Cyanobuta‐1,3‐dienes as Novel Electron Acceptors for Photoactive Multicomponent Systems

Abstract

The synthesis, electrochemical, and photophysical properties of five multicomponent systems featuring a Zn(II) porphyrin (ZnP) linked to one or two anilino donor-substituted pentacyano- (PCBD) or tetracyanobuta-1,3-dienes (TCBD), with and without an interchromophoric bridging spacer (S), are reported: ZnP-S-PCBD (1), ZnP-S-TCBD (2), ZnP-TCBD (3), ZnP-(S-PCBD)2 (4), and ZnP-(S-TCBD)2 (5). By means of steady-state and time-resolved absorption and luminescence spectroscopy (RT and 77 K), photoinduced intramolecular energy and electron transfer processes are evidenced, upon excitation of the porphyrin unit. In systems equipped with the strongest acceptor PCBD and the spacer (1, 4), no evidence of electron transfer is found in toluene, suggesting ZnP→PCBD energy transfer, followed by ultrafast (<10 ps) intrinsic deactivation of the PCBD moiety. In the analogous systems with the weaker acceptor TCBD (2, 5), photoinduced electron transfer occurs in benzonitrile, generating a charge-separated (CS) state lasting 2.3 μs. Such a long lifetime, in light of the high Gibbs free energy for charge recombination (ΔG(CR)=-1.39 eV), suggests a back-electron transfer process occurring in the so-called Marcus inverted region. Notably, in system 3 lacking the interchromophoric spacer, photoinduced charge separation followed by charge recombination occur within 20 ps. This is a consequence of the close vicinity of the donor-acceptor partners and of a virtually activationless electron transfer process. These results indicate that the strongly electron-accepting cyanobuta-1,3-dienes might become promising alternatives to quinone-, perylenediimide-, and fullerene-derived acceptors in multicomponent modules featuring photoinduced electron transfer.

Linear Optical and Third‐Order Nonlinear Optical Properties of Some Fluorenyl‐ and Triarylamine‐Containing Tetracyanobutadiene Derivatives

“…In line with the scant luminescence data available for such compounds,, the TCBD derivatives 1–3 and 2′ were found to be totally nonemissive in solution at 25 °C (Table ). This behavior can be traced back to the fact that their lowest singlet excited state is presently a CT state mostly localized on the TCBD.…”

Systematic Variation of Cyanobuta‐1,3‐dienes and Expanded Tetracyanoquinodimethane Analogues as Electron Acceptors in Photoactive, Rigid Porphyrin Conjugates

“…Therefore, the resulting behavior is the sum of the behavior of the components. Considering that the same trend was observed in our previous multicomponent systems, [11] we conclude that the structural rigidification of the linker and modulation of the acceptor moieties in this work did not alter the independent redox behavior of the individual components.…”

“…[11] In the latter, we connected a zinc porphyrin, meso-substituted with 3,5-di-tert-butylphenyl groups to provide sufficient solubility, to a tetracyano-or pentacyanobuta-1,3-diene electron acceptor through a flexible linker. To obtain precise control over the electron donor-acceptor distance and orientation, we identified two possible new linker structures, namely a phenylScheme 1.…”

“…A first step towards this goal was their successful integration into a "molecular heterojunction", namely a zinc(II) porphyrin electron donor covalently linked to an anilino-substituted tetracyanobuta-1,3-diene electron acceptor. [11] This system underwent photoinduced electron transfer from the porphyrin donor to the multicyanobuta-1,3-diene acceptor, generating a long-lived charge-separated state with a lifetime of 2.3 μs in PhCN. To further explore the structural space of potentially photoactive push-pull chromophores and to examine the versatility of the CA-RE reaction in complex molecular systems, we prepared a homologous series of covalently linked multicomponent electron donoracceptor conjugates.…”

Porphyrin Donor and Tunable Push–Pull Acceptor Conjugates—Experimental Investigation of Marcus Theory

“…Our first foray into the field of photoinduced electron transfer was the successful linking of a Zn II –porphyrin electron donor to an anilino‐substituted alkyne activated towards the CA‐RE reaction allowing for the formation of a tetracyanobuta‐1,3‐diene electron acceptor . Notably, this system underwent photoinduced electron transfer to afford a long‐lived charge‐separated state of 2.3 μs in benzonitrile.…”