Linear and Two-Photon Absorption Properties of Interacting Polar Chromophores: Standard and Unconventional Effects

Aggregates of Cyanine Dyes: When Molecular Vibrations and Electrostatic Screening Make the Difference

Aggregates of cyanine dyes are currently investigated as promising materials for advanced electronic and photonic applications. The spectral properties of aggregates of cyanine dyes can be tuned by altering the supramolecular packing, which is affected by the length of the dye, the presence of alkyl chains, or the nature of the counterions. In this work, we present a joint experimental and theoretical study of a family of cyanine dyes forming aggregates of different types according to the length of the polymethinic chain. Linear and nonlinear optical spectra of aggregates are rationalized here in terms of an essential-state model accounting for intermolecular interactions together with the molecular polarizability and vibronic coupling. A strategy is implemented to properly account for screening effects, distinguishing between electrostatic intermolecular interactions relevant to the ground state (mean-field effect) and the interactions relevant to the excited states (excitonic effects). To the best of our knowledge, this is the first attempt to simulate nonlinear spectral properties of aggregates of symmetric dyes accounting for molecular vibrations.

Fluorescent phosphorus dendrimers excited by two photons: synthesis, two-photon absorption properties and biological uses

Different types of two-photon absorbing (TPA) fluorophores have been synthesized and specifically functionalized to be incorporated in the structure of phosphorus dendrimers (highly branched macromolecules). The TPA fluorophores were included in the periphery as terminal functions, in the core, or in the branches of the dendrimer structures, respectively. Also the functionalization in two compartments (core and surface, or branches and surface) was achieved. The consequences of the location of the fluorophores on the fluorescence and TPA properties have been studied. Several of these TPA fluorescent dendrimers have water-solubilizing functions as terminal groups, and fluorophores at the core or in the branches. They have been used as fluorescent tools in biology for different purposes, such as tracers for imaging blood vessels of living animals, for determining the phenotype of cells, for deciphering the mechanism of action of anticancer compounds, and for safer photodynamic therapy.

Dual-wavelength efficient two-photon photorelease of glycine by π-extended dipolar coumarins

Photolabile protecting groups (PPGs) releasing bioactive compounds upon two-photon excitation have emerged as increasingly popular tools to control and study physiological processes. Yet the limited two-photon photosensitivity of many cages is still a critical issue for applications. We herein report the design, synthesis and photophysical study of polarized extended coumarinyl derivatives which show large two-photon sensitivity (up to 440 GM) at two complementary wavelengths in the NIR spectral range. DFT calculations demonstrate that subtle tuning of polarization in the ground-state and confinement of the photo-induced intramolecular charge transfer upon excitation is responsible for enhancing two-photon absorption while maintaining large uncaging efficiency. These findings open a new engineering route towards efficient coumarinyl PPGs.

Aggregates of quadrupolar dyes for two-photon absorption: the role of intermolecular interactions

We present a theoretical investigation of small aggregates of quadrupolar (A-π-D-π-A or D-π-A-π-D) charge-transfer dyes, with attention focused on the role of intermolecular interactions in determining their optical properties. We tackle the theoretical issue by adopting essential-state models (ESMs), which describe an isolated molecule in terms of a minimal number of electronic states, corresponding to the resonance structures. ESMs quite naturally describe intermolecular interactions relaxing the dipolar approximation and accounting for molecular polarizabilities. The approach is applied to curcuminoid and squaraine dyes, two families of chromophores with weak and strong quadrupolar character, respectively. The method is validated against experiment and for curcuminoids also against time-dependent density functional theory. ESMs rationalize the strong ultra-excitonic effects recurrently observed in the experimental optical spectra of aggregates of highly polarizable quadrupolar dyes, offering a valuable tool to exploit the supramolecular design of material properties.

Superlinear amplification of the first hyperpolarizability of linear aggregates of DANS molecules

A comprehensive study of optical properties of DANS in different environments explains the observed ∼30-fold enhancement of the hyper-Rayleigh signal of DANS@CNT vs. DANS in solution in terms of collective and cooperative phenomena occurring in aggregates of less than 10 aligned molecules.

