Positional Isomers of Tetramethoxypyrene-based Mono- and Biradicals

The positional isomers of <i>tert</i>-butylnitroxide (NO) substituted 4,5,9,10-tetramethoxypyrene-based mono- and biradical are synthesized. While the biradical 2,7-TMPNO in which two NO radical moieties are attached at the nodal plane of pyrene adopts a semiquinoid structure, the 1,6- and 1,8-isomers of the same exist in biradical form. The tuning of the antiferromagnetic exchange interactions is achieved by synthesizing the positional isomers of the biradical while maintaining the same radical moiety as well as the π spacer

Mesitylboron-Substituted Ladder-Type Pentaphenylenes: Charge-Transfer, Electronic Communication, and Sensing Properties

A series of dimesitylboron (B)- or ditolylamino (N)-substituted ladder-type pentaphenylenes (PP) has been designed and synthesized. The UV−vis absorption spectra of compounds BPPN, BPPB, and NPPN reveal an identical maximum wavelength at 432 nm, which indicates that the B and N centers have very similar contributions to the extended conjugation. A rather weak solvatochromism in the UV−vis absorption spectra is observed for compound BPPN, while a remarkable solvatochromic emission is achieved even though the distance between the B and the N centers is as huge as 22 Å. The photoluminescence of BPPN shows a bathochromic shift of 108 nm when the solvent polarity is increased from cyclohexane (453 nm) to acetone (561 nm). Compound BPPN acts as a colorimetric and fluorescent chemosensor with high sensitivity (10−5 M) and selectivity for F− over other halogen ions. By inhibiting the charge transfer (CT) from the N center to the B center, the intense green CT emission of compound BPPN rapidly switches into the sky-blue emission of PP when F− is bound to the B center. Furthermore, a CT emission can be switched “on” and “off” when compound BPPB is used as F− sensory material. Such an intramolecular CT emission between the two B centers has so far never been reported. Corresponding studies by cyclic voltammetry and differential pulse voltammetry reveal a two-step reduction of the two bridged B centers in compound BPPB, which might suggest that the charge delocalizes through the whole molecule and that the terminal redox centers communicate through the pentaphenylene bridge

Condensed Derivatives of Thiadiazoloquinoxaline as Strong Acceptors

Three novel thiadiazoloquinoxaline (TQ) derivatives, <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, were synthesized by introducing two triisopropylsilylethynyl groups and alternating the aromatic ring units in the condensed moiety of TQ. The synthetic route is very efficient, providing high yields. Cyclic voltammetry suggests high electron affinity values of −3.82, −3.95, and −3.99 eV for <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, respectively. Single-crystal X-ray diffraction reveals that three molecules form corresponding dimers by intermolecular S–N interaction and have very similar two-dimensional π-stacking. The π-stacking distances between them are as close as 3.34–3.46 Å

Arylamine-Substituted Oligo(ladder-type pentaphenylene)s: Electronic Communication between Bridged Redox Centers

Novel bis(arylamine-substituted) oligo(ladder-type pentaphenylene)s 1−3, with bridge lengths estimated to be 2.2, 4.2, and 6.3 nm, respectively, have been developed, and the model compound 4 with a mono-arylamine substituent was also synthesized. Their absorption spectra in different solvents are almost identical, while distinct bathochromic shifts of the photoluminescence (PL) spectra were observed with increasing solvent polarity due to the polarized excited states. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) spectra display a two-step oxidation of the bridged diamines in compound 1, which suggests that the electron and charge delocalize in mixed-valence (MV) cation 1+• and that both redox centers can communicate through the pentaphenylene bridge. Only unresolved curves in CV and DPV spectra were observed in the first two oxidation processes of diamines 2 and 3, indicating that the bridges are too long for efficient delocalization over the entire molecules and the radical cations localize at each arylamine center. This finding was further supported by chemical oxidation with SbCl5 and studies of the corresponding UV−vis−NIR absorption spectra of compounds 1−4. A significant intervalence charge-transfer (IVCT) band around 5283 cm-1 (1893 nm) was observed in 1+•. This is the first report of such a highly intense IVCT band in the NIR region with intensity similar to that of the visible band of the radicals, enabling further analysis of the CT process and the coupling matrix element V, classifying 1+• as a class II derivative (V = 1.6 kcal/mol). This study may offer an effective way to improve the understanding of charge transfer and charge-carrier transport in various conjugated oligomers or polymers and facilitate their ongoing exploration in optoelectronic applications

