Designing Multifunctional Expanded Pyridiniums: Properties of Branched and Fused Head-to-Tail Bipyridiniums

Abstract

The multifaceted potentialities of expanded pyridiniums (EPs), based on one pyridinium core bearing a 4-pyridyl or 4-pyridylium as the N-pyridinio group, are established at both experimental and theoretical levels. Two classes of head-to-tail (htt) EPs were designed, and their first representative elements were synthesized and fully characterized. The branched (B) family is made up of 2,6-diphenyl-4-aryl-1,4'-bipyridin-1-ium (or 1,1'-diium) species, denoted 1B and 2B for monocationic EPs (with aryl = phenyl and biphenyl, respectively) and 1B(Me) and 2B(Me) for related quaternarized dicationic species. The series of fused (F) analogues comprises 9-aryl-benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium species, denoted 1F and 2F, and their 2,15-diium derivatives referred to as 1F(Me) and 2F(Me). Electrochemistry (in MeCN vs SCE) reveals that branched EPs undergo a single reversible bielectronic reduction at ca. -0.92 V for 1B/2B and -0.59 V for 1B(Me)/2B(Me), whereas pericondensed species show two reversible monoelectronic reductions at ca. -0.83 and -1.59 V for 1F/2F and ca. -0.42 and -1.07 V for 1F(Me)/2F(Me). Regarding electronic absorption features, all htt-EP chromophores show absorptivity in the range of ca. 1-4 × 10(4) M(-1) cm(-1), with red-edge absorptions extending toward 450 and 500 nm (in MeCN) for 2B(Me) and 2F(Me), respectively. These lowest-energy pi-pi* transitions are ascribed to intramolecular charge transfer between the electron-releasing biphenyl group and the htt-bipyridinium electron-withdrawing subsystems. EPs display room-temperature photoemission quantum yields ranging from 10% to 50%, with the exception of 1B, and branched luminophores are characterized by larger Stokes shifts (8000-10 000 cm(-1)) than fused ones. Lastly, a method to predict the efficiency of photobiscyclization of branched EPs into fused ones, based on the analysis of computed difference maps in total electron density for singlet excited states, is proposed.

Kinetics of Multielectron Transfers and Redox-Induced Structural Changes in N-Aryl-Expanded Pyridiniums: Establishing Their Unusual, Versatile Electrophoric Activity

“…6 The present work deals with the determination and analysis of standard potentials and ET rate constants of differently congested (mono)pyridinium and htt-bipyridinium derivatives in order to assess to what extent intramolecular steric hindrance is a valuable parameter to tune the features of ET processes, including their overall layout (successive or simultaneous ETs) and their intrinsic kinetic rates. Therefore, mapping the kinetic landscape of pyridinium-based versatile electrophores is also of prime interest.…”

“…The scope of the paper is limited to the simplest case study of the first two ETs, starting from the respective monocationic and dicationic native forms of various model electrophores. From our previous work 5,6,17 it was expected that all compounds would be reduced by at least two electrons. relaxation of skeletal geometry), could be addressed, assessed and tested.…”

“…Indeed, in the typical case of the two-electron reduction of a pyridinium core, the process can occur stepwisely or in an apparent single step, depending on the substitution pattern of the electron-accepting cation. 6,17,18 At this point, it is worth noting that, despite the lack of steric protection, in dimethyl sulfoxide MV is reduced in two separate and reversible one-electron steps at -0.37 V and -0.70 V against Ag|AgCl|1 M LiCl reference electrode. 6 In the case of the unsubstituted N-methylpyridinium cation, the atomic contributions to the LUMO (Chart 1), which governs the first reduction process, originate mainly from the nitrogen atom and carbon atoms in the 2-, 6-and 4-positions (α-and γ-positions with respect to the nitrogen atom).…”

“…at the same (apparent) potential, is an important phenomenon, which operates in many efficient natural processes, in particular in catalytic systems. 6 In the case of the unsubstituted N-methylpyridinium cation, the atomic contributions to the LUMO (Chart 1), which governs the first reduction process, originate mainly from the nitrogen atom and carbon atoms in the 2-, 6-and 4-positions (α-and γ-positions with respect to the nitrogen atom). Multi-electron transfers are involved in applications spanning from biosensors and catalysis to solar energy conversion and storage (including man-made photosynthesis) and molecular electronics.…”

“…5,6 For such species, conveniently balanced intramolecular steric congestion and the involvement of the orbitals centered on the two pyridinium rings in the LUMO and SOMO, in the case of dicationic "head-to-tail" (htt) bipyridinium derivatives (htt-EPs, Chart 1), are instrumental in assisting and/or amplifying structural changes triggered by the first electron transfer (ET), that, in their turn, have a significant impact on the energetics of the second ET process. Under certain conditions the transfer of these two electrons at the same potential is also possible, as in the typical case of the above-mentioned EPs.…”