change in the dipole moment (Δμ) between ground and excited states of the dye. We calculate the Δμ and observe that it passes through a maximum at a surfactant concentration half of the critical micelle concentration of SDS. This observation imparts support to earlier quantum mechanical calculation, which infers deviation from the parallel orientation of the dye during surfactant-induced aggregation. We extended our measurements and varied the carbon chain length of the anionic surfactant, and we found that all of them exhibit a maximum in Δμ, while their relative magnitude is dependent on the surfactant carbon chain length.

Anisotropic Two-Photon Absorption and Second Harmonic Generation in Single Crystals of Silver(I) Coordination Complexes

Metal complexes have been gaining attention in recent times over the traditional inorganic materials such as nonlinear optical materials. Here, we report both two-photon absorption (2PA) and second harmonic generation (SHG) from single crystals of two Ag(I) complexes with considerable optical anisotropy. We demonstrate that by controlling the incident light polarization, the tunability between these two nonlinear optical processes can be achieved. The d_eff values of the observed SHG from one complex are determined to be one order of magnitude greater than β-BBO crystals.

Effects of the Mixing Protocol on the Self-Assembling Process of Water Soluble Porphyrins

The hierarchical self-assembling kinetics of the porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS₄⁴⁻) into J-aggregates at high ionic strength under acidic conditions and eventually in the presence of an added chiral templating agent (tartrate) were investigated through UV/Vis spectroscopy, resonance light scattering, and circular dichroism (CD). The effect of changing the mixing order of the various components in the solution on the kinetic parameters and the expression of chirality on the final J-aggregates was evaluated. In this latter case, only when the chiral tartrate anion is premixed with the porphyrin, the resulting nano-architectures exhibit CD spectra that reflect the handedness of the chiral inducer. We discuss a general mechanistic scheme, with the involvement of ion pairs or dimers that offer an alternative pathway to the aggregation process.

Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin

Under moderate acidic conditions, the cationic (+3) complexes ions tris(1,10-phenanthroline)cobalt(III), [Co(phen)₃]³⁺, and hexamminecobalt(III), [Co(NH₃)₆]³⁺, efficiently promote the self-assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS₄) into J-aggregates. The growth kinetics have been analyzed according to a well-established autocatalytic model, in which the rate determining step is the initial formation of a nucleus containing m porphyrin units (in the range 2–3), followed by a stage whose rate constant k_c evolves as a power of time. The observed catalytic rate constants and the extent of J-aggregation increase on increasing the metal complex concentration, with the phen complex being the less active. The UV/Vis extinction spectra display quite broad envelops at the J-band, especially for the amino-complex, suggesting that electronic dipolar coupling between chromophores is operative in these species. The occurrence of spontaneous symmetry breaking has been revealed by circular dichroism and the measured dissymmetry g-factor decreases on increasing the aggregation rates. The role of these metal complexes on the growth and stabilization of porphyrin nano-assemblies is discussed in terms of the different degree of hydrophilicity and hydrogen bonding ability of the ligands present in the coordination sphere around the metal center.

Kinetic Investigation on Tetrakis(4-Sulfonatophenyl)Porphyrin J-Aggregates Formation Catalyzed by Cationic Metallo-Porphyrins

Under mild acidic conditions, various metal derivatives of tetrakis(4-N-methylpyridinium)porphyrin (gold(III), AuT₄; cobalt(III), CoT₄; manganese(III), MnT₄ and zinc(II), ZnT₄) catalytically promote the supramolecular assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS₄) into J-aggregates. The aggregation kinetics have been treated according to a well-established model that involves the initial formation of a critical nucleus containing m porphyrin units, followed by autocatalytic growth, in which the rate evolves as a power of time. An analysis of the extinction time traces allows to obtain the rate constants for the auto-catalyzed pathway, k_c, and the number of porphyrins involved in the initial seeding. The aggregation kinetics have been investigated at fixed H₂TPPS₄ concentration as a function of the added metal derivatives MT₄. The derived rate constants, k_c, obey a rate law that is first order in [MT₄] and depend on the specific nature of the catalyst in the order AuT₄ > CoT₄ > MnT₄ > ZnT₄. Both resonance light scattering (RLS) intensity and extinction in the aggregated samples increase on increasing [MT₄]. With the exception of AuT₄, the final aggregated samples obtained at the highest catalyst concentration exhibit a negative Cotton effect in the J-band region, evidencing the occurrence of spontaneous symmetry breaking. The role of the nature of the metal derivative in terms of overall charge and presence of axial groups will be discussed.

