Alkene-Bridged Ionic Covalent Organic Nanosheets (iCONs) Based on D‑π-A for Photocatalytic Hydrogen Evolution

Hydrophilicity, dispersivity, and charge-carrier separation are major factors that vitally affect photocatalytic hydrogen evolution (PHE). However, there has been relatively little research on considering these factors simultaneously. In this work, a series