A BODIPY-Bridged Bisphenoxyl Diradicaloid: Solvent-Dependent Diradical Character and Physical Properties

Abstract

We report a new boron dipyrromethene (BODIPY)-bridged bisphenoxyl diradicaloid (2), which showed closed-shell diamagnetic character in less polar solvents such as dichloromethane but open-shell diradical character with paramagnetic activity in the very polar solvent N,N-dimethylformamide. X-ray crystallographic analysis of 2 revealed an anti-parallel stacked dimer structure via intermolecular dipole–dipole interaction, and the observed solvent-dependent diradical character can be explained by the different dihedral angles between the phenoxyl units and the BODIPY bridge, and structural flexibility of the molecule in different solvents. Compound 2 also exhibited solvent-dependent optical and electrochemical properties.

Intramolecular Hydrogen Bonds in Selected Aromatic Compounds: Recent Developments

“…The growing popularity of fluorescent dyes rests upon the possibility of chemical modification of both the chromophore core and its substituents, which can manifest in a variety of spectroscopic properties [70][71][72][73][74][75][76][77][78][79][80][81][82][83]. The main objective of fluorescent BODIPY (4,4′-difluoro-4-boron-3a,4a-diaza-s-indacene) dye studies has been to explore the influence of the tautomeric equilibrium sensor on the spectroscopic parameters of the dye [84][85][86][87][88].…”

Tuning phenoxyl-substituted diketopyrrolopyrroles from quinoidal to biradical ground states through (hetero-)aromatic linkers

“…[14][15][16][17] Within this material class, biradical(oid)s composed of p-systems substituted with two phenoxyl units are among the most outstanding representatives, due to the broad variety of accessible structures, which makes them an ideal model system for experimental and theoretical investigations (Scheme 1). [18][19][20][21][22][23][24][25] In order to gain insight into the biradical character of these organic compounds, it is crucial to experimentally distinguish the biradical and quinone singlet state. As energy differences between these states tend to be very narrow, an even more careful choice of analytical methods is essential to uncover hidden discrepancies and draw solid conclusions.…”

“…21,22 Additionally, the (in)equivalency of phenyl protons a (Scheme 1, top) can be used as an indicator for the molecules' rigidity. 20,22 Furthermore, in order to experimentally access the singlet-triplet energy gap (DE ST ), SQUID (superconducting quantum interference device) or variable temperature (VT) EPR can be utilised, of which the latter one commonly is performed in solution. 19,22 In the solid state, further effects like enhanced intermolecular interactions have to be considered.…”

“…In the past, several factors inuencing the electronic ground state and hence enabling a spin crossover from closed shell quinoidal to open shell biradical of p-conjugated compounds derived from quinodimethanes in general and twofold phenoxyl substituted chromophores in particular have been investigated. In this regard, the impact of the number of bridging phenyl units, 28,29 the connecting p-core (Scheme 1, bottom), [18][19][20][21]24,25 hetero atom effects, the number of attached (donor) substituents and the steric demand of ortho-positioned alkyl chains protecting the radical centres in such systems [30][31][32] has been investigated. Furthermore, based on numerous longitudinally and laterally extended zethrenes, [33][34][35][36] Wu and coworkers as well as Juríček and coworkers have demonstrated, that the size of the p-scaffold can signicantly inuence the biradical character of these polycyclic aromatic hydrocarbons (PAH).…”

“… 23 X-ray analysis is a powerful method to shine light on the electronic ground state, since it allows an experimental bond length determination. Accordingly, the appearance of a distinct bond length alternation (BLA) is commonly a clear hint towards closed shell quinones, 20,22 whereas its absence indicates a dominating open shell biradical character. 24…”