Anilino-Substituted Multicyanobuta-1,3-diene Electron Acceptors: TICT Molecules with Accessible Conical Intersections.

A theoretical investigation based on DFT, TD-DFT, and CASSCF/CASPT2 methods has been carried out to elucidate the photophysics of two anilino-substituted pentacyano- and tetracyanobuta-1,3-dienes (PCBD and TCBD, respectively). These molecules exhibit exceptional electron-accepting properties, but their effective use in multicomponent systems for photoinduced electron transfer is limited because they undergo ultrafast (∼1 ps) radiationless deactivation. We show that the lowest-energy excited states of these molecules have a twisted intramolecular charge-transfer character and deactivate to the ground state through energetically accessible conical intersections (CIs). The topology of the lowest-energy CI, analyzed with a linear interpolation of the two branching-space vectors (g and h), indicates it is a sloped CI, ultimately responsible for the ultrafast deactivation of this class of compounds.

Cyanobuta-1,3-dienes as novel electron acceptors for photoactive multicomponent systems.

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.

Switching from separated to contact ion-pair binding modes with diastereomeric calix[4]pyrrole bis-phosphonate receptors.

We describe the design, synthesis and conformational assignment of three diasteromeric bis-phosphonate cavitands based on an aryl extended calix[4]pyrrole tetrol scaffold. The diastereoisomers differ in the relative spatial orientation of the P═O groups installed at their upper rims. We demonstrate that these compounds act as heteroditopic receptors for ion pairs forming ion-paired 1:1 complexes with alkylammonium (quaternary and primary) chloride salts in dichloromethane (DCM) solution and in the solid-state. (1)H NMR titrations indicate that the complexes are highly stable thermodynamically and kinetically. In the case of tetraalkyl-phosphonium/ammonium chloride guests, the host featuring the two P═O groups directed outwardly with respect to the aromatic cavity, 4oo, produces the most thermodynamically stable complexes. Conversely, for the primary alkyl ammonium chloride, the most effective receptor is the diastereoisomer 4ii with the two P═O groups converging on top of the aromatic cavity. In the nonpolar DCM solvent, the size of the quaternary cation has a strong impact in the thermodynamic stability of the complexes and their binding geometry. We use 2D-ROESY experiments to map out the binding geometries of the 1:1 complexes formed in solution. The 1:1 complexes of the 4oo host with the chloride salts have a separated arrangement of the bound ion-pair. In contrast, those of the 4ii host display a close-contact arrangement. We also investigate the same complexation processes in acetonitrile (ACN) solution. Both the salt and the initially formed anionic complex are fully dissociated in this more polar solvent. The receptors show an analogous trend in their binding affinities for quaternary phosphonium/ammonium chloride salts to the one seen in DCM solution. However, in ACN solution, the magnitudes of the binding affinities are reduced significantly and the size of the cation does not play a role. In addition, the inversion in the trend of relative binding affinities of the complexes, which was revealed in DCM solution, is eradicated in ACN when changing the cation substitution from quaternary to primary.

Host-guest-driven copolymerization of tetraphosphonate cavitands.

The outstanding complexing properties of tetraphosphonate cavitands towards N-methylpyridinium salts were exploited to realise a new class of linear and cyclic AABB supramolecular polymers through host-guest interactions. The effectiveness of the selected self-association processes was tested by (1)H NMR studies, whereas microcalorimetric analyses clarified the binding thermodynamics and revealed the possibility of tuning entropic contributions by acting on the flexibility of the guest linker. Although the formation of linear polymeric chains for a rigid system was demonstrated by X-ray analysis, the presence of a concentration-dependent ring-chain equilibrium was indicated by solution viscosity measurements in the case of a very flexible ditopic BB guest co-monomer.

Ion-pair complexation with a cavitand receptor.

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K(ass)) in the range of 10(3)-10(4) M(-1). Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H(down) protons converging in the lower pocket, thereby maximizing the CH-anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen-bonding-acceptor P=O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by (1)H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.

Hierarchical self-assembly on silicon.

A set of modular components was designed, synthesized, and combined to yield an innovative, robust, and reliable methodology for the self-assembly of large supramolecular structures on silicon wafers. Specific host-guest and H-bonding motifs were embedded in a single molecule by exploiting the remarkable complexing properties of tetraphosphonate cavitands toward methylammonium and methylpyridinium salts and the outstanding homo- and hetero-dimerization capability of the ureidopyrimidone moiety. An assembly/disassembly sequence in solution was devised to assess the orthogonality and reversibility of H-bonding and host-guest interactions. The entire process was fully tested and characterized in solution and then successfully transferred to the solid state. The selected binding motifs resulted to be fully compatible in the assembly mode and individually addressable in the disassembly mode. The complete orthogonality of the two interactions allows the molecular level control of each step of the solid-state assembly and the predictable response to precise external stimuli. Complementary surface analysis techniques, such as atomic force microscopy (AFM), ellipsometry, and fluorescence, provided the univocal characterization of the realized structures in the solid state.

Electrochemically controlled formation/dissociation of phosphonate-cavitand/methylpyridinium complexes.

The phosphorus-bridged cavitand 1 self-assembles very efficiently in CH2Cl2 with either the monopyridinium guest 2+ or the bispyridinium guest 3(2+). In the first case a 1:1 complex is obtained, whereas in the second case both 1:1 and 2:1 host-guest complexes are observed. The association between 1 and either one of the guests causes the quenching of the cavitand fluorescence; in the case of the adduct between 1 and 3(2+), the fluorescence of the latter is also quenched. Cavitand complexation is found to affect the reduction potential values of the electroactive guests. Voltammetric and spectroelectrochemical measurements show that upon one-electron reduction both guests are released from the cavity of 1. Owing to the chemical reversibility of such redox processes, the supramolecular complexes can be re-assembled upon removal of the extra electron from the guest. Systems of this kind are promising for the construction of switchable nanoscale devices and self-assembling supramolecular materials, the structure and properties of which can be reversibly controlled by electrochemical stimuli.