Relationship between the Molecular Structure of Cyanine Dyes and the Vibrational Fine Structure of their Electronic Absorption Spectra

Vibronic sub‐bands in the electronic absorption spectra of symmetrical cyanine dyes (see picture) are attributed to the symmetric C? C valence vibration of the polymethine chain in the excited state. The ³J(H,H) coupling constants in the polymethine chain can be used to characterize the bond localization within the chain in the ground state and thus to explain the intensity distribution of the sub‐bands.

     

The Effects of Substituents and Solvents on the Ground‐State π‐Electronic Structure and Electronic Absorption Spectra of a Series of Model Merocyanine Dyes and Their Theoretical Interpretation

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR ¹³C chemical shifts between adjacent C atoms in the polymethine chain and the ³J(H,H) coupling constants for trans‐vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating π‐electron density in the polymethine chain. In addition, the prediction of zero‐π‐bond order alternation (or zero‐bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck‐Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v=0 to v′, where v is the vibrational quantum number of the totally symmetric C?C valence vibration of the polymethine chain in the electronic ground state and v′ is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.     

Indachlorins: Nonplanar Indanone‐Annulated Chlorin Analogues with Panchromatic Absorption Spectra between 300 and 900 nm

Indaphyrins, pyrrole‐modified porphyrins containing a cleaved pyrrole β,β′‐bond and two annulated indanone moieties, possess unusually broadened and redshifted UV/Vis spectra because of their π‐expanded chromophores. The parent free base indaphyrin has been crystallographically characterized, highlighting its strongly ruffled conformation incorporating a helimeric twist. It was shown to be susceptible to regiospecific derivatizations at the opposite side of the ring‐cleaved pyrrole (dihydroxylation, followed by functional group transformations of the resulting diol functionality), generating indaphyrin‐based chlorin analogues, indachlorins, that incorporate a dihydroxypyrroline, pyrrolindione, oxazolone, or a morpholine moiety. Structural modifications resulted in further broadening and hyper‐ and bathochromic shifts of the optical spectra, some of which possess a nearly panchromatic absorption between 300 to well above 900 nm. The extents to which these modifications affect their solid‐state conformations were analyzed.     

Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes

Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM ). This feature allows the optical absorption spectra of the cell‐membrane‐associated proteins to be monitored in vivo in the course of extracellular respiratory electron‐transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.

     

Analysis of Coherent Heteroclustering of Different Dyes by Use of Threoninol Nucleotides for Comparison with the Molecular Exciton Theory

To test the molecular exciton theory for heterodimeric chromophores, various heterodimers and clusters, in which two different dyes were stacked alternately, were prepared by hybridizing two oligodeoxyribonucleotides (ODNs), each of which tethered a different dye on D ‐threoninol at the center of the strand. NMR analyses revealed that two different dyes from each strand were stacked antiparallel to each other in the duplex, and were located adjacent to the 5′‐side of a natural nucleobase. The spectroscopic behavior of these heterodimers was systematically examined as a function of the difference in the wavelength of the dye absorption maxima (Δλ_max). We found that the absorption spectrum of the heterodimer was significantly different from that of the simple sum of each monomeric dye in the single strand. When azobenzene and Methyl Red, which have λ_max at 336 and 480 nm, respectively, in the single strand (Δλ_max=144 nm), were assembled on ODNs, the band derived from azobenzene exhibited a small hyperchromism, whereas the band from Methyl Red showed hypochromism and both bands shifted to a longer wavelength (bathochromism). These hyper‐ and hypochromisms were further enhanced in a heterodimer derived from 4′‐methylthioazobenzene and Methyl Red, which had a much smaller Δλ_max (82 nm; λ_max=398 and 480 nm in the single‐strand, respectively). With a combination of 4′‐dimethylamino‐2‐nitroazobenzene and Methyl Red, which had an even smaller Δλ_max (33 nm), a single sharp absorption band that was apparently different from the sum of the single‐stranded spectra was observed. These changes in the intensity of the absorption band could be explained by the molecular exciton theory, which has been mainly applied to the spectral behavior of H‐ and/or J‐aggregates composed of homo dyes. However, the bathochromic band shifts observed at shorter wavelengths did not agree with the hypsochromism predicted by the theory. Thus, these data experimentally verify the molecular exciton theory of heterodimerization. This coherent coupling among the heterodimers could also partly explain the bathochromicity and hypochromicity that were observed when the dyes were intercalated into the duplex.     