Practical two-photon-absorption cross sections and spectra of eosin and hematoxylin

We experimentally investigate the two-photon absorption cross sections and spectra of eosin and hematoxylin for applications in nonlinear microscopy. The experiments are carried out on pure samples of the two dyes in DI-water solvent with different concentrations, in the typical range employed in standard staining procedures. Nonlinear fluorescence is excited by a line-shaped beam emitted by a Ti:Sapphire mode-locked laser in the wavelength range from 740 to 880 nm and is detected through a microscope setup. The two-photon absorption spectral response is systematically analyzed and discussed. Finally, the staining is applied on biological tissue samples, which are imaged by two-photon microscopy. Our results show that the employed dyes are fully suitable for applications in nonlinear imaging.

Novel Nanohybrids Based on Supramolecular Assemblies of Meso-tetrakis-(4-sulfonatophenyl) Porphyrin J-aggregates and Amine-Functionalized Carbon Nanotubes

The ability of multiwalled carbon nanotubes (MWCNTs) covalently functionalized with polyamine chains of different length (ethylenediamine, EDA and tetraethylenepentamine, EPA) to induce the J-aggregation of meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) was investigated in different experimental conditions. Under mild acidic conditions, protonated amino groups allow for the assembly by electrostatic interaction with the diacid form of TPPS, leading to hybrid nanomaterials. The presence of only one pendant amino group for a chain in EDA does not lead to any aggregation, whereas EPA (with four amine groups for chain) is effective in inducing J-aggregation using different mixing protocols. These nanohybrids have been characterized through UV/Vis extinction, fluorescence emission, resonance light scattering and circular dichroism spectroscopy. Their morphology and chemical composition have been elucidated through transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). TEM and STEM analysis evidence single or bundles of MWCNTs in contact with TPPS J-aggregates nanotubes. The nanohybrids are quite stable for days, even in aqueous solutions mimicking physiological medium (NaCl 0.15 M). This property, together with their peculiar optical features in the therapeutic window of visible spectrum, make them potentially useful for biomedical applications.

Comprehensive investigation of the triplet state electronic structure of free-base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin by a combined advanced EPR and theoretical approach

The nature of the photoexcited triplet state of free-base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS^4-) has been investigated by advanced Electron Paramagnetic Resonance (EPR) techniques combined with quantum chemical calculations. The zero-field splitting (ZFS) parameters, D and E, the orientation of the transition dipole moment in the ZFS tensor frame, and the proton hyperfine couplings have been determined by magnetophotoselection-EPR and pulse electron-nuclear double resonance spectroscopy. Both time-resolved and pulse experiments exploit the electron spin polarization of the photoexcited triplet state. Comparison of the magnetic observables with computational results, including CASSCF calculations of the ZFS interaction tensor, provides an accurate picture of the triplet-state electronic structure. The theoretical investigation has been integrated with a systematic analysis on the parent free-base porphyrin molecule to assess the effect of the sulfonatophenyl substituents on the magnetic tensors. Additionally, the magnetophotoselection effects are discussed in terms of tautomerization in the excited singlet state of H₂TPPS^4-.

From Simplicity to Complexity in Grafting Dawson-Type Polyoxometalates on Porphyrin, Leading to the Formation of New Organic-Inorganic Hybrids for the Investigation of Third-Order Optical Nonlinearities

