Synthesis and Spectral Properties of a Highly Soluble Push−Pull Type of Quinoidal Thiophenes

An Acceptor-Substituted N-Heterocyclic ortho-Quinodimethane: Pushing the Boundaries of Polarization in Donor-Acceptor-Substituted Polyenes

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted ortho-quinodimethane (oQDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted ortho-N-heterocyclic quinodimethane (oNHQ). We have examined the extent of polarization of the conjugated π-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations. The bond lengths in the phenyl linker do not exhibit the alternation typical of oQDMs. In addition, the 13C and 15N NMR shifts suggest significant charge separation, an interpretation supported by the diatropic ring current determined by NICSZZ(r) computations, which is characteristic of aromatic compounds. DFT calculations show that polarization is an electronic effect that is amplified by steric influences. More strikingly, the oxidation and reduction potentials of the push-pull substituted oQDM are virtually identical to those of authenticated anionic and cationic derivatives. The results therefore indicate that an aromatic zwitterionic structure represents the electronic structure more accurately than a neutral quinoidal Lewis structure, which indicates that the acceptor-substituted oNHQ is a rare example of an organic zwitterion in which the centers of charge are in conjugation. The ambiphilic reactivity of the acceptor-substituted oNHQ, which is evidenced by the dehydrogenation of ammonia borane and the addition of phenylacetylene via heterolytic C-H bond cleavage, further supports its notation as an organic zwitterion and is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted oNHQ can be considered to be an intramolecular carbogenic FLP in terms of its reactivity.

Novel Dissymmetric Formal Quinoidal Molecules End-Capped by Dicyanomethylene and Triphenylphosphonium

A number of quinoidal molecules with symmetric end-capping groups, particularly dicyanomethylene units, have been synthesized for organic optoelectronic materials. In comparison, dissymmetric quinoidal molecules, characterized by end-capping with different groups, are less explored. In this paper, we present the unexpected formation of new formal quinoidal molecules, which are end-capped with both dicyanomethylene and triphenylphosphonium moieties. The structures of these dissymmetric quinoidal molecules were firmly verified by single crystal structural analyses. On the basis of the control experiments and DFT calculations, we proposed the reaction mechanism for the formation of these dissymmetric quinoidal molecules. The respective zwitterionic forms should make contributions to the ground state structures of these quinoidal molecules based on the analysis of their bond lengths and aromaticity and Mayer Bond Orbital (MBO) calculation. This agrees well with the fact that negative solvatochromism was observed for these quinoidal molecules. Although these new quinoidal molecules are non-emissive both in solutions and crystalline states, they become emissive with quantum yields up to 51.4 % after elevating the solvent viscosity or dispersing them in a PMMA matrix. Interestingly, their emissions can also be switched on upon binding with certain proteins, in particular with human serum albumin.

π-Extended S-Heterocyclic Naphthoquinodimethane with Dual Diradical and Dipolar Character

The 2,6-naphthoquinodimethane (2,6-NQDM) containing S-heterocyclic molecule 6-S was synthesized, and its dual open-shell diradical and dipolar characters were revealed via both experimental and theoretical studies. Unlike the shorter p-quinodimethane (p-QDM)-containing analogue 5-S, which possesses a closed-shell ground state (y₀ = 0%) with small dipolar character, 6-S possesses enhanced dipolar character with a singlet diradical ground state (y₀ = 23.3%) and a thermally accessible triplet excited state (ΔE_ST = -4.13 kcal/mol). Despite this, it displays good stability (t_1/2 = 41days) under ambient air and light conditions due to its distinctive dipolar character and kinetic blocking of reactive sites.

Photoinduced Generation of the π-Conjugated Zwitterionic State in the ESIPT Fluorophore of 2,4-Bisimidazolylphenol

We demonstrate that 2,4-bis(4,5-diphenyl-1H-imidazol-2-yl)phenol (2,4-bImP) undergoes photoinduced conversion into the so-called "π-conjugated zwitterion" after causing an excited-state intramolecular proton transfer (ESIPT) reaction. The powder sample of 2,4-bImP exhibits largely Stokes-shifted fluorescence characteristics to ESIPT fluorophores. On the other hand, its originally colorless solutions become colored when exposed to UV light for several minutes, whose color depends on the type of solvent. In particular, the CHCl₃ solution rapidly turns dark green with the absorption maximum around 700 nm, and the colored solution is nearly restored to original by alternating addition of acid and base. To explain such drastic and reversible color changes, we hypothesized that the occurrence of ESIPT (i.e., deprotonation of the phenol and protonation of the imidazolyl group at its 2-position) triggered the charge-separated structure between the negatively charged phenolate and the positively charged imidazoliumyl group at its 4-position, which allowed resonance with the neutral p-quinoid structure. The formation of this π-conjugated zwitterion was strongly supported by the results of ¹H and ¹⁵N NMR and Raman measurements.

