Theoretical study of two-photon absorption in donor-acceptor chromophores tetraalkylammonium halide/carbon tetrabromide

Two-photon absorption properties of a series of donor-acceptor chromophores of tetraalkylammonium halide/carbon tetrabromide ([NR4h.CBr4], h = Cl, Br, I; R = Me, Et, Pr) complexes are investigated in terms of the calculated results by the time-dependent density functional theory (TDDFT) technique combined with the sum-over-states (SOS) method. The modeling two-photon absorption spectra show that these charge-transfer complexes have large two-photon absorption (TPA) cross sections and the [NEt4I.CBr4] has the largest TPA cross section delta with the value of 5.0 x 10(-45) cm4 s photon(-1). The maximum values of delta increase with increasing separations between the donor/acceptor in the order Cl...Br < Br...Br < I...Br for [NEt4h.CBr4] complexes; however, the TPA cross sections delta vary slightly as the size of the alkyl group increases from methyl to propyl for the bromide as a donor, and the maximum wavelength of the TPA peak lambdamax indicates a bathochromic shift. The charge transfers from the halide anion to the carbon tetrabromide make a significant contribution to the excited states, and the donor-acceptor charge transfer plays an important role in the TPA activity, whereas changes in size of alkyl group do not make a substantial contribution to TPA.     

Conformationally tuned large two-photon absorption cross sections in simple molecular chromophores

We investigate here the relationship between molecular architecture and two-photon absorption (TPA) processes in a class of alkyl-substituted 4-quinopyran chromophores. We find that TPA cross sections diverge as the one-photon gap energy nears one-half of the two-photon gap. The molecular strategy proposed here to tune these two-excitation gaps for maximizing TPA cross sections is to twist the molecule about the bond connecting the chromophore donor and acceptor phenylene fragments. Extremely large TPA cross sections, determined by the absorption bandwidth, can then be realized (imaginary part of the third-order polarizability approximately 2.6 x 10(5) x 10(-36) esu) for fundamental photon energies near 1.0 eV, when the torsional angle approaches 104 degrees. The required torsional angle is achieved by introduction of sterically encumbered 2,2',2' ',2' " tertiary alkyl substituents.     

Two-photon absorption properties of proquinoidal D-A-D and A-D-A quadrupolar chromophores

We report the synthesis, one- and two-photon absorption spectroscopy, fluorescence, and electrochemical properties of a series of quadrupolar molecules that feature proquinoidal π-aromatic acceptors. These quadrupolar molecules possess either donor-acceptor-donor (D-A-D) or acceptor-donor-acceptor (A-D-A) electronic motifs, and feature 4-N,N-dihexylaminophenyl, 4-dodecyloxyphenyl, 4-(N,N-dihexylamino)benzo[c][1,2,5]thiadiazolyl or 2,5-dioctyloxyphenyl electron donor moieties and benzo[c][1,2,5]thiadiazole (BTD) or 6,7-bis(3',7'-dimethyloctyl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (TDQ) electron acceptor units. These conjugated structures are highly emissive in nonpolar solvents and exhibit large spectral red-shifts of their respective lowest energy absorption bands relative to analogous reference compounds that incorporate phenylene components in place of BTD and TDQ moieties. BTD-based D-A-D and A-D-A chromophores exhibit increasing fluorescence emission red-shifts, and a concomitant decrease of the fluorescence quantum yield (Φ(f)) with increasing solvent polarity; these data indicate that electronic excitation augments benzothiadiazole electron density via an internal charge transfer mechanism. The BTD- and TDQ-containing structures exhibit blue-shifted two-photon absorption (TPA) spectra relative to their corresponding one-photon absorption (OPA) spectra, and display high TPA cross sections (>100 GM) within these spectral windows. D-A-D and A-D-A structures that feature more extensive conjugation within this series of compounds exhibit larger TPA cross sections consistent with computational simulation. Factors governing TPA properties of these quadrupolar chromophores are discussed within the context of a three-state model.     