Four new organic-inorganic hybrids, [N(C₄H₉)₄]₅H[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}C₄₄N₄H₂₉O₃] (TPPOH-1POM), [N(C₄H₉)₄]₁₀H₂[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}₂C₄₄N₄H₂₈O₂] (TPPOH-2POM trans), [N(C₄H₉)₄]₁₀H₂[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}₂C₄₄N₄H₂₈O₂] (TPPOH-2POM cis), and [N(C₄H₉)₄]₁₅H₃[{HNC₂O(CH₂O)₃P₂W₁₅V₃O₅₉}₃C₄₄N₄H₂₇O] (TPPOH-3POM), have been synthesized by covalently grafting the Dawson-type polyoxometalate (POM) [N(C₄H₉)₄]₅[H₄P₂W₁₅V₃O₆₂] onto new porphyrins N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]-4-[10,15,20-tris(4-hydroxyphenyl)porphyrin-5-yl]benzamide (TPPOH-1Tris), 4,4'-[5,15-bis(4-hydroxyphenyl)porphyrin-10,20-diyl]bis[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-2Tris trans), 4,4'-[10,15-bis(4-hydroxyphenyl)porphyrin-5,20-diyl]bis[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-2Tris cis). and 4,4',4″-[20-(4-hydroxyphenyl)porphyrin-5,10,15-triyl]tris[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-3Tris), respectively, in N, N-dimethylacetamide under nitrogen. The hybrid compounds were thoroughly characterized by elemental analysis, FT-IR, NMR (¹H, 2D NOESY, ³¹P, and ⁵¹V), ESI-MS, MALDI-TOF-MS, UV-vis, and fluorescence spectroscopy. All porphyrins and their corresponding hybrids exhibited remarkable third-order nonlinear-optical responses under laser irradiation, with the intensity of light at focus E₀ being 4.2 μJ at wavelength 532 nm, pulse duration τ = 6 ns, and repetition rate 10 Hz. The molecular second hyperpolarizability (γ) values of the hybrids (TPPOH-1POM, TPPOH-2POM trans, TPPOH-2POM cis, TPPOH-3POM) were calculated to be 8.48 × 10^-28 esu, 8.94 × 10^-28 esu, 10.16 × 10^-28 esu, and 8.93 × 10^-28 esu, while the molecular second hyperpolarizability (γ) of their corresponding porphyrin precursors (TPPOH-1Tris, TPPOH-2Tris trans, TPPOH-2Tris cis, and TPPOH-3Tris) were 7.19 × 10^-28 esu, 8.09 × 10^-28 esu, 9.07 × 10^-28 esu, and 7.24 × 10^-28 esu, respectively. The γ values of the cis forms were found to be superior to those of all other compounds, indicating the significant role of the greater dipole moment and low symmetry of the cis form compared to other compounds. It was also found that the fluorescence intensity was decreased by 81.52%, 90.37%, 93.93%, and 84.62% for the hybrids TPPOH-1POM, TPPOH-2POM trans, TPPOH-2POM cis, and TPPOH-3POM with respect to their corresponding precursors, respectively, which reveals the Dawson-type POM as a strong quencher. This quenching phenomenon not only indicates again unequivocally the formation of hybrid compounds but also may imply a photoinduced electron/energy-transfer process favored from a porphyrin moiety to a Dawson-type POM moiety.

Mechanism for Copper(II)-Mediated Disaggregation of a Porphyrin J-Aggregate

J-aggregates of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin form at intermediate pH (2.3-3.1) in the presence of NiSO₄ or ZnSO₄ (ionic strength, I.S. = 3.2 M). These aggregates convert to monomeric porphyrin units via metallation with copper(II) ions. The kinetics for the disassembly process, as monitored by UV/vis spectroscopy, exhibits zeroth-order behavior. The observed zeroth-order rate constants show a two-term dependence on copper(II) ion concentrations: linear and second order. Also observed is an inverse dependence on hydrogen ion concentration. Activation parameters have been determined for the disassembly process leading to ΔH ^≠ = (+163 ± 15) kJ·mol^-1 and ΔS ^≠ = (+136 ± 11) J·K^-1. A mechanism is proposed in which copper(II) cation is in pre-equilibrium with a reactive site at the rim of the J-aggregate. An intermediate copper species is thus formed that eventually leads to the final metallated porphyrin either through an assisted attack of a second metal ion or through a direct insertion of the metal cation into the macrocycle core.

Cooperative porphyrin-quadrupolar based triad for combined two-photon induced fluorescence and singlet oxygen generation

The design and synthesis of a cooperative multichromophoric triad system which combines the large two-photon absorption properties of fluorene-cored bis-donor quadropolar dyes and the remarkable sensitizing properties of the porphyrin subunit (i.e. high intersystem crossing and ability to produce singlet oxygen by energy transfer to oxygen from its triplet excited state) is described. After irradiation the energy can be transferred from the quadrupolar chromophores to the porphyrin with an estimated 80% efficiency via a FRET process. Moreover both the two-photon absorption properties of the quadrupolar subunits and the sensitizing and fluorescence properties of the porphyrin are retained indicating that deleterious competing processes (such as photo-induced electron transfer) are prevented in such molecular architectures thanks to the implemented design. As a result, the two-photon absorption induced singlet oxygen generation efficiency of the triad in the NIR region is found to be enhanced by an order of magnitude as compared to the porphyin subunit. Potential applications of these porphyrin-based multichromophoric systems for photodynamic therapy based upon two-photon excitation in the NIR region might be possible since it overcomes the low two-photon absorption response of porphyrin while fully retaining their remarkable photosensitizing properties.