Rational Molecular Design towards Vis/NIR Absorption and Fluorescence by using Pyrrolopyrrole aza‐BODIPY and its Highly Conjugated Structures for Organic Photovoltaics

Pyrrolopyrrole aza‐BODIPY (PPAB) developed in our recent study from diketopyrrolopyrrole by titanium tetrachloride‐mediated Schiff‐base formation reaction with heteroaromatic amines is a highly potential chromophore due to its intense absorption and fluorescence in the visible region and high fluorescence quantum yield, which is greater than 0.8. To control the absorption and fluorescence of PPAB, particularly in the near‐infrared (NIR) region, further molecular design was performed using DFT calculations. This results in the postulation that the HOMO–LUMO gap of PPAB is perturbed by the heteroaromatic moieties and the aryl‐substituents. Based on this molecular design, a series of new PPAB molecules was synthesized, in which the largest redshifts of the absorption and fluorescence maxima up to 803 and 850 nm, respectively, were achieved for a PPAB consisting of benzothiazole rings and terthienyl substituents. In contrast to the sharp absorption of PPAB, a PPAB dimer, which was prepared by a cross‐coupling reaction of PPAB monomers, exhibited panchromatic absorption across the UV/Vis/NIR regions. With this series of PPAB chromophores in hand, a potential application of PPAB as an optoelectronic material was investigated. After identifying a suitable PPAB molecule for application in organic photovoltaic cells based on evaluation using time‐resolved microwave conductivity measurements, a maximized power conversion efficiency of 1.27 % was achieved.     

Quantum Chemical Study Aimed at Modeling Efficient Aza-BODIPY NIR Dyes: Molecular and Electronic Structure,Absorption, and Emission Spectra

A comprehensive study of the molecular structure of aza-BODIPY and its derivatives, obtained by introduction of one or more substituents, was carried out. We considered the changes in the characteristics of the electronic and geometric structure of the unsubstituted aza-BODIPY introducing the following substituents into the dipyrrin core; phenyl, 2-thiophenyl, 2-furanyl, 3-pyridinyl, 4-pyridinyl, 2-pyridinyl, and ethyl groups. The ground-state geometries of the unsubstituted Aza-BODIPY and 27 derivatives were computed at the PBE/6-31G(d) and CAM-B3LYP/6-31+G(d,p) levels of theory. The time-dependent density-functional theory (TDDFT) together with FC vibronic couplings was used to investigate their absorption and emission spectra.     

Experimental and Theoretical Investigation of the First‐Order Hyperpolarizability of Octupolar Merocyanine Dyes

Hyper‐Rayleigh scattering experiments and quantum chemical calculations are combined to investigate the second‐order nonlinear optical responses of a series of three‐arm merocyanine derivatives. They exhibit an octupolar hyperpolarizability response with lower amplitude than crystal violet due to a lower extent of the photoinduced charge transfer and reduced bond length alternation. Strong effects on the second‐order optical response measured close to the two‐photon absorption level are clearly evidenced; for example, the effective measured polarization ratio deviates below the ideal octupolar value of 3/2 even at very low excitation power. These effects are attributed to two‐photon absorption resonance, which we believe modifies dynamically the population of the ground state versus that of the excited state.     

