Three Redox States of a Diradical Acceptor–Donor–Acceptor Triad: Gating the Magnetic Coupling and the Electron Delocalization

Abstract

The diradical acceptor-donor-acceptor triad 1 ·· , based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1 ·-, and triradical cation species, 1 ···+ , obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of 2 electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations. TOC GRAPHICSIn the last few years, there is an increasing interest in the design and synthesis of novel organic materials exhibiting intramolecular electron transfer (IET) phenomena. Challenging are dyadic systems formed by donor (D) and acceptor (A) units connected by conjugated bridges due to their potential applications as molecular wires or switches. [1][2][3][4][5][6][7][8] Organic mixed-valence (MV) compounds containing at least two redox sites with two different oxidation states connected through a conjugated bridge are good examples for this kind of compounds. [9][10][11][12][13][14] On the other hand, research on organic molecules containing several stable radical centres linked through ferromagnetic coupling bridges have also received much attention during the past decades. [15][16][17][18][19][20][21][22] In fact, there are recent examples of diradical compounds that can switch the magnetic interaction between the radical centres by changing the chemical nature of the bridge. 19,20 Thus, switching the magnetic response of organic molecular materials through the application of an external stimulus is an interesting phenomenon since they mimic elemental electronic operations.In our group, we have recently reported an organic radical D-A dyad, based on the electron π-donor tetrathiafulvalene (TTF) connected to the electron acceptor 3 polychlorotriphenylmethyl (PTM) radical, that exhibits bistability in solution through the application of external stimuli such as the polarity of the solvent or temperature. Figure S1). DFT calculations show that E and Z isomers of 4 lie extremely close in energy (ca. 0.1 kcal/mol). In addition the computational analysis suggests that the cis configuration of the vinylene units is ca. 2 kcal/mol disfavoured with respect to the trans one (see Table S4). Finally, subsequent deprotonation of 4 with two equivalents of TBAOH and oxidation of corresponding dicarbanions using silver nitrate yield diradical 1 ·· (E + Z) in 71%, as a brownish powder.5 Scheme 1. Three-step synthesis of 1 ··. TBAOH = tetrabutylammonium hydroxide. showing an intense band at 385 nm, characteristic of PTM radical chromophores. The shoulders around 420 and 540 nm are attributed to the electronic conjugation of the unpaired electron into the π-framework. Regarding the low-energy region, the weak broad band that appears around 900 nm is assigned to the intramolecular charge transfer process from the e...

TTF–PTM dyads: from switched molecular self assembly in solution to radical conductors in solid state

“…In view of the interesting electronic properties of some organic acceptor-donor-acceptor (A-D-A) triads, [57][58][59] we have also studied the switching properties of a A-D-A diradical triad based on two PTM radicals connected through a TTF-vinylene bridge (2). 60 This molecule exhibited an electrochemical reversible switching by one-electron reduction and oxidation modifying its optical, magnetic and electronic properties (Fig. 16).…”

Design of Perchlorotriphenylmethyl (PTM) Radical‐Based Compounds for Optoelectronic Applications: The Role of Orbital Delocalization

“…In this context, quantum‐chemical calculations can prove very useful to rationalize the absorption spectra of PTM‐based derivatives. Over the recent years, Time‐Dependent Density Functional Theory (TD‐DFT) has been largely exploited to analyze the optical properties of medium‐size systems,, such as PTM compounds ,,. Approaches based on Coupled Cluster (CC) or Complete Active Space (CAS‐SCF) calculations have not been tested due to their large prohibitive computational cost.…”

Tetrathiafulvalene–Polychlorotriphenylmethyl Dyads: Influence of Bridge and Open‐Shell Characteristics on Linear and Nonlinear Optical Properties

“…[28] In view of the donor and acceptor capabilities of both TTF and radical PTM molecules, it is of high interest to combine both units in a D-π-A molecule and theoretically evaluate its NLO response. Recently, we have reported different D-π-A systems based on a PTM radical electron-acceptor linked to a TTF electron-donor through conjugated vinylene bridges that exhibit a reversible switching between a neutral and a zwitterionic state in solution through the application of external stimuli [29][30][31][32] or conductivity in solid state. [33][34][35] In order to study the optical properties of this kind of radical dyads and evaluate the influence of the open-shell character and the length of the bridge, we have synthesized and characterized a family of TTF-π-PTM radical derivatives (1a-1c) and their non-radical analogues (2a-2c) (Scheme 1).…”

“…Recently, we have reported other D‐π‐A systems based on a PTM radical electron acceptor linked to a TTF electron donor through conjugated vinylene bridges. These D‐π‐A systems exhibit either reversible switching between neutral and zwitterionic states in solution upon the application of external stimuli, or conductivity in solid state . To study the optical properties of these radical dyads and to evaluate the influence of the open‐shell character and the length of the vinylene bridge, we have synthesized and characterized a family of TTF‐π‐PTM radical derivatives ( 1 a – 1 c ) and their non‐radical analogues ( 2 a – 2 c ) (Scheme ).…”