Comparison Study of the Site-Effect on Regioisomeric Pyridyl–Pyrene Conjugates: Synthesis, Structures, and Photophysical Properties

Abstract

To investigate the "site effect" of pyridyl substituents on a pyrene core, four regioisomeric monopyridyl-pyrene (1−4) and five regioisomeric dipyridyl-pyrene (5−9) conjugates were synthesized and characterized and their structures confirmed by single-crystal X-ray diffraction. The photophysical properties and related frontier orbital features of these compounds have been studied both experimentally and theoretically and demonstrate the dependence of the properties of the compounds on the position of substitution of the pyridyl moieties connecting to the pyrene core. It was found that the absorption spectra of 2-and 4substituted pyrene derivatives display similar and weak influence on the S 2 ← S 0 excitations, whereas they are quite different from those of 1-substituted isomers. The emission spectra of 1and 4-substituted pyrenes are quite similar, whereas those of 2-substituted isomers display the largest bathochromic shift. The 1,6-disubstituted compound 5 exhibits a near-unity emission quantum yield in solution, which is nearly three times higher than those of other regioisomeric dipyridyl-pyrenes. In addition, the tetrasubstituted 1,6-dipyridyl-3,8-di-n-butyl-pyrene (10) exhibits the highest solid-state quantum yield of 0.24 among all of the 10 pyridyl-pyrenes prepared in this study.

2‐ and 2,7‐Substituted para‐N‐Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double‐Stranded (ds) DNA

“…These show strong π‐stacking interactions between pyridinium and pyrene moieties as a result of protonation, which determine the parallel head‐to‐tail stacking in 1M and 1H and the offset parallel stacking in 2M and 2H . A similar π‐stacking interaction between pyridyl and pyrene moieties is not observed in the neutral compounds 1 and 2 , in which π‐stacking is only present between the pyrene moieties ( 1 ) or both the pyrene and the pyridyl moieties ( 2 ) [20] . In 1M , 1H , 2M and 2H , cation stacks weakly interact with the anions, which are located in between the stacks.…”

“…In summary, the mono‐ or dications are almost planar in all compounds in the solid state (torsion angles: 0.8–2.5°), except for 1M (torsion angles: 18.2 and 22.7°). It is important to note that the observed torsion angles in the corresponding neutral compounds 1 (38.3°) and 2 (12.1° for α‐form and 38.6° for β‐form) [20] are larger than those in the cationic compounds, which are strongly diminished by protonation or methylation (except for 1M ). The smaller torsion angles may also be a result of the large degree of π‐stacking interactions in the near‐planar cationic compounds.…”

“…The neutral compounds 1 and 2 were synthesised via the Suzuki–Miyaura coupling reaction of 2‐(Bpin)pyrene and 2,7‐bis(Bpin)pyrene, respectively, with 4‐bromopyridine, as we have reported previously; [20] however, [Pd(dppf)Cl 2 ] was used as the catalyst precursor instead of [Pd(PPh 3 ) 4 ]. The methylation reactions yielding compounds 1M and 2M were carried out with MeOTf in dry toluene under an argon atmosphere, and single crystals of 1M and 2M were obtained by slow evaporation of concentrated solutions of the compounds in MeCN.…”

“…The localised double bond (C=C) character at the 4‐, 5‐, 9‐, and 10‐positions can be exploited to derivatise pyrene at its K‐region [27, 28] . Thus, various compounds with substituents at the 4‐, 5‐, 9‐, and 10‐positions with interesting photophysical and electrochemical properties have recently been reported [20, 29, 30] …”

“…Recently, we have reported the syntheses and photophysical properties of several mono‐ and bis‐pyridyl‐pyrene compounds substituted at various positions of pyrene [20] . It has been demonstrated that the protonation of pyridyl‐pyrene compounds shifts the emission bathochromically [45] .…”