Excited-state deactivation of branched two-photon absorbing chromophores: a femtosecond transient absorption investigation

Branched macromolecular structures are now an important area of research for enhanced two-photon absorption (TPA) cross sections. The mechanism of this enhancement has been suggested as a complex interplay between intramolecular interactions and the extent of charge-transfer character in the branches. In order to probe these processes more clearly, excited-state dynamics of multibranched chromophores by means of femtosecond transient absorption spectroscopy are reported. Investigations have been carried out on the PRL dye series (PRL-101, PRL-501, PRL-701), which have shown cooperative enhancement of the TPA cross section. Upon photoexcitation, transient absorption measurements have shown the presence of a localized charge-transfer (intramolecular charge transfer, ICT) state independent of branching. The results point to ultrafast localization of charge in this particular system of chromophores. Pump-probe measurements in highly polar solvents have shown the presence of a nonemissive charge-transfer state which is a solvent stabilized and conformationally relaxed state. The population of this nonemissive state increases from monomer to trimer, and thus, it has been used as indicator of the polar nature of the Franck-Condon state. These results have shown an increase of charge-transfer character of the excited state with an increase in branching, and this explains the relative increase in the two-photon cross section of the PRL series.     

Theoretical study of one- and two-photon absorption properties of octupolar D2d and D3 bipyridyl metal complexes

The molecular equilibrium structures, electronic structures, and one- and two-photon absorption (TPA) properties of C2v (Zn(II), Fe(II) and Cu(I)) dipolar and D2d (Zn(II) and Cu(I)) and D3 (Zn(II)) octupolar metal complexes featuring different functionalized bipyridyl ligands have been studied by the ZINDO-SOS method. The calculated results show that one- and two-photon absorption properties of metal complexes are strongly influenced by the nature of the ligands (donor end groups and pi linkers) and metal ions as well as by the symmetry of the complexes. The length of the pi-conjugated backbone, the Lewis acidity of the metal ions, and the increase of ligand-to-metal ratio result in a substantial enhancement of the TPA cross sections of metal complexes. Substitution of C=N and N=N for C=C plays an important role in altering the maximum TPA wavelengths and the maximum TPA cross sections of metal complexes. Of them, the C=N substituted metal complexes have relatively large TPA cross sections. Replacing styryl with thienylvinyl makes the one-photon absorption wavelength red shift and at the same time leads to a great decrease of the maximum TPA cross sections of metal complexes. The possible reason is discussed. In the range 500-1250 nm, octupolar metal complexes exhibit intense TPAs and therefore are promising candidates for TPA materials.     

Electronic and vibronic contributions to two-photon absorption in donor-acceptor-donor squaraine chromophores

Many squaraines have been observed to exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions. Here, the electronic and vibronic contributions to these low-energy two-photon absorptions are elucidated by performing correlated quantum-chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two-photon active states approach those of the lowest energy one-photon active (1B(u)) states. This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these types of chromophores. On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low-lying electronic states of appropriate symmetry for TPA. Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1B(u) state with b(u) vibrational modes. Through implementation of Herzberg-Teller theory, we are able to identify the vibrational modes responsible for the low-energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.     

Synthesis and Characterization of Dipolar Two-Photon Absorption Chromophores

It is a new interest to explore novel organic two-photon absorption (TPA) chromophores with large TPA cross section for their potential applications in various fields, such as three-dimensional optical data storage, micro- and nano-fabrication, biological imaging and optical limiting, and so on. As far as asymmetrical D-π-A type chromophores were concerned, larger TPA cross section can be obtained with the enhancement of electron-donation/electron-acception ability.[1] Recently, we have synthesized a series of new D-π-A type compounds. In these chromophores, stilbene or phenyl cyclo-bridged hexatriene is served as π-bridge, which terminated with a number of amino groups and dicyanomethylene unit (Scheme 1). All the chromophores can be obtained by the condensation reaction of aldehydes and active methyl or methenyl groups in good yields (72%～81%). The chromophores have been characterized by 1H NMR,IR, UV-vis, PL and EA. All these five chromophores in dilute solutions show strong red fluorescence (Table 1). The studies on the TPA properties are in progress.     