The Effect of the Buffering Capacity of the Supporting Electrolyte on the Electrochemical Oxidation of Dopamine and 4‐Methylcatechol in Aqueous and Nonaqueous Solvents

Dopamine was electrochemically oxidized in aqueous solutions and in the organic solvents N,N‐dimethyl‐formamide and dimethylsulfoxide containing varying amounts of supporting electrolyte and water, to form dopamine ortho‐quinone. It was found that the electrochemical oxidation mechanism in water and in organic solvents was strongly influenced by the buffering properties of the supporting electrolyte. In aqueous solutions close to pH 7, where buffers were not used, the protons released during the oxidation process were able to sufficiently change the localized pH at the electrode surface to reduce the deprotonation rate of dopamine ortho‐quinone, thereby slowing the conversion into leucoaminochrome. In N,N‐dimethylformamide and dimethylsulfoxide solutions, in the absence of buffers, dopamine was oxidized to dopamine ortho‐quinone that survived without further reaction for several minutes at 25 °C. The voltammetric data obtained in the organic solvents were made more complicated by the presence of HCl in commercial sources of dopamine, which also underwent an oxidation process.     

Selective Cycloaddition of Tetracyanoethene (TCNE) and 7,7,8,8‐Tetracyano‐p‐quinodimethane (TCNQ) to Afford meso‐Substituted Phenylethynyl Porphyrins

π‐Extended TCBD‐porphyrins that contained a 1,1,4,4‐tetracyanobuta‐1,3‐diene unit were prepared by a highly efficient [2+2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8‐tetracyano‐p‐quinodimethane (TCNQ) with meso‐substituted trans‐A₂B₂‐porphyrins that contained two phenylethynyl groups, followed by a retro‐electrocyclization reaction. Depending on the electronic properties of the arylethynyl groups, the cycloaddition reaction took place exclusively on either one or two ethynyl moieties with high yield. The addition of TCNQ proceeded with complete regioselectivity. The resulting π‐expanded TCBD‐porphyrins had a hypsochromically shifted Soret band and showed unique, broad absorption in the visible region.     

Identification of Triplet States in Carotenoids by Intracavity Absorption Spectroscopy and Measurements of Delayed Fluorescence

Owing to the low quantum yield of phosphorescence , triplet states of carotenoids have been very difficult to identify. These states can be characterized by intracavity absorption spectroscopy, which allows the direct measurement of the spin-forbidden singlet–triplet transitions in low concentrated solutions, and by delayed fluorescence measurements.     

Synthesis,Spectrothermal Behaviour and Molecular Structure of Aquaorotatotriethanolaminenickel(II) Monohydrate

The aquaorotatotriethanolaminenickel(II) monohydrate, [Ni(HOr)(H₂O)(tea)]·H₂O ( 1 ), was synthesized and characterized by means of elementel analysis, IR and UV‐Vis, spectroscopy, magnetic susceptibility, thermal analysis and X‐ray diffraction techniques. The nickel ion in [Ni(C₅H₂N₂O₄)(H₂O)(N(C₂H₄OH)₃)] is chelated to the deprotonated N3 pyrimidine atom and to the carboxylate oxygen atom of the bidentate orotate dianion, and to the one nitrogen and two oxygen atoms of the tridentate triethanolamine molecule and its octahedral geometry is completed by an aqua ligand. It crystallizes in the monoclinic system, space group P2₁/c with lattice parameters a = 7.1528(5) Å, b = 19.4903(14) Å, c = 11.8085(8) Å, β = 106.237(5)°, V = 1580.55(19) ų, Z = 4. An extensive three dimensional network of O_w‐H…O, N‐H…O and O‐H…O hydrogen bonds, π‐π and π‐ring interactions are responsible for crystal stabilization. The decomposition reaction take places in the temperature range 20‐1000 °C in the static air atmosphere. Thermal decomposition of 1 proceeds in three stages.     