Spectroscopic investigation on porphyrins nano-assemblies onto gold nanorods

The interaction between gold nanorods (Au NRs), synthesized by a conventional seeded growth protocol, and the anionic tetrakis-(4-sulfonatophenyl)porphyrin (TPPS₄) has been investigated through various spectroscopic techniques. At neutral pH, the formation of H-aggregates and the inclusion of porphyrin monomers in CTAB micelles covering the nanorods have been evidenced. Under mild acidic conditions (pH=3) a nano-hybrid assembly of porphyrin J-aggregates and Au NRs has been revealed. For the sake of comparison, Cu(II) and Zn(II) metal porphyrin derivatives as well as a cationic porphyrin have been studied in the same experimental conditions, showing that: i) CuTPPS₄ forms porphyrin H-dimers onto the Au NRs; ii) ZnTPPS₄ undergoes to demetallation, followed by acidification of the central core and eventually aggregation onto Au NRs; iii) cationic porphyrin does not interact with Au NRs.

Nonlinear optical properties, upconversion and lasing in metal–organic frameworks

The building block modular approach that lies behind coordination polymers (CPs) and metal–organic frameworks (MOFs) results not only in a plethora of materials that can be obtained but also in a vast array of nonlinear optical properties that could be aimed at.

Correlated Fluctuations and Intraband Dynamics of J-Aggregates Revealed by Combination of 2DES Schemes

The intraband exciton dynamics of molecular aggregates is a crucial initial step to determine the possibly coherent nature of energy transfer and its implications for the ensuing interband relaxation pathways in strongly coupled excitonic systems. In this work, we fully characterize the intraband dynamics in linear J-aggregates of porphyrins, good model systems for multichromophoric assemblies in biological antenna complexes. Using different 2D electronic spectroscopy schemes together with Raman spectroscopy and theoretical modeling, we provide a full characterization of the inner structure of the main one-exciton band of the porphyrin aggregates. We find that the redistribution of population within the band occurs with a characteristic time of 280 fs and dominates the modulation of an electronic coherence. While we do not find that the coupling to vibrations significantly affects the dynamics of excitonic coherence, our results suggest that exciton fluctuations are nevertheless highly correlated.

Partial Unfolding of Tubulin Heterodimers Induced by Two-Photon Excitation of Bound meso-Tetrakis(sulfonatophenyl)porphyrin

The water-soluble porphyrin meso-tetrakis(p-sulfonatophenyl)porphyrin (TSPP) can be noncovalently bound to tubulin and used as a photosensitizer, which upon irradiation triggers photochemical reactions that lead to conformational changes of the protein. These conformational changes in turn inhibit tubulin's primary function of polymerizing into microtubules. We explored the possibility of using two-photon excitation of the bound porphyrin to induce photosensitized protein unfolding. Although TSPP has a relatively low cross section (∼30 GM) our results did find that two-photon excitation of the ligand causes partial unfolding of the tubulin host and the inhibition of the in vitro formation of microtubules. Conversely, irradiating tubulin alone caused no such effects despite the large irradiance per pulse (97-190 GW/cm(2)). The conformational changes were characterized using spectroscopic studies and provide a promising protocol for the future application of non-native photosensitization of proteins.

Systematic Study of Locally Dense Basis Sets for NMR Shielding Constants

This paper presents a systematic study of partitioning schemes for locally dense basis sets in the context of NMR shielding calculations. The partitionings explored were based exclusively on connectivity and utilized the basis sets from the pcS-n series. Deviations from pcS-4 shieldings were calculated for a set of 28 organic molecules at the HF, B3LYP, and KT3 levels of theory, with the primary goal being the determination of an efficient scheme that achieves maximal deviations of 0.1 ppm for (1)H and 1 ppm for (13)C. Both atom based and group based divisions of basis sets were examined, with the latter providing the most promising results. It is demonstrated that for the systems studied, at least pcS-1 is required for all parts of the molecule. This, coupled with pcS-3 on the group of interest and pcS-2 on the adjacent groups, is sufficient to achieve the desired level of accuracy at a minimal computational expense. In addition, the suitability of the pcS-n basis sets for post-SCF methods was confirmed through a comparison with other standard basis sets at the MP2 level.