Two-photon absorption in solution by means of time-dependent density-functional theory and the polarizable continuum model

We present the first study of two-photon absorption (TPA) of solvated molecules based on direct evaluation of TPA cross sections from the quadratic response of time-dependent perturbations. A set of prototypical two-photon (TP) chromophores has been selected and analyzed: a pure pi system (t-stilbene) and its substituted homologs obtained employing a donor (D) and an acceptor (A) group to probe the solvent effects along the series pi, D-pi-D, A-pi-D, and A-pi-A. For the selected systems we have calculated the TPA cross sections in different environments by means of the polarizable continuum model. The data have been analyzed to evaluate how the structural and environmental parameters contribute to the final two-photon absorption cross section. These include molecular structure, geometry relaxation in solution, polarity, and refractive index of the solvent. The performances of the three common functionals SVWN, BLYP, and B3LYP have been compared. The results show a significant solvent dependence of the TPA cross section and an unusual trend when passing from cyclohexane to water. The data have also been rationalized in terms of the main orbital excitations leading to the transitions. Finally, trends along the series have been described and comparison with experiments and previous calculations has been drawn.     

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.     

Concise Synthesis and Two‐Photon‐Excited Deep‐Blue Emission of 1,8‐Diazapyrenes

Efficient violet–blue‐emitting molecules are especially useful for applications in full‐color displays, solid‐state lighting, as well as in two‐photon absorption (TPA) excited frequency‐upconverted violet–blue lasing. However, the reported violet–blue‐emitting molecules generally possess small TPA cross sections. In this work, new 1,8‐diazapyrenes derivatives 3 with blue two‐photon‐excited fluorescence emission were concisely synthesized by the coupling reaction of readily available 1,4‐naphthoquinone O,O‐diacetyl dioxime ( 1 ) with internal alkynes 2 under the [{RhCl₂Cp*}₂]–Cu(OAc)₂ (Cp*=pentamethylcyclopentadienyl ligand) bimetallic catalytic system. Elongation of the π‐conjugated length of 1,8‐diazapyrenes 3 led to the increase of TPA cross sections without the expense of a redshift of the emission wavelength, probably due to the rigid planar structure of chromophores. It is especially noteworthy that 2,3,6,7‐tetra(4‐bromophenyl)‐1,8‐diazapyrene ( 3c ) has a larger TPA cross section than those of other molecules reported so far. These experimental results are explained in terms of the effects of extension of the π‐conjugated system, intramolecular charge transfer, and reduced detuning energy.     

Theoretical study on the photophysical properties of hexapyrrolidine C60 adducts with Th, D3, and S6 symmetries

The equilibrium geometries of three isomeric hexapyrrolidine C(60) adducts with T(h), D(3), and S(6) symmetries are optimized by means of the B3LYP method at the 6-31G basis sets in this paper. On the basis of the optimized structures, the excited state and third-order nonlinear optical properties, such as third-harmonic generation (THG), electric-field-induced second-harmonic generation (EFISHG), and degenerate four-wave mixing (DFWM), and two-photon absorption (TPA) cross sections, delta, are calculated by using the TDB3LYP model based on the 6-31G level coupled with the sum-over-states (SOS) method. The computational results show that the transition energies from S(0) to S(1) of the T(h) hexaadduct and the D(3) hexaadduct have a remarkable blue shift by comparison with that of the C(60) parent. These results are in agreement with experimental ones. However, the first singlet excitation energy of the S(6) hexaadduct has a red shift compared with that of the C(60) parent. Accordingly, we predict that different positions located by six addends may result in the different spectrum properties. Finally, the two-photon absorption cross sections indicate that the largest average value of resonant TPA, delta, of the D(3) hexaadduct has a red shift compared with those of the T(h) and S(6) hexaadducts.     

Single- and two-photon properties of a dye-derivatized Roussin's red salt ester (Fe2(mu-RS)2(NO)4) with a large TPA cross section

The synthesis, characterization, photochemistry, and two-photon photophysical properties of a new dye-derivatized iron sulfur nitrosyl cluster Fe2(mu-RS)2(NO)4 (AFX-RSE, RS = 2-thioethyl ester of N-phenyl-N-(3-(2-ethoxy)phenyl)-7-(benzothiazol-2-yl)-9,9-diethyl-fluoren-2-yl-amine) were investigated. Under continuous photolysis, AFX-RSE decomposes with modest quantum yields (Phi(diss) = (4.9 +/- 0.9) x 10(-3) at lambda(irr) = 436 nm) as measured from the loss of the nitrosyl bands in the IR absorbance spectrum. Nitric oxide (NO) was qualitatively demonstrated to be photochemically produced via single-photon excitation through the use of an NO-specific electrode. Steady-state luminescence measurements have shown that AFX-RSE fluorescence is about 88% quenched relative to the model compound AF-tosyl. This is attributed to a relatively efficient energy transfer from the excited states of the antenna chromophores to the dinuclear metal center, with the subsequent production of NO. In addition, the two-photon absorption (TPA) cross sections (delta) were measured for the AF-chromophores via the two-photon excitation (TPE) photoluminescence technique using a femtosecond excitation source. The TPA cross section of AFX-RSE was found with this technique to be delta = 246 +/- 8 GM (1 GM = 10(-50) cm4 s photon(-1) molecule(-1)).     