Push-pull quinoidal porphyrins

A family of push-pull quinoidal porphyrin monomers has been prepared from a meso-formyl porphyrin by bromination, thioacetal formation, palladium-catalyzed coupling with malononitrile and oxidation with DDQ. Attempts at extending this synthesis to a push-pull quinoidal/cumulenic porphyrin dimer were not successful. The crystal structures of the quinoidal porphyrins indicate that there is no significant contribution from singlet biradical or zwitterionic resonance forms. The crystal structure of an ethyne-linked porphyrin dimer shows that the torsion angle between the porphyrin units is only about 3°, in keeping with crystallographic results on related compounds, but contrasting with the torsion angle of about 35° predicted by computational studies. The free-base quinoidal porphyrin monomers form tightly π-stacked layer structures, despite their curved geometries and bulky aryl substituents.

The ratio and topology effects of benzodithiophene donor-benzooxadiazole acceptor fragments on the optoelectronic properties of donor molecules toward solar cell materials

A series of conjugated donor molecules (DmAnSq where m = 1-4, n = 1-7 while D = benzodithiophene, A = benzooxadiazole and S denotes ethyne spacers between D and A or D and D fragments) with various ratios of D/A fragments and topologies have been designed and investigated for OPV applications. An increase in the ratio of the acceptor fragment with respect to the donor fragment decreases the LUMO energy level and narrows the Eg for the designed molecule. More vertically (C4 and C8 substituted phenyl ring positions) bonded acceptor fragments than linearly (C2 and C6 substituted thiophene ring positions) bonded fragments result in a significant red shift in the maximum absorption wavelength. While, linearly bonded fragments lead to stronger absorption bands. Molecules with D-A-D topology exhibit more significant optical and electronic characteristics than those with D-D topology. All donor molecules (m = 2-4) of the D-A-D type show lower λh values than those of 1 donor containing (DAn) molecules. D-D type molecules show only lower λe values than DAn molecules because of the presence of a second donor fragment. The charge transfer phenomenon is shape dependent. The branched or anisotropic X, H, π, n, and square shaped molecules display higher charge transfer rates than the corresponding linear isomers due to better dimensionality. On the basis of these results, we suggest that designed donor and corresponding matched acceptor molecules have potential to act as promising candidates in solar cell devices.

Bright, Red Single-Molecule Emitters: Synthesis and Properties of Environmentally Sensitive Dicyanomethylenedihydrofuran (DCDHF) Fluorophores with Bisaromatic Conjugation

A group of new fluorescent dye materials for single-molecule imaging applications comprised of an amine donor, a π-system comprised of phenyl and thiophene rings and a 2-dicyanomethylene-3-cyano-2,5-dihydrofuran acceptor group have been synthesized. Relative to comparable single-ring compounds these doubly aromatic conjugated fluorophores have red-shifted absorption and emission usually accompanied by significantly increased quantum yields. Solvatochromism studies indicate that the photophysical properties of these dyes are sensitive to the solvent polarity and environmental rigidity. Photophysical studies demonstrate that these DCDHF dye materials are strong single-molecule emitters and the total number of detected photons per molecule is among the highest thus far for this family of fluorophores.

Classification of dynamic vibronic couplings in vibrational real-time spectra of a thiophene derivative by few-cycle pulses

Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.

Confined breather-type excitation in a quinoidal thiophene after sub-5fs pulse excitation

A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5 fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the C[Double Bond]C stretching was observed for the first time. It generates two sidebands of 2238 and 700 cm(-1) with a separation of 769 cm(-1) corresponding to the modulation frequency. The latter mode is very close to the C-S-C ring deformation, which indicate that this mode mediates coupling between the breather mode and the C(beta)-C(beta) stretching mode. It was also found that the breather-type excitation has a longer lifetime (2-3 ps) than in all-trans-polyacetylene (50 fs) due to its confinement.