Two-photon Absorption Response Research Articles

Two-Photon Transitions in Quadrupolar and Branched Chromophores: Experiment and Theory

A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related pi-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.

Strongly Interacting Organic Conjugated Dendrimers with Enhanced Two-Photon Absorption

We report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. This report focuses on the mechanism of enhancement of the two-photon absorption (TPA) response of this material as a function of the dendrimer generation number. This enhancement is strongly correlated to the size and geometry of the spectroscopic unit in a strongly interacting macromolecular system. These systems are investigated by a wide variety of time-resolved spectroscopy methods including time-resolved fluorescence, transient absorption, three-pulse photon echo peak shift measurements as well as TPA cross section measurements. The extensive combination of modern spectroscopic methods strongly indicates that the spectroscopic unit (domain) in high generations covers the G1 (nine linear building blocks) and exceeds the trimer size.

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

Investigations into the effects of electrostatic interactions on linear and two-photon absorption (TPA) properties are probed using a reference polar chromophore and different related compounds. Compounds in which a passive charge is present near the active chromophore, as well as dimers of the reference molecule, were studied. The combined experimental and theoretical investigation demonstrates that strong modulation of the properties can be attained. In particular, the TPA response of dimers is heavily affected by unconventional excitonic effects, that is, beyond the standard Heitler-London approximation. These effects, which stem from purely electrostatic interactions, lower the nonlinear optical response in the investigated case by an additional amount (up to 20%), and are expected to enhance the TPA cross section in other supramolecular alignments. Electrostatic interactions cannot be overlooked when modelling or investigating highly concentrated and/or confined samples, as are usually needed in many applications. The correct knowledge of their effects can be exploited to guide engineering at the molecular and supramolecular level.

An investigation on the two-photon absorption activity of various terpyridines and related homoleptic and heteroleptic cationic Zn(ii) complexes

The two-photon absorption (TPA) properties of various terpyridines of the kind [4′-(C6H4-p-X)-2,2′∶6′,2″-terpyridine] and related homoleptic and heteroleptic bis(terpyridine) cationic zinc(II) complexes were investigated by the TPA induced photoluminescence (TPA-PL) method in a femtosecond regime. It appeared that terpyridines bearing an X donor group are characterized by TPA cross sections among the largest ever reported for a molecule with a dipole symmetry whereas coordination to a Zn(II) center leads to a decrease of the TPA response.

Role of Donor−Acceptor Strengths and Separation on the Two-Photon Absorption Response of Cytotoxic Dyes: A TD-DFT Study

Time-dependent density functional theory (TD-DFT) is applied to model one-photon (OPA) and two-photon (TPA) absorption spectra in a series of conjugated cytotoxic dyes. Good agreement with available experimental data is found for calculated excitation energies and cross sections. Calculations show that both OPA and TPA spectra in the molecules studied are typically dominated by two strong peaks corresponding to different electronic states. We find that donor-acceptor strengths and conjugated bridge length have a strong impact on the cross-section magnitudes of low- and high-frequency TPA maxima, respectively. These trends are analyzed in terms of the natural transition orbitals of the corresponding electronic states. Observed structure-property relationships may have useful implications on design of organic conjugated chromophores with tunable two-photon absorption properties for photodynamic therapy applications.

One- and two-photon absorption properties of a series of derived pyrazolines

The one-photon absorption and two-photon absorption (TPA) properties of a series of pyrazoline derivatives have been theoretically investigated by means of density functional theory and ZINDO-SOS method. On the basis of optimized geometry and UV–vis spectrum, the position and strength of the TPA for molecules are reported. The theoretical findings are consistent with the experimental observations. It is found that a most crucial role in increasing TPA cross-section is played by the conjugated length and the introduction of donors. The calculated maximum TPA cross-section values are in the range of 360–1760 × 10−50 cm4 s/photon. The TPA response will be as another possible application for derived pyrazoline.

Two-photon absorption in quadrupolar pi-conjugated molecules: influence of the nature of the conjugated bridge and the donor-acceptor separation.

Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.

Two-photon absorption properties of star-shaped molecules containing peripheral diarylthienylamines

Two-photon absorption (TPA) properties of several novel donor–acceptor–donor (D–A–D) quadrupolar and octupolar star-shaped molecules containing peripheral diarylthienylamines were investigated and are discussed in this Letter. The effects of donor–acceptor strength, conjugation length, and molecular symmetry on the effective TPA cross-section of these novel molecules were studied. It was found that incorporation of a triazine acceptor center in octupolar molecules can effectively enhance TPA response. This provides a strategy for designing novel octupolar molecules with a large TPA response but without red-shifting their linear absorption wavelengths.