Molecular-Based Fluorescent Nanoparticles Built from Dedicated Dipolar Thienothiophene Dyes as Ultra-Bright Green to NIR Nanoemitters

Fluorescent Organic Nanoparticles (FONs), prepared by self-aggregation of dedicated dyes in water, represent a promising green alternative to the toxic quantum dots (QDs) for bioimaging purposes. In the present paper, we describe the synthesis and photophysical properties of new dipolar push-pull derivatives built from thieno[3,2-b]thiophene as a π-conjugated bridge that connects a triphenylamine moiety bearing various bulky substituents as electron-releasing moiety to acceptor end-groups of increasing strength (i.e., aldehyde, dicyanovinyl and diethylthiobarbiturate). All dyes display fluorescence properties in chloroform, which shifts from the green to the NIR range depending on the molecular polarization (i.e., strength of the end-groups) as well as a large two-photon absorption (TPA) band response in the biological spectral window (700-1000 nm). The TPA bands show a bathochromic shift and hyperchromic effect with increasing polarization of the dyes with maximum TPA cross-section reaching 2000 GM for small size chromophore. All dyes are found to form stable and deeply colored nanoparticles (20-45 nm in diameter) upon nanoprecipitation in water. Although their fluorescence is strongly reduced upon aggregation, all nanoparticles show large one-photon (up to 10⁸ M(-1)·cm(-1) in the visible region) and two-photon (up to 10⁶ GM in the NIR) brightness. Interestingly, both linear and non-linear optical properties are significantly affected by interchromophoric interactions, which are promoted by the molecular confinement and modulated by both the dipolar strength and the presence of the bulky groups. Finally, we exploited the photophysical properties of the FONs to design optimized core-shell nanoparticles built from a pair of complementary dipolar dyes that promotes an efficient core-to-shell FRET process. The resulting molecular-based core-shell nanoparticles combine large two-photon absorption and enhanced emission both located in the NIR spectral region, thanks to a major amplification (by a factor of 20) of the core fluorescence quantum yield. These novel nanoparticles, which combine huge one-and two-photon brightness, hold major promise for in vivo optical bioimaging.

Cooperative Veratryle and Nitroindoline Cages for Two-Photon Uncaging in the NIR

Tandem uncaging systems in which a two-photon absorbing module and a cage moiety, linked via a phosphorous clip, that act together by Förster resonance energy transfer (FRET) have been developed. A library of these compounds, using different linkers and cages (7-nitroindolinyl or nitroveratryl) has been synthesized. The investigation of their uncaging and two-photon absorption properties demonstrates the scope and versatility of the engineering strategy towards efficient two-photon cages and reveals surprising cooperative and topological effects. The interactions between the 2PA module and the caging moiety are found to promote cooperative effects on the 2PA response while additional processes that enhance the uncaging efficiency are operative in well-oriented nitroindoline-derived dyads. These synergic effects combine to lead to record two-photon uncaging cross-section values (i.e., up to 20 GM) for uncaging of carboxylic acids.

Polarization modulation of two-photon excited fluorescence in a V-shaped dipicolinate-triphenylamine compound

Polarization modulation of two-photon excited fluorescence in a V-shaped dipicolinate-triphenylamine compound was investigated with 100 fs 800 nm laser pulses. The peak fluorescence intensity versus the input irradiance was measured to meet a square dependence, which offered evidence for two-photon excited fluorescence. The variations of the two-photon excited fluorescence intensity showed strong response to the different polarized incident lights and were tightly dependent on the linearly polarized component of the incident light. Furthermore, the polarization modulation efficiency of the two-photon excited fluorescence had an obvious concentration dependence when the concentration of solution was under 2.5×10(-4)  mol/L. The enhancement of modulation efficiency was attributed to the concentration dependence of the two-photon absorption cross section.

Two-dimensional electronic-vibrational spectra: modeling correlated electronic and nuclear motion

2D-EV spectra of solvated organic dyes modeled in terms of a reduced set of electronic diabatic states.

Large two-photon absorption of terpyridine-based quadrupolar derivatives: towards their applications in optical limiting and biological imaging

Developing organic chromophores with large two-photon absorption (TPA) in both organic solvents and aqueous media is crucial owing to their applications in solid-state photonic devices and biological imaging. Herein, a series of novel terpyridine-based quadrupolar derivatives have been synthesized. The influences of electron-donating group, type of conjugated bridge, as well as solvent polarity on the molecular TPA properties have been investigated in detail. In contrast to the case in organic solvents, bis(thienyl)-benzothiadiazole as a rigid conjugated bridge will completely quench molecular two-photon emission in aqueous media. However, the combination of alkylcarbazole as the donor and bis(styryl)benzene as a conjugation bridge can enlarge molecular TPA cross-sections in both organic solvent and aqueous media. The reasonable two-photon emission brightness for the organic nanoparticles of chromophores 3-5 in the aqueous media, prepared by the reprecipitation method, enables them to be used as probes for in vivo biological imaging.

