A library of imidazo[1,2-a]pyridines was synthesized by using the Gevorgyan method and their linear and non-linear optical properties were studied. Derivatives that contained both electron-donating and electron-withdrawing groups at the 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2-0.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron-donating substituents improved the luminescence performance of these compounds, whereas electron-withdrawing substituents led to a more erratic behavior. Substitution on the six-membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2-a]pyridine units on its periphery and a 1,4-dicyanobenzene unit at its center. This system exhibited a large Stokes-shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge-transfer character. This quadrupolar feature also led to an acceptable two-photon absorption response in the NIR region.

p-p Stackingi so mnipresent not only in nature but in aw ide variety of practical fields appliedt oo ur lives. Becauseo fi ts importance in ap erformance of natural and artificial systems, such as light harvesting system and working layer in device, many researchers have put intensive effort into identifying its underlying nature. However,f or the case of p-p stacked systemsc omposed of antiaromatic units, the understanding of the fundamentalm echanisms is still unclear.H erein, we synthesized an ew type of planar b,b'-phenylene-bridgedh exaphyrin (1.0.1.0.1.0),r eferred as naphthorosarin whichp ossesses the 24p-electronc onjugated pathway.E specially,t he corresponding antiaromaticp orphyrinoid shows the unique property to form dimeric species adopting the face-to-face geometry which is unprecedentedi nc ases of known annulated naphthorosarins. In order to elucidate the intriguing properties derived from the stacked dimer,t he current study focuses on the experimental support to rationalizet he observed p-p interactions betweent he two subunits.

Peripherally substituted antiaromatic naphthorosarins have been synthesized for the first time. The synthesis was accomplished by acid‐catalyzed condensation of naphthobipyrrole building blocks with aromatic aldehydes. The naphthobipyrrole building blocks were synthesized by simple oxidative coupling of the corresponding pyrrole substituted aromatics. Solid‐state structural analyses of the synthesized naphthorosarins revealed that the presence of meso‐2,6‐dichlorophenyl‐ and 5,6‐difluoro‐substitution substantially alter the geometry and properties of the naphthorosarins. The substituents affect the redox potentials as well and, in turn, the proton‐coupled electron‐transfer processes leading to the formation of one‐ and two‐electron reduced forms of the corresponding naphthorosarins. One particular naphthorosarin that bears both peripheral fluorine and meso‐2,6‐dichlorophenyl substituents forms a stable 25 π‐electron species upon treating with TFA that was characterized by single‐crystal X‐ray diffraction analysis. The current study underscores how structural modifications can be used to fine‐tune the electronic features of naphthorosarins, including stabilization of odd electron species.

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpol'skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.

The anion-radical coupling of structurally diverse series of aromatic compounds possessing biaryl linkages led to seven fused, polycyclic heterocycles in reasonable yields. The yield of the key step (K, toluene, O2) depends on both electronic and steric factors. The whole strategy consists of just two steps starting from an unsubstituted imidazo[1,2-a]pyridine, giving target compounds in an overall yield of 4-34%. The same strategy also works for derivatives of imidazo[1,5-a]pyridine. A new process has been discovered for such vertically-expanded imidazo[1,2-a]pyridines, consisting of a sequential Diels-Alder reaction followed by a retro-Diels-Alder reaction. The optical properties of the library of π-expanded imidazo[1,2-a]pyridines were for the first time fully characterized, showing that fluorescence quantum yields are significantly lower than for the singly-linked compounds.