Lifetime shortening and fast energy-tansfer processes upon dimerization of a A-π-D-π-A molecule

Time-resolved fluorescence and transient absorption experiments uncover a distinct change in the relaxation dynamics of the homo-dimer formed by two 2,5-bis[1-(4-N-methylpyridinium)ethen-2-yl)]-N-methylpyrrole ditriflate (M) units linked by a short alkyl chain when compared to that of the monomer M. Fluorescence decay traces reveal characteristic decay times of 1.1 ns and 210 ps for M and the dimer, respectively. Transient absorption spectra in the spectral range of 425-1050 nm display similar spectral features for both systems, but strongly differ in the characteristic relaxation times gathered from a global fit of the experimental data. To rationalize the data we propose that after excitation of the dimer the energy localizes on one M branch and then decays to a dark state, peculiar only of the dimer. This dark state relaxes to the ground state within 210 ps through non-radiative relaxation. The nature of the dark state is discussed in relation to different possible photophysical processes such as excimer formation and charge transfer between the two M units. Anisotropy decay traces of the probe-beam differential transmittance of M and the dimer fall on complete different time scales as well. The anisotropy decay for M is satisfactorily ascribed to rotational diffusion in DMSO, whereas for the dimer it occurs on a faster time scale and is likely caused by energy-transfer processes between the two monomer M units.

Kinetic control of chirality in porphyrin J-aggregates

Detailed kinetic investigations demonstrate the fundamental role of kinetic parameters in the expression and transmission of chirality in supramolecular systems. The rate of the aggregation process leading to the formation of J-aggregates strongly affects the size of these nanoassemblies and the chiral induction.

Significantly enhanced adsorption of bulk self-assembling porphyrins at solid/liquid interfaces through the self-assembly process

Controlling the adsorption behavior of bulk-phase self-assembling dye molecules at solid/liquid interfaces is of importance for application to various devices. Herein, we report an unexpected phenomenon on the adsorption behaviors of bulk J-aggregating water-soluble porphyrin diacids. A comparative study on the adsorption amounts of J-aggregated meso-tetrakis(4-sulfonatophenyl)porphyrin diacid from freshly prepared and pre-aged solutions revealed enhanced adsorption through the self-assembly process (EASAP). The aggregate structure formed by EASAP is almost identical to the one from preformed J-aggregate solutions. The generation ratio of J-aggregates at an interface and in bulk strongly depends on the interface-to-volume ratio of the solutions. The surface property of cuvettes and coexisting inorganic ions has no significant effects on EASAP. While EASAP occurs in the J-aggregations of the other water-soluble porphyrin diacids, it is suggested that self-assembly properties play an important role in the adsorption proportion. These results will provide new insight into the adsorption equilibrium of bulk self-assembling molecules at solid/liquid interfaces.

Porphyrin aggregates in the form of nanofibers and their unusual aggregation induced emission

We present here the simple procedure for synthesizing elongated fibers like porphyrin aggregates. Usually whenever the aggregation in dye molecules takes place the emission always tends to quench. In this work we explore and discuss the unusual enhanced emission property of these aggregates. The nanofibers of porphyrin were characterized with the help of atomic force microscopy and UV-vis spectroscopy whereas photoluminescence spectroscopy was used to check their emission property.

Kinetic effects of tartaric acid on the growth of chiral J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin

The kinetics of growth for chiral J-aggregates of H(4)TPPS(4) porphyrin have been investigated under different experimental conditions in the presence of tartaric acid. The observed rate constants and the anisotropy factor g show a defined dependence on the enantiomer used as a chiral templating agent.

Spectroscopic and kinetic investigations on porphyrin J-aggregates induced by polyamines

A non-covalent supramolecular approach to build up mesoscopic fractal J-aggregates of meso-tetrakis(4-sulfonatophenyl)porphyrin by using different polyamines under mild acidic conditions is reported. Spermine-induced J-aggregates have been treated as a model system for a kinetic investigation. The kinetic analysis has been performed through two non-conventional approaches as function of a different reagent mixing protocol. In particular, an autocatalytic pathway in which the formation of aggregation nuclei is the rate-determining step is found when porphyrin is added to the reaction mixture as first reagent. In order to gain information on this early stage, a kinetic investigation has been carried out as function of different parameters, such as pH, and both spermine and porphyrin concentration. The nucleation becomes particularly evident when polyamines with fewer than three protonable nitrogen atoms are used. In these cases, a sort of "YES/NO" effect for the aggregation process has been found as a function of the reagent mixing order protocol. Most importantly, the observed kinetic behavior strongly affects the mesoscopic structure of the final aggregates.

Shape of the Q band in the absorption spectra of porphyrin nanotubes: Vibronic coupling or exciton effects?

