Centrosymmetric Chromophore Research Articles

Dicyanomethylene-functionalized s-indacene-based D-π-A-π-D dyes exhibiting large near-infrared two-photon absorption cross-section

A series of centrosymmetric chromophores with D-π-A-π-D molecular architecture has been synthesized and characterized with respect to their third-order nonlinear optical (NLO) properties. The investigated compounds feature novel central electron acceptor fragments composed of derivatives of s-indacene-1,3,5,7(2H,6H)-tetraone (Janus dione), in which the carbonyls have been substituted by either two or four dicyanomethylene groups. Due to the increased electron acceptor strength, up to twofold increase in two-photon absorption (2 PA) cross-section is observed for the dyes in comparison to the structural analogues based on the parent compound. The best performing dye exhibits intensive 2 PA absorption band in 1000–1300 nm range with a peak cross-section value of 11,000 GM. Optical limiting was successfully demonstrated using the compound, marking the presented chemical design as a promising direction for optical power-limiting applications in the important near-infrared (NIR) spectral region.Electronic Supplementary Information (ESI) available: experimental procedures, results of quantum chemical calculations, NMR spectra.

Triplet state structure-property relationships in a series of platinum acetylides: effect of chromophore length and end cap electronic properties.

To develop quantitative structure-spectroscopic property relationships in platinum acetylides, we investigated the triplet state behavior of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C[triple bond, length as m-dash]C-Phenyl-X)2, where X = diphenylamino, NH2, OCH3, t-Bu, CH3, H, F, benzothiazole, CF3, CN, and NO2. We measured ground state absorption, phosphorescence, excitation and triplet state absorption spectra and triplet lifetimes. By DFT we calculated the phosphorescence emission energy (ET), the spin density on the end cap (SD(X)), triplet state geometry and the distance between the triplet centroid and the central platinum atom (RS-Pt(X)). Compounds with electron-donating X have smaller triplet state lifetime, blue-shifted phosphorescence and larger triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. Compounds with electron-withdrawing X have larger triplet lifetime, red-shifted phosphorescence and smaller triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. The range of spin-orbit-coupling between the platinum atom and the triplet centroid was determined to be 6 Å. The quantity RS-Pt is shown to be a linear function of one-dimensional well length calculated from experimental ET. The multiple examples demonstrate RS-Pt is a useful descriptor for analyzing triplet state behavior.

Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking.

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C≡C-Phenyl-X)2, where X = diphenylamino (DPA), NH2, OCH3, t-Bu, CH3, H, F, benzothiazole (BTH), CF3, CN, and NO2. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S1 state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S1 state. The values are small Δμ < 1.0 D for neutral substituents such as CH3, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO2. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation ( Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014 , 118 , 3749 - 3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.

Chiroptical Dissymmetries in Fluorescence Excitation from Single Molecules of (M-2) Helicene Dimers

We report on the single-molecule chiroptical properties of "right"-handed bridged triaryl amine helicene dimers, MH2. Using an experimental setup to precisely define the circular excitation polarization at the sample plane, we investigated the circular dichroic response in luminescence from individual molecules in which induced ellipticity from microscope optics is minimized. Our results comparing circular anisotropies in fluorescence excitation from MH2 and perylene diimide (PDI), an achiral, centrosymmetric chromophore, demonstrate a significant reduction in the breadth of the distribution of circular dissymmetry parameters obtained from modulation of the circularly polarized excitation source (457 nm). For PDI, we observe a symmetric distribution of circular anisotropy parameters centered about zero, with a fwhm of 0.25. For MH2, we observe an asymmetric distribution peaked at g = -0.09, with a slightly larger width as the corresponding PDI distribution. These results indicate that the large dissymmetry parameters (|g| > 0.5) in fluorescence excitation described in our original report (Hassey, R.; et al. Chirality 2008, 20, 1039-1046 and Hassey, R.; et al. Science 2006, 314, 1437-1439) were indeed affected by (at the time, unknown) linear polarization artifacts. However, the present results on MH2 provide compelling evidence for single-molecule circular dissymmetries much larger than solution or thin-film ensemble values, defined primarily by the enhanced rotatory strength (relative to the monomer), and restricted orientation at the sample surface.

