Property tuning in charge-transfer chromophores by systematic modulation of the spacer between donor and acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.     

A distinctive example of the cooperative interplay of structure and environment in tuning of intramolecular charge transfer in second-order nonlinear optical chromophores

The strongly enhanced cooperative influence of medium polarity and organic structural design on the first hyperpolarizability beta of a novel family of highly polarizable azinium-(CH=CH-thienyl)-dicyanomethanido chromophores 1-3 is described. The dyes can be efficiently synthesized by regioselective protonation/alkylation of the corresponding bidentate anion precursors. Consecutive annelation of the pyridyl ring of 1 (pyridine-->quinoline-->acridine) and medium polarity effects are responsible for an extraordinarily variable range of intramolecular charge transfer (ICT), leading to a large set of pi-electron distribution patterns. Accordingly, systems with remarkably different zwitterionic/quinoid character in the ground and excited states present beta values in a broad range, eventually switching from negative to positive. Our investigation is based on a combination of experimental (UV/Vis spectroscopy, multinuclear NMR spectroscopy, and electrooptical absorption measurements) and computational (ab initio) approaches. It is shown that: 1) beta and mubeta are dramatically influenced, even by orders of magnitude, by a complex, non-monotonic interplay of structure and medium action, which in turn affects molecular ICT and bond length alternation (BLA), 2) the computations, validated by different experimental data, are to be recommended as an extremely useful tool in the search for a greatly improved set of molecular nonlinear optical (NLO) responses (in the case of 1-3 they show that such conditions may be attained only in a narrow and limited range of dielectric constants in which the annelation effect operates most efficiently), and 3) the search for the most favorable molecular NLO response of a highly polarizable chromophore both in solution and in solid matrices should simultaneously take into account not only the molecular design supplemented by annelation effects but also the polarity of the medium.     

Preparation of polymer optical fibers doped with nonlinear optical active organic chromophores

We synthesized two novel organic nonlinear optical chromophores—chiral S(+)‐N‐[p‐(4‐nitrostyryl) phenyl] prolinol and non‐chiral [p‐(4‐nitrostyryl) phenyl] piperdine—as potential laser‐active dyes for photonic applications. Both materials show good optical transmittance in the telecommunication frequency region, desirable solubility in acrylic polymer optical fiber matrices, and attractive fluorescence properties that are advantageous for laser‐gain materials and devices. Subsequently, these two chromophores were incorporated into poly(methyl methacrylate) and poly(ethyl methacrylate) and drawn into polymer optical fibers. The relevant properties of these organic dye‐doped fibers have been studied, revealing essential attributes of laser‐active characteristics. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1794–1801, 2001     

Electronic origin for enhanced nonlinear optical response of complexes from tetraalkylammonium halide and carbon tetrabromide: electrostatic potentials of intermolecular donor-acceptor dyads

Electronic origin for nonresonant enhancement of nonlinear optical response in the complexes formed from tetraalkylammonium halide and carbon tetrabromide is provided in view of electrostatic potentials of intermolecular donor (halide ion)-acceptor (CBr(4)). The calculated electrostatic potentials of donor-acceptor range from -4.83 to -7.70 kcal mol(-1) and show a decreasing order of [Et(4)Cl(-)Br] > [Et(4)Br(-)Br] congruent with [Et(4)I(-)Br] > [Bu(4)Br(-)Br]. The calculated second-order susceptibilities of solid complexes are in an increasing order of [NEt(4)ClCBr(4)] < [NEt(4)BrCBr(4)] congruent with [NEt(4)ICBr(4)] < [NBu(4)BrCBr(4)C(3)H(6)O]. It has been shown that the donor/acceptor dyads make the exclusive contribution to nonlinear optical response. A large size of halide or tetraalkylammonium ion results in a small electrostatic potential and large nonlinear optical response in these charge-transfer complexes. It indicates that a small supermolecular interaction will create a large nonlinear optical response, and it gives a clue to design the molecular complexes with large non-linear optical susceptibility.     

Polymer matrix effects on the nonlinear optical response of incorporated chromophore: new analytical models.