Absorption and linear dichroism spectra of self-assembled tubular aggregates of TPPS(4) porphyrin are studied theoretically with special emphasis on the low energy part of the spectra (the Q band region) where the coupling with intramolecular vibrations is pronounced. The model Hamiltonian includes both the excitonic coupling between four molecular electronic excited states contributing to the porphyrin Q and B bands as well as the intermediate-strength linear exciton-phonon coupling to one effective high-frequency molecular vibrational mode. Good agreement between the calculated and experimental spectra is obtained. The results allow us to identify the nature of the peaks observed in the Q band region of the aggregate's absorption spectrum; we show that the two most prominent peaks within the Q band originate from two different excitonic subbands. It is shown that the coupling between the Q and B bands plays an important role and the vibronic coupling affects the details of the absorption lineshape.

Holstein-Peirls-Hubbard trimer as a model for quadrupolar two-photon absorbing dyes

The linear and nonlinear optical properties of a Donor-Acceptor-Donor system have been investigated by using a two-electron three-point-site model system. Some basic features of electron correlations are included in the model by means of a bi-electronic density matrix. The polarizabilities and second hyperpolarizabilities have been computed with a modified version of the Collective Electronic Oscillators (CEO) method which allowed us to include the electron-phonon coupling. Both singly- and doubly-excited states are taken into account in the computation of (hyper-)polarizabilities. The effects of electron-phonon coupling on the two-photon absorption and on the third harmonic generation in the infrared region are discussed.

Spectroscopic investigation and molecular modeling on porphyrin/PAMAM supramolecular adduct

Noncovalent adducts (TPPC@PAMAM) between meso-tetrakis(4-carboxyphenyl)porphyrin (TPPC) and polyamidoamine PAMAM dendrimer (generation 2.0) have been obtained by simply mixing the two components at different stoichiometric amount. The resulting species are readily soluble and stable in aqueous solution up to millimolar concentration. Electrostatic interactions between the anionic carboxylate groups of TPPC and the protonated amino groups of the PAMAM dendrimer play an important role in the stabilization of these adducts. UV/Vis absorption, steady state and time-resolved fluorescence emission and anisotropy measurements suggest the presence of equilibria involving different species as function of the [PAMAM]/[TPPC] ratio. At low ratios the observed spectroscopic behavior evidence the presence of H-aggregates, while at higher ratios well-defined species containing monomeric TPPC strongly interacting with the charged dendrimer are formed. Docking of the binary supramolecular adduct further supports the experimental results showing a favorable interaction with the porphyrin being completely included in the dendrimer. The interaction of the binary TPPC@PAMAM adduct (1/1 ratio) with calf-thymus DNA has been investigated through spectroscopic and photophysical techniques. All the experimental results point to the formation of a ternary complex between the binary adduct and the DNA backbone.

Linear and nonlinear photophysics and bioimaging of an integrin-targeting water-soluble fluorenyl probe

Linear photophysical characterization and two-photon absorption (2PA) properties of a new water-soluble fluorene derivative, 3-(9-(2-(2-methoxyethoxy)ethyl)-2,7-bis{3-[2-(polyethyleneglycol-550-monomethylether-1-yl)]-4-(benzo[d]thiazol-2-yl)styryl}-9H-fluoren-9-yl)propanoic acid (1), were investigated in several organic solvents and water at room temperature. A comprehensive analysis of the steady-state absorption, emission and excitation anisotropy spectra revealed electronic structures of 1, including mutual orientation of the transition dipoles, relatively weak solvatochromic effects, high fluorescence quantum yield (approximately 0.5-1.0), and strong aggregation in water. The 2PA spectra of 1 were obtained in the 600-900 nm spectral range by two-photon induced fluorescence (2PF) and open aperture Z-scan methods using femtosecond laser sources. No discrete 2PA bands were apparent and values of the corresponding 2PA cross sections monotonically increased in the short wavelength range up to 3000 GM in organic solvents and approximately 6000 GM in aqueous solution, reflecting relatively high two-photon absorptivity. The 2PA efficiency of in water increased 2-3 times relative to aprotic solvents and can be explained by cooperative electronic effects of molecular aggregates of 1 produced in aqueous media. The carboxylic acid fluorenyl probe 1 was conjugated with the cyclic peptide RGDfK. Two-photon fluorescence microscopy (2PFM) imaging of U87MG cells (and MCF-7 as control), incubated with fluorene-RGD peptide conjugate 2, demonstrated high alpha(v)beta(3) integrin selectivity, making this probe particularly attractive for integrin imaging.