Electrochemical and spectroelectrochemical properties of building blocks for molecular arrays: reactions of quinoxalino[2,3-b]porphyrins containing metal(II) ions

Abstract

Quinoxalino[2,3-b]porphyrins are laterally-extended porphyrins with aromatic ring systems fused to the β,β'-positions of a pyrrolic ring of the macrocycle. They are building blocks for coplanar laterally-extended oligoporphyrins with applications in molecular electronics. The electrochemistry and spectroelectrochemistry of four such quinoxalinoporphyrins containing metal(II) ions and one free-base quinoxalinoporphyrin dissolved in nonaqueous media have been investigated and the data are compared to that seen for the same derivatives of the parent macrocycle lacking the fused quinoxaline ring. The investigated compounds are represented as (P)M and (PQ)M, where P = 5 ,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin, PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin and M = 2 H , Zn , Cu , Ni or Pd . The complexes all undergo two ring-centered reductions and one or two ring-centered oxidations in PhCN , CH 2 Cl 2, pyridine or THF . Additional redox reactions are also seen for the quinoxaline group in THF . The fusion of an electroactive quinoxaline group to the porphyrin macrocycle results in an 80 to 270 mV shift of E 1/2 towards easier reductions and the appearance of a third reduction which is assigned as a quinoxaline-centered redox process. The average HOMO-LUMO gap for the ( P ) M and ( PQ ) M derivatives is 2.26 ± 0.09 V and 2.14 ± 0.08 V , respectively. Both values are smaller than the average separation of 2.33 ± 0.13 V for the corresponding derivatives of tetraphenylporphyrin. The electrochemistry and UV-visible spectroelectrochemcal data indicate that moderate communication exists between the quinoxaline unit and the porphyrin π-ring system.

Electrochemistry of mono- and bis-porphyrins containing a β,β′-fused tetraazaanthracene group

“…The electrochemistry of (PTA)H 2 in non-aqueous media is similar in many respects to that which has been previously described for (PQ)H 2 under similar solution conditions [7,22]. By way of example, cyclic voltammograms for (P)H 2 , (PQ)H 2 and (PTA)H 2 in PhCN, 0.1 M TBAP are shown in Fig.…”

“…It has also been shown that electron distribution in the molecule can be altered by both metalation and through chemical substitution [14]. In the current study we have extended our previous electrochemical analyses of quinoxalinoporphyrins (PQ)M [7,10,22] by the fusing of a larger ring system to a tetraazaanthracene-linked porphyrin monomer (PTA)M, and we have investigated its electronic character conjointly with its p-conjugated bis-porphyrin parent.…”

“…In this regard a number of ring-extended porphyrins have been synthesized for use as molecular wires and switches [4][5][6][7][8][9][10][11][12][13][14]. The "simplest" ring-extended derivatives among these compounds are the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrins containing a b,b′-fused quinoxaline (Q) or tetraazaanthracene (TA) group.…”

“…It is also a perfect model compound for understanding what role the bridge plays in mediating electronic communication between porphyrins, which theoretical work has indicated is a critical point in determining whether molecules of this type are viable as molecular wires [28,29]. Extensive research has been undertaken in understanding the electronic character of the porphyrinbridge interaction in quinoxalinoporphyrins where delocalization of electron density upon electrochemical reduction has been demonstrated [7,22]. It has also been shown that electron distribution in the molecule can be altered by both metalation and through chemical substitution [14].…”

Electronic conduction and electrocatalysis by supramolecular tetraruthenated copper porphyrazine films

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Their main characteristics are associated with the π-conjugated ring which imparts interesting physical and chemical properties, including the facility of stacking and yielding molecular films. The possibility of rationally manipulating their properties at the nanoscale is a relevant issue to be pursued, aiming the generation of functional molecular materials for nanotechnology devices.…”