Electronic and magnetic properties of bimetallic ytterbocene complexes: the impact of bridging ligand geometry

Bimetallic ytterbocene complexes with bridging N-heterocylic ligands have been studied extensively in recent years due to their potential applications ranging from molecular wires to single-molecule magnets. Herein, we review our recent results for a series of ytterbocene polypyridyl bimetallic complexes to highlight the versatility and tunability of these systems based on simple changes in bridging ligand geometry. Our work has involved structural, electrochemical, optical, and magnetic measurements with the goal of better understanding the electronic and magnetic communication between the two ytterbium metal centers in this new class of bimetallics.     

9,10‐Diaminoanthracenes Revisited: The Influence of N‐Substituents on Their Electronic States

The electronic and molecular structures of 9,10‐diamino‐substituted anthracenes with different N‐substituents have been re‐examined. In particular, different N‐substituents influence both the electronic and molecular structures of the oxidized species of 9,10‐diaminoanthracenes. The anthrylene moiety of 9,10‐bis(N,N‐di(p‐anisyl)amino)anthracene retains its planarity during the course of two successive one‐electron oxidations, whereas 9,10‐bis(N,N‐dimethylamino)anthracene and 9,10‐bis(N‐p‐anisyl‐N‐methylamino)anthracene undergo a substantial structural change to a butterfly‐like structure through a two‐electron oxidation process. The structural changes observed for the oxidized states are ascribed to significant differences in the frontier molecular orbitals of the above‐mentioned three kinds of 9,10‐diaminoanthracenes due to different extents of mixing between the amine‐localized and anthrylene‐localized orbitals.     

A Joint Theoretical and Experimental Study of the Behavior of the DIDS Inhibitor and its Derivatives

4,4′‐Diisothiocyanostilbene‐2,2′‐disulfonic acid (DIDS) is a well‐known ion‐exchange inhibitor targeting cardiac functions and indirectly impeding both radio‐ and chemo‐resistance. A joint computational and experimental study is presented to provide deeper insights into DIDS and other members of this family of compounds. To this end, we applied state‐of‐the‐art density functional theory (DFT) and time‐dependent DFT methods, in addition to measuring the optical properties. The experimental data show that such compounds are highly sensitive to their environment and that the optical properties change within as little time as 7 h. However, the optical properties of DIDS are similar in various acidic/basic environments, which were confirmed by pK_a computations on both cis and trans isomers. The protonation analysis also highlights that the singly protonated form of DIDS behaves like a proton sponge compound. The experimentally observed redshift that can be seen when going from water to DMSO was reproduced solely by using the solvation model based on density, although the polarization continuum model and implicit/explicit hybrid schemes were also tested. The characteristic broadening of the absorption peak in water and the vibronic fine structure in DMSO were also reproduced thanks to vibronic coupling simulations associated with the solvent reorganization energy. For other stilbene derivatives, a correlation is found between the maximum absorption wavelength and the Hammett parameters.     

Control of Chromophore Symmetry by Positional Isomerism of Peripheral Substituents

The first example of the control of porphyrinoid chromophore symmetry based on the positional isomerism of peripheral substituents has been achieved by preparing tetraazaporphyrins (TAPs) with C_4h, D_2h, C_2v, and C_s symmetry due to the relative arrangement of peripheral tert‐butylamino and cyano groups as push and pull substituents, respectively. The four structural isomers were successfully isolated and characterized by ¹H NMR spectroscopy and X‐ray crystallography. The band morphology in the Q‐band region varies depending on the molecular symmetry due to the significant perturbation introduced into the chromophore by the push and pull substituents. The C_4h and C_2v isomers exhibit a single Q band, whereas the Q bands of the D_2h and C_s isomers show a marked splitting. The magnetic circular dichroism spectra indicate that the push–pull TAPs retain the properties of the 16‐membered 18π‐electron perimeter generally observed for porphyrinoids. Theoretical calculations have demonstrated that the perturbation introduced by the substituents lowers the D_4h symmetry of the parent TAP π‐conjugated system, and this results in significant spectral changes. A novel approach to the fine‐tuning of the spectral properties of porphyrinoids based on changes in the chromophore symmetry is described.