Solvatochromic shift mechanisms of centrosymmetric chromophores in polar liquids

Origins of absorption shifts in polar liquids are considered for centrosymmetric chromophores devoid of dipole moments. The refractive index (n) dependent bathochromism shows that the excited state is always more polarizable than ground state. With increasing solvent dielectric constant (ɛ), mainly red shifts occur, but there are exceptions. Hypsochromism of n–π* transition in s-tetrazine is ascribed to a decrease of quadrupole moment in the excited state. In this case the upper level is pushed up in the reaction field of the ground-state quadrupole. Red shifts of intense bands in polyenes (11Bu or S2) and polyarenes (1Bb or β), reaching ∼−250 cm−1 are assigned to local polarization in the reaction fields of chromophore bond dipoles. The hyperbolic-like ɛ-dependence of solvent shift, approximated to an empirical function (ɛ − n2)/(ɛ − n2 + c), is much steeper (with “stretching” factor c = 3 ± 0.5) in this case, as compared to that for the linear effect (for s-tetrazine, c = 9). Polarization mechanism also causes less band broadening in polar environments. Contrary to a common view, the spectral shift is probing local reaction fields of chromophores, rather than random fields in the cavities accommodating the dye. Negligible polarization in solvent “cavity” fields is in accordance with earlier hole-burning study of matrix induced dipole moments in tetrapyrrolic pigments that depend surprisingly little on the polarity of polymeric hosts (R.B. Altmann, I. Renge, L. Kador, D. Haarer, J. Chem. Phys. 97 (1992) 5316 [30]).

First Divalent Metal Complexes of the Polyether Ionophore Monensin A:X-Ray Structures of [Co(Mon)2(H2O)2] and [Mn(Mon)2(H2O)2] and their Bactericidal Properties

The complexation of carboxylic acid Monensin A (MonH, 1a) with CoCl2.6H2O and MnCl2.4H2O leads to the formation of mononuclear complexes [Co(Mon)2(H2O)2], 2a and [Mn(Mon)2(H2O)2], 2b, respectively. The unique crystal structures of 2a and 2b were determined by X-ray crystallography. The complexes crystallize in the monoclinic space group P2 1 with an octahedrally coordinated transition metal center forming the crystallographically centrosymmetric chromophore CoO6 or MnO6, respectively. Two molecules of Monensin A act bidentately through their carboxylate moiety and a hydroxyl group, and two water molecules are additionally trans-coordinated. Although the transition metal ions are not bound into the cavity of the ligand, the unusual bidentate coordination mode of the ionophore induces its "pseudo-cyclization" forming 22-membered cycles further stabilized by a number of H-bonds. The complexes are the first example of divalent metal complexes of the monovalent polyether ionophore. The parallel study on the complexation ability of the potassium complex of Monensin A (MonK, 1b) towards Co(II) and Mn(II) showed the formation of the isostructural complexes 2a and 2b accompanied by loss of the potassium ion due to the new coordination mode of the ligand. The biological tests performed with the antibiotic MonH and the corresponding metal(II) complexes show greatly enhanced antimicrobial activity of complexes 2a-b against Gram(+)-bacteria.

Centrosymmetric Chromophores: Second-Harmonic Generation from Langmuir−Blodgett and Spun-Coated Films of Hydroxy-Substituted Squaraines

The linear and nonlinear optical properties of 2,4-bis[4-(N-methyl-N-alkylamino)-2,6-dihydroxyphenyl]-squaraine (1), 2,4-bis[4-(N,N-dialkylamino)-2-hydroxyphenyl]squaraine (II), and 2,4-bis(8-hydroxy-9-julolidinyl)squaraine (III) are reported. Electrospray ionization mass spectrometry (ESI-MS) has provided evidence of association in dilute solution (ca. 2 x 10 -5 mol dm -3 ) with m/z values which may be assigned to dimeric aggregates for dye II. Solid solutions of this dye in poly(vinyl acetate) show second-harmonic generation (SHG), albeit very weak, and therefore indicate that the dimeric aggregate is noncentrosymmetric. In contrast, dyes I and III form heptameric aggregates with their solid solutions being SHG-inactive. There is some correlation between the type of aggregate from ESI-MS and the Langmuir film spectra, but van der Waals interactions between the hydrophobic groups play a more significant role in determining the structure at the air-water interface. The absorption maxima of the Langmuir and deposited Langmuir-Blodgett films vary from 530 nm to the near-infrared, but only those films with a maximum at 650- 700 nm, or a significant shoulder in this range, are SHG-active. The second-order behavior is not an inherent property of the molecule itself but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.