A new approach aimed at the modeling of the nonlinear optical (NLO) response of a dipole chromophore incorporated into a locally anisotropic, deformable, polarizable polymer matrix is suggested. The general continuum methodology is used with a specific cavity ansatz being employed; the cavity is chosen to be conformal to the characteristic ellipsoid of the generalized permittivity tensor of the polymer medium. The suggested ansatz allows the electrostatic boundary value problem to be solved analytically, and the local field experienced by the chromophore in the polymer electret to be found. Four analytically solvable models, which correspond to two singular and two nonsingular models, are considered; in two of them the chromophore is characterized by singular dipole and quadrupole moments; the other two use the approximation of the electric moment of the chromophore with that of the polarized ellipsoid. The relation between the macroscopic polymer properties and the microscopic characteristics of the NLO chromophore is established. The compliance of the obtained formulas for the local field with those of the classical Onsager approach is analyzed, and their specific features are considered.     

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.     

Effects of dipolar interactions on linear and nonlinear optical properties of multichromophore assemblies: a case study

Interchromophore interactions in flexible multidipolar structures for nonlinear optics were addressed by a combined experimental and theoretical study on two series of one-, two-, and three-chromophore systems in which identical push-pull chromophores are assembled through covalent and flexible linkers in close proximity. The photophysical and nonlinear optical properties (quadratic hyperpolarizability) of the multichromophore systems were investigated and compared to those of the monomeric chromophores. Multimers have larger dipole moments than their monomeric analogues, that is, the dipolar subchromophores self-orientate within the multimeric structures. This effect was found to depend on the intersubchromophore distance in a nontrivial manner, which confirms that molecular engineering of such flexible systems is more complex than in completely geometrically controlled systems. Electric-field-induced second-harmonic generation (EFISHG) measurements in solution revealed increased figures of merit as compared to the monomeric analogue. This effect increases with increasing number and polarity of the individual subchromophores in the nanoassembly and increasing spacing between dipolar subchromophores. Experimental results are interpreted by a theoretical model for interacting polar and polarizable chromophores. The properties of multidipolar assemblies are shown to be related to the relative orientation of chromophores, which is imposed by interchromophore interactions. The supramolecular structure is thus a result of self-organization. The proposed theoretical model was also used to predict the properties of multichromophore structures made up of more polar and polarizable push-pull chromophores, and showed that stronger interchromophore interactions can heavily affect the individual optical responses. This suggests new routes for engineering highly NLO responsive multichromophore systems.     

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.     

Theoretical investigation of the second-order nonlinear optical properties of helical pyridine-pyrimidine oligomers

The structure and second-order nonlinear optical properties of a series of helical pyridine-pyrimidine oligomers recently synthesized by Barboiu and Lehn have been investigated theoretically by combining molecular mechanics and quantum chemistry approaches. In the absence of substituents, the hyper-Rayleigh scattering response (betaHRS) and the projection of the first hyperpolarizability on the dipole moment (beta||) exhibit periodic variations with chain length, but these nonlinear optical responses remain small. The first hyperpolarizabilities can, however, be enhanced by adding substituents. The greatest enhancement is obtained by substituting the pyrimidine groups by donor groups. Moreover, regular distributions of the donor groups around the helices enable the design of supramolecular structures exhibiting dipolar, octupolar or Lambda-shaped nonlinear optical characters, evident from the values of the depolarization ratios in hyper-Rayleigh scattering. Therefore this theoretical investigation demonstrates the potential of helical structures for the organization of chromophores in such a way that they exhibit large and specific second-order nonlinear optical responses.     

Difluorenyl carbo‐Benzenes: Synthesis,Electronic Structure,and Two‐Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores

The synthesis, crystal and electronic structures, and one‐ and two‐photon absorption properties of two quadrupolar fluorenyl‐substituted tetraphenyl carbo‐benzenes are described. These all‐hydrocarbon chromophores, differing in the nature of the linkers between the fluorenyl substituents and the carbo‐benzene core (C?C bonds for 3 a , C?C?C?C expanders for 3 b ), exhibit quasi–superimposable one‐photon absorption (1PA) spectra but different two‐photon absorption (2PA) cross‐sections σ_2PA. Z‐scan measurements (under NIR femtosecond excitation) indeed showed that the C?C expansion results in an approximately twofold increase in the σ_2PA value, from 336 to 656 GM (1 GM=10^?50 cm⁴ s molecule^?1 photon^?1) at λ=800 nm. The first excited states of A_u and A_g symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum‐over‐state estimations of σ_2PA(λ_i), in which λ_i=2 hc/E_i, h is Planck’s constant, c is the speed of light, and E_i is the energy of the 2PA‐allowed transition. The calculated σ_2PA values of 227 GM at 687 nm for 3 a and 349 GM at 708 nm for 3 b are in agreement with the Z‐scan results.     

Energy flow in push-pull chromophores: vibrational dynamics in para-nitroaniline.

para-Nitroaniline (PNA) plays an essential role as the prototype model of push-pull chromophores. The nature and degree of participation of vibrational degrees of freedom in the charge-transfer and internal-conversion processes are current issues of great theoretical and practical importance. Ultrafast time-resolved anti-Stokes resonance Raman spectroscopy (TRARRS) experiments on PNA in dimethyl sulfoxide with three different excitation wavelengths were performed to probe these dynamical influences. The vibrational dynamics associated with S0 were independent of incident wavelength, and this supports the picture that the S1 dynamics are fast relative to the rate of intersystem crossing. The phenyl breathing mode nu(19) (860 cm(-1)) and the symmetric NO2 stretch nu(29) (1310 cm(-1)) exhibited vibrational lifetimes in S0 of 8.1 and 5.2 ps, respectively. No evidence for inhomogeneous broadening of the charge-transfer band in the UV/Vis absorption spectrum was found.     

Structure–Property Relationships in Click‐Derived Donor–Triazole–Acceptor Materials

To shed light on intramolecular charge‐transfer phenomena in 1,2,3‐triazole‐linked materials, a series of 1,2,3‐triazole‐linked push–pull chromophores were prepared and studied experimentally and computationally. Investigated modifications include variation of donor and/or acceptor strength and linker moiety as well as regioisomers. Photophysical characterization of intramolecular charge‐transfer features revealed ambipolar behavior of the triazole linker, depending on the substitution position. Furthermore, non‐centrosymmetric materials were subjected to second‐harmonic generation measurements, which revealed the high nonlinear optical activity of this class of materials.     

Donor–Acceptor (D–A)‐Substituted Polyyne Chromophores: Modulation of Their Optoelectronic Properties by Varying the Length of the Acetylene Spacer

A series of donor–acceptor‐substituted alkynes, 2 a – f , was synthesized in which the length of the π‐conjugated polyyne spacer between the N,N‐diisopropylanilino donor and the 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) acceptor was systematically changed. The effect of this structural change on the optoelectronic properties of the molecules and, ultimately, their third‐order optical nonlinearity was comprehensively investigated. The branched N,N‐diisopropyl groups on the anilino donor moieties combined with the nonplanar geometry of 2 a – f imparted exceptionally high solubility to these chromophores. This important property allowed for performing INADEQUATE NMR measurements without ¹³C labeling, which, in turn, resulted in a complete assignment of the carbon skeleton in chromophores 2 a – f and the determination of the ¹³C–¹³C coupling constants. This body of data provided unprecedented insight into characteristic ¹³C chemical shift patterns in push–pull‐substituted polyynes. Electrochemical and UV/Vis spectroscopic studies showed that the HOMO–LUMO energy gap decreases with increasing length of the polyyne spacer, while this effect levels off for spacers with more than four acetylene units. The third‐order optical nonlinearity of this series of molecules was determined by measuring the rotational averages of the third‐order polarizabilities (γ_rot) by degenerate four‐wave mixing (DFWM). These latter studies revealed high third‐order optical nonlinearities for the new chromophores; most importantly, they provided fundamental insight into the effect of the conjugated spacer length in D–A polyynes, that can be exploited in the future design of suitable charge‐transfer chromophores for applications in optoelectronic devices.     

Synthesis,X-ray crystal structure,UV/visible linear and nonlinear (optical limiting) spectral properties of symmetrical and unsymmetrical porphyrazines with annulated 1,2,5-thiadiazole and 1,4-diamyloxybenzene moieties

Co-cyclization of 1,2,5-thiadiazole-3,4-dicarbonitrile and 3,6-diamyloxyphthalodinitrile in the presence of magnesium or lithium amylate in amyl alcohol leads to mixtures containing the Mg derivatives of the symmetrical species tetrakis(1,2,5-thiadiazolo)porphyrazine, (S(4))PzH(2), and tetrakis(1,4-diamyloxybenzo)porphyrazine, (A(4))PzH(2), and the low-symmetry macrocycles bearing peripheral 1,2,5-thiadiazole and 1,4-diamyloxybenzene rings in the ratio 1:3, 2:2 (cis and trans), and 3:1, that is, (SA(3))PzH(2), (S(2)A(2))PzH(2), (SASA)PzH(2), and (S(3)A)PzH(2), respectively. The basic Mg materials were converted to the corresponding free-base macrocycles by treatment with CF(3)COOH. The species were separated from the mixtures by chromatography, either as Mg complexes or demetalated materials. With results on (S(4))PzH(2) and (SA(3))PzH(2) in hand, including crystallographic work on the latter, a general chemical physical investigation has been carried out of all the symmetrical and unsymmetrical free-base macrocycles. The structures of the species (S(2)A(2))PzH(2) and (A(4))PzH(2). were elucidated by single-crystal X-ray crystallography. The effect of the progressive variation of the macrocyclic structure along the series, from the symmetrical (S(4))PzH(2) to its symmetrical partner (A(4))PzH(2) via the low-symmetry 3:1, 2:2 (cis and trans), and 1:3 macrocycles, was studied by IR, (1)H NMR, and UV/Vis linear and nonlinear (optical limiting) measurements. The results are interpreted on the basis of intra- and intermolecular interactions between the electron-deficient 1,2,5-thiadiazole and the electron-donating 1,4-diamyloxybenzene moieties.     

Cooperative interactions in supramolecular aggregates: linear and nonlinear responses in calix[4]arenes.

The linear and nonlinear optical polarizabilities of donor-acceptor (D-pi-A) chromophores in confined geometries of calix[4]arenes are investigated through a model for interacting polar-polarizable molecules. Both the linear polarizability (alpha) and the first hyperpolarizability (beta) decrease with increasing the interdipolar angle, as expected in the oriented-gas picture. However, within the polar-polarizable model we predict deviations from the additive result, irrespective of the interdipolar angle. Depending on the nature of the chromophore, electrostatic intermolecular interactions between polar and polarizable chromophores lead to cooperative damping or enhancement of the optical responses. Specifically, for chromophores whose ground state is dominated by the neutral D-pi-A structure both alpha and beta are suppressed with respect to the prediction of the oriented-gas model, whereas the opposite holds true for chromophores whose ground state is dominated by the zwitterionic D(+)-pi-A(-). These results explain recent experimental data on a calix[4]arene functionalized with a donor-acceptor dye for nonlinear optical applications. Density functional theory calculations on the relevant crystal structure further support our interpretation.     

Influence of the Delocalization Error and Applicability of Optimal Functional Tuning in Density Functional Calculations of Nonlinear Optical Properties of Organic Donor–Acceptor Chromophores

Nonempirically tuned hybrid density functionals with range‐separated exchange are applied to calculations of the first hyperpolarizability (β_∥) and charge‐transfer (CT) excitations of linear “push–pull” donor–acceptor‐substituted organic molecules with extended π‐conjugated bridges. An unphysical delocalization with increasing chain length in density functional calculations can be reduced significantly by enforcing an asymptotically correct exchange‐correlation potential adjusted to give frontier orbital energies representing ionization potentials. The delocalization error for a number of donor–acceptor systems is quantified by calculations with fractional electron numbers and from orbital localizations. Optimally tuned hybrid variants of the PBE functional incorporating range‐separated exchange can produce similar magnitudes for β_∥ as Møller–Plesset second‐order perturbation (MP2) correlated calculations. Improvements are also found for CT excitation energies, with results similar to an approximate coupled‐cluster model (CC2).