Dipolar versus octupolar triphenylamine-based fluorescent organic nanoparticles as brilliant one- and two-photon emitters for (bio)imaging

Two related triphenylamine-based dipolar and octupolar fluorophores are used to prepare aqueous suspensions of fluorescent organic nanoparticles (FONs) via the reprecipitation method. The obtained spherical nanoparticles (30-40 nm in diameter) are fluorescent in aqueous solution (up to 15% fluorescence quantum yield) and exhibit extremely high one- and two-photon brightness, superior to those obtained for quantum dots. Despite the two chromophores showing similar fluorescence in solution, the fluorescence of FONs made from the octupolar derivative is significantly red-shifted compared to that generated by the dipolar FONs. In addition, the maximum two-photon absorption cross section of the FONs made from the octupolar derivative is 55% larger than that of the dipolar derivative FONs. The experimental observations provide evidence that the different molecular shape (rodlike versus three-branched) and charge distribution (dipolar versus octupolar) of the two chromophores strongly affect the packing inside the nanoparticles as well as their spectroscopic properties and colloidal stability in pure water. The use of these FONs as probes for biphotonic in-vivo imaging is investigated on Xenopus laevis tadpoles to test their utilization for angiography. When using FONs made from the octupolar dye, the formation of microagglomerates (2-5 μm scale) is observed in vivo, with subsequent lethal occlusion of the blood vessels. Conversely, the nanoparticles of the dipolar dye allow acute imaging of blood vessels thanks to their suitable size and brightness, while no toxic effect is observed. Such a goal cannot be achieved with the dissolved dye, which permeates the vessel walls.

Synthesis, photophysical, and two-photon absorption properties of elongated phosphane oxide and sulfide derivatives

A series of rod-shaped and related three-branched push-pull derivatives containing phosphane oxide or phosphane sulfide (PO or PS)-as an electron-withdrawing group conjugated to electron-donating groups, such as amino or ether groups, with a conjugated rod consisting of arylene-vinylene or arylene-ethynylene building blocks-were prepared. These compounds were efficiently synthesized by a Grignard reaction followed by Sonogashira coupling. Their photophysical properties including absorption, emission, time-resolved fluorescence, and two-photon absorption (TPA) were investigated with special attention to structure-property relationships. These fluorophores show high fluorescence quantum yields and solvent-dependent experiments reveal that efficient intramolecular charge transfer occurs upon excitation, thereby leading to highly polar excited states, the polarity of which can be significantly enhanced by playing on the end groups and conjugated linker. Rod-shaped and related three-branched systems show similar fluorescence properties in agreement with excitation localization on one of the push-pull branches. By using stronger electron donors or replacing the arylene-ethynylene linkers with an arylene-vinylene one induces significant redshifts of both the low-energy one-photon absorption and TPA bands. Interestingly, a major enhancement in TPA responses is observed, whereas OPA intensities are only weakly affected. Similarly, phosphane oxide derivatives show similar OPA responses than the corresponding sulfides but their TPA responses are significantly larger. Finally, the electronic coupling between dipolar branches promoted by common PO or PS acceptor moieties induces either slight enhancement of the TPA responses or broadening of the TPA band in the near infrared (NIR) region. Such behavior markedly contrasts with triphenylamine-core-mediated coupling, which gives evidence for the different types of interactions between branches.

Solvent Effects on the Resonance Raman and Hyper-Raman Spectra and First Hyperpolarizability of N,N-Dipropyl-p-nitroaniline

Linear absorption spectra, resonance Raman spectra and excitation profiles, and two-photon-resonant hyper-Rayleigh and hyper-Raman scattering hyperpolarizability profiles are reported for the push-pull chromophore N,N-dipropyl-p-nitroaniline in seven solvents spanning a wide range of polarities. The absorption spectral maximum red shifts by about 2700 cm(-1), and the symmetric -NO2 stretch shifts to lower frequencies by about 11 cm(-1) from hexane to acetonitrile, indicative of significant solvent effects on both the ground and excited electronic states. The intensity patterns in the resonance Raman and hyper-Raman spectra are similar and show only a small solvent dependence except in acetonitrile, where both the Raman and hyper-Raman intensities are considerably reduced. Quantitative modeling of all four spectroscopic observables in all seven solvents reveals that the origin of this effect is an increased solvent-induced homogeneous broadening in acetonitrile. The linear absorption oscillator strength is nearly solvent-independent, and the peak resonant hyperpolarizability, beta(-2omega;omega,omega), varies by only about 15% across the wide range of solvents examined. These results suggest that the resonant two-photon absorption cross sections in this chromophore should exhibit only a weak solvent dependence.