E/Z Molecular Photoswitches Activated by Two-Photon Absorption: Comparison between Different Families

Nonlinear optical techniques as two-photon absorption (TPA) have raised relevant interest within the last years due to the capability to excite chromophores with photons of wavelength equal to only half of the corresponding one-photon absorption energy. At the same time, its probability being proportional to the square of the light source intensity, it allows a better spatial control of the light-induced phenomenon. Although a consistent number of experimental studies focus on increasing the TPA cross section, very few of them are devoted to the study of photochemical phenomena induced by TPA. Here, we show a design strategy to find suitable E/Z photoswitches that can be activated by TPA. A theoretical approach is followed to predict the TPA cross sections related to different excited states of various photoswitches’ families, finally concluding that protonated Schiff-bases (retinal)-like photoswitches outperform compared to the others. The donor-acceptor substitution effect is therefore rationalized for the successful TPA activatable photoswitch, in order to maximize its properties, finally also forecasting a possible application in optogenetics. Some experimental measurements are also carried out to support our conclusions.     

Two-photon absorption in V-type chromophores with electron-rich heterocyclevinylene bridges

A series of V-type chromophores with electron-rich heterocyclevinylene bridges have been synthesized via Wittig-Horner-Emmons and Vilsmeier reactions.All the target chromophores showed strong one-photon and two-photon excited emission,and their electron properties could be tuned by using different heterocycles such as furan,thiophene and pyrrole moieties in I-III.The maximum two-photon absorption (TPA) cross sections occurred at 760 nm and were measured to be in the range of 400-800 GM.     

Two-photon absorption in linear bis-dioxaborine compounds-the impact of correlation-induced oscillator-strength redistribution.

Quantum-chemical calculations of the two-photon absorption (TPA) cross-sections are used to determine the characteristics of the electronic excited states responsible for the observed peaks in the TPA spectra of two bis-dioxaborine-substituted biphenyl derivatives. We find two distinct TPA-active states with very different TPA cross-sections: the difference is explained on the basis of electron correlation. These effects, on the one hand, lead to TPA cross-sections of up to 500 x 10(-50) cm4 s photon(-1) for the state favored by correlation; on the other hand, they limit the overall cross-sections achievable in this class of materials.     

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.     

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.     

Dependence of the two-photon absorption cross section on the conjugation of the phenylacetylene linker in dipolar donor-bridge-acceptor chromophores

The nonlinear optical properties of four isomeric dipolar two-photon chromophores are compared. The chromophores consist of a carbazole electron donor coupled to a naphthalimide electron acceptor by a phenylacetylene bridge. By variation of the connectivity of the bridge at the phenyl groups, four compounds with 0, 1, and 2 meta linkages are synthesized. The linear and nonlinear optical properties of these compounds are measured. Despite similar linear absorption cross sections, the two-photon absorption cross section delta of the all-meta compound is almost a factor of 10 lower than the all-para compound. By taking the detailed molecular conformations into account in order to calculate accurate dipole moment changes, we find that the decrease in delta results largely from the decreased charge transfer ability with increasing number of meta linkages. We find that a two-state model can be used to predict semiquantitatively the observed trend in delta on the basis of the linear optical properties of the molecules. This work illustrates the dramatic effect the ground-state polarizability can have on the nonlinear optical response of organic compounds and also provides a way to quantify the ability of meta linkages to inhibit charge transfer in their ground-state configuration.     

Strong two photon absorption and photophysical properties of symmetrical chromophores with electron accepting edge substituents

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.     

Theoretical study of two-photon absorption properties of a series of ferrocene-based chromophores

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.