Detection of parity-forbidden excited states by preresonance Raman excitation: Diphenyldecapentaene

The presence of dipole-forbidden (‘‘hidden’’) excited electronic states in centrosymmetric chromophores can in principle be inferred from preresonance Raman excitation profiles (REPs). As the excitation radiation is tuned through the appropriate energy range, vibronic coupling between the state of interest and nearby ‘‘allowed’’ electronic states will produce interference effects in the ground state scattering intensity. We have used the Kramers–Heisenberg dispersion relation to establish a one-to-one correspondence between the vibrational modes of the 2 1Ag excited state and the interference features in the experimental preresonance REP of all-trans diphenyldecapentaene (DPDP). The parameters utilized to predict the REP of DPDP were obtained without adjustment from absorption and fluorescence excitation spectra. The satisfactory representation and interpretation of the structure in the experimental spectrum establishes preresonance Raman excitation as a viable technique for characterizing such hidden states in non-fluorescing molecules.

Synthesis, characterization, crystal structures and magnetic exchange in dinuclear copper complexes with 3-amino-1-propanol as terminal and bridging ligand

The synthesis, X-ray structures and spectroscopic and magnetic properties are described for two groups of dinuclear Cu(II) compounds with the ligand 3-amino-1-propanol (Hap). The formulae of the compounds are for group A: [Cu(ap)(anion)] 2, in which ap is the dehydronated Hap and the anions are formate, nitrate, chloride and bromide and for group B: [Cu(ap)(Hap)] 2(anion) 2, with anion = iodide, bromide, chloride, nitrate and tetrafluoroborate. The structure of group A compounds consists of dinuclear units with the co-planar centrosymmetric chromophore ANCuOO′CuNA, in which the A ligands (anions) bridge to neighbouring units as axial ligands, thereby forming infinite chains. Dimer Cu…Cu distances are about 295 pm. The X-ray structure of [Cu(ap)(NO 3)] 2 ( I) is described in detail. Dark blue crystals of I were studied with single crystal X-ray measurements and refined to a final R value of 0.025 for 1440 reflections. The compound crystallizes in the monoclinic space group P2 1/ c with two dimeric molecules in a cell of dimensions a = 886.56(5), b = 815.92(6), c = 928.71(3) pm and β = 104.671(4)°. The structure consists of centrosymmetric di-alkoxy bridged dimers in which the Cu…Cu distance is 293.91(3) pm. The dimers are polymerized along the c axis into chains via NHO hydrogen bonds. These chains are joined together along the b axis by CuONOCu bridges and weak NHO hydrogen bonds. This results in layers, which are held together by van der Waals forces. The copper(II) ions have a distorted square pyramidal geometry, which is close to a tetragonally distorted octahedral coordination sphere. The structure of group B compounds consists of dinuclear units with the centrosymmetric chromophore [LNCuOO′CuNL] in which L is an N-coordinating Hap ligand. The alcoholic function of Hap coordinates as an axial ligand to neighbouring dimeric units, forming again an infinite chain structure. Dimeric Cu…Cu distances are 303 pm. The anions in structure B are not coordinated to copper, but are hydrogen bonded to the ligand NH and OH groups. Ligand-field spectra agree with square-pyramidal (4 + 1) or distorted octahedral (4 + 1 + 1) coordination geometries for Cu(II) in all cases. Small differences in ligand-field spectra are ascribed to differences in anions and axial CuO distances. The compounds are all very strongly magnetically coupled, as seen from the fact that they are diamagnetic at low temperatures. Both groups of compounds have been studied with magnetic susceptibility measurements in the 2–400 K region and the data could be fitted with the Bleaney-Bowers equation for dimers. Compounds of type A have much smaller values of -2 J than those of type B. This nicely agrees with the larger value of the CuOCu angles in the case of group B (99.5–7° in A, against 103.5°). EPR spectra of the compounds show mainly small paramagnetic impurities (below 1%) in agreement with the dimeric structure.