Thiadiazoloquinoxalines: Tuning Physical Properties through Smart Synthesis

The synthesis of π-conjugated acceptors based on thiadiazoloquinoxaline (TQ) derivatives is described. Apart from reporting on the functionalization of the TQ core, the influence of the substituents was studied by UV−vis absorption and emission spectroscopy, cyclic voltammetry measurements, and DFT calculations. By changing the donor as well as the π-spacer, a fine-tuning of the photo- and electrochemical properties was achieved

Condensed Derivatives of Thiadiazoloquinoxaline as Strong Acceptors

Three novel thiadiazoloquinoxaline (TQ) derivatives, <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, were synthesized by introducing two triisopropylsilylethynyl groups and alternating the aromatic ring units in the condensed moiety of TQ. The synthetic route is very efficient, providing high yields. Cyclic voltammetry suggests high electron affinity values of −3.82, −3.95, and −3.99 eV for <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, respectively. Single-crystal X-ray diffraction reveals that three molecules form corresponding dimers by intermolecular S–N interaction and have very similar two-dimensional π-stacking. The π-stacking distances between them are as close as 3.34–3.46 Å

Highly Ordered Phenanthroline-Fused Azaacene

A new synthetic route to prepare a centrosymmetric phenanthroline-fused azaacene derivative, <b>TIPS-BisPhNPQ</b>, is described. Another axialsymmetric analogue, <b>TIPS-PhNTQ</b>, is also synthesized for comparison. Cyclic voltammetry measurements indicate high electron affinity values of −4.03 and −4.01 eV for <b>TIPS-PhNTQ</b> and <b>TIPS-BisPhNPQ</b>, respectively. Single-crystal X-ray diffraction reveals that <b>TIPS-PhNTQ</b> forms dimers by intermolecular S–N and N–N interaction, while <b>TIPS-BisPhNPQ</b> shows a highly ordered arrangement via two-dimensional brickwork packing and intermolecular hydrogen bonding. The synthetic protocol established in this paper should be highly applicable to the preparation of more azaacene derivatives with extended π-conjugations

Phenylene Bridged Boron−Nitrogen Containing Dendrimers

The synthesis and characterization of novel phenylene bridged boron−nitrogen containing π-conjugated dendrimers N3B6 and N3B3, with peripheral boron atoms and 1,3,5-triaminobenzene moiety as a core, are presented. UV−vis absorption and emission measurements reveal that the optical properties of the resulting compounds can be controlled by changing the donor/acceptor ratio: a 1:1 ratio results in a more efficient charge transfer than the 1:2 ratio. This was proven by the red shift of the emission maxima and the stronger solvatochromic effect in N3B3 compared to N3B6

Condensed Derivatives of Thiadiazoloquinoxaline as Strong Acceptors

Three novel thiadiazoloquinoxaline (TQ) derivatives, <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, were synthesized by introducing two triisopropylsilylethynyl groups and alternating the aromatic ring units in the condensed moiety of TQ. The synthetic route is very efficient, providing high yields. Cyclic voltammetry suggests high electron affinity values of −3.82, −3.95, and −3.99 eV for <b>TIPS-APhTQ</b>, <b>TIPS-PhTQ</b>, and <b>TIPS-BDTTQ</b>, respectively. Single-crystal X-ray diffraction reveals that three molecules form corresponding dimers by intermolecular S–N interaction and have very similar two-dimensional π-stacking. The π-stacking distances between them are as close as 3.34–3.46 Å

Study on the Heteroatom Influence in Pyridine-Based Nitronyl Nitroxide Biradicals with Phenylethynyl Spacers on the Molecular Ground State

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV−vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing |aN|= 7.36 G. This indicates strong, intramolecular exchange coupling (J ≫ 7 × 10-4 cm-1; J/aN ≫ 1) of the biradicals with in the limit of EPR. The temperature dependence on the Δms = ±2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV−vis studies of 1−5 show characteristic differences in the extinctions of n−π* transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P21/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet−triplet (ΔEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results