Limitations of Essential-State Models for the Description of Two-Photon Absorption Processes: The Example of Bis(dioxaborine)-Substituted Chromophores

We report spectroscopic and quantum-chemical investigations comparing the two-photon absorption (TPA) properties of a bis(dioxaborine)-substituted derivative of biphenyl with those of a bis(dioxaborine) carbazole derivative. The former molecule is close to linear and centrosymmetric, while the dioxaborine groups of the latter are in a V-shaped arrangement, due to their linkage to the 3 and 6 positions of the bridging group. For both systems, we find sizable TPA cross sections (on the order of 360−530 × 10-50 cm4 s/photon). Interestingly, while the TPA response in the biphenyl-based system can be well described on the basis of the traditional three-state model, a significantly larger number of excited states needs to be considered for the carbazole derivative. We present a detailed comparison of the convergence of the theoretical approaches and an analysis of the various channels that contribute to the TPA response in molecules with low effective symmetries.

Ultrafast Two-Photon Absorption Approach to Optical Line Shape Measurements

An analysis of optical heterodyne detected (OHD) two-photon absorption (TPA) resonant signals in transparent materials is presented. The formal analogies between the vibrationally resonant Raman and electronically resonant TPA P ( 3 ) signals and response functions are given. Fourier transform procedures are shown to allow recovery of the complex TPA response function and eliminate residual Raman contributions from these spatially selected OHD TPA responses. The phase selectivity and wavelength dependence of this technique can be exploited to reveal both the real and imaginary parts of the line shape function describing the solute-solvent response to the two-photon resonant electronic excitation. In-phase and in-quadruture measurements at any resonant wavelength yield the absorption band shape. Intramolecular vibronic structure can be determined for totally diffuse absorption bands. These effects are illustrated by the analysis of two model electronic absorption systems: the MD-simulated methyl iodide B-state origin band in Ar and a multimode Brownian-oscillator-modeled absorption line shape with simple vibronic structure. Electronic excitations less accessible to one-photon resonant ultrafast studies such as UV/VUV transition, electronic excitations in inherently optically dense media, and dipole-forbidden states are well suited for study by this TPA approach.

Role of dimensionality on the two-photon absorption response of conjugated molecules: The case of octupolar compounds

A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.

Tuning the two-photon absorption response of quadrupolar organic molecules

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model.

Two-photon absorption resonance effects in the third-order responses of transparent liquids

Two-photon absorption (TPA) resonance effects on the ultrafast pump–probe and transient grating (TG) responses of some transparent liquids excited with 400 nm pulses are demonstrated. The contribution of TPA and ground state Raman responses are separated in a TG experiment when the third-order polarization signal is viewed in the appropriate phase-matched direction as shown for the homodyne, TG response of CS 2. The pump–probe dichroic responses of liquids with (CS 2 and CH 3I) and without (H 2O, CCl 4, and CH 3OH) TPA resonances at 200 nm are compared. These results are discussed in the context of a perturbative third-order polarization treatment of TPA.

Optimization of quadrupolar chromophores for molecular two-photon absorption

Multiphoton absorption is attracting considerable attention owing to applications in various fields including optical power limitation, and high resolution imaging of biological tissues. Within this framework, our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections in the visible and/or NIR region and high fluorescence quantum yield. Our strategy was based on the push–push or pull–pull functionalization of rigid conjugated cores built from a rigidified biphenyl or bithiophene moiety. A series of push–push and pull–pull elongated chromophores have been prepared and their nonlinear absorption properties investigated by determining their two-photon excited fluorescence (TPEF) characteristics. The TPA spectra of theses derivatives can be tuned and their TPA responses boosted by playing on the nature of the rigid core and electroactive end groups. As a result, quadrupolar molecules showing very large TPA cross-sections (up to 1.3×10 −47 cm 4 s photon −1 ) in the visible red and/or NIR region have been obtained. Such derivatives are promising for optical limitation applications as well as for TPEF imaging of cellular membranes.

On the nature of electronic excitations in poly(paraphenylenevinylene): A quantum-chemical investigation

Correlated quantum-chemical calculations are performed on phenylenevinylene oligomers containing up to eleven repeat units, to characterize the nature of the electronic excitations relevant for the photophysical properties of the corresponding polymer. The focus is first on the nonlinear optical response of model conjugated chains and the simulation of their frequency-dependent (third-harmonic generation, electroabsorption, and two-photon absorption) response. From the assignment of the calculated resonance features, the excited states dominating the third-order nonlinear polarizability are identified and their chain-length dependence is investigated. On that basis, we build an essential-state single-chain model (that includes the 1Bu, 2Ag, mAg, and nBu states) and apply it to the interpretation of recent experimental data reported for poly(paraphenylenevinylene) and derivatives. We then examine how the exciton binding energy, here defined as the difference between the energies of the charge-separated nBu and the strongly optically allowed 1Bu excited states, is affected by both intrachain and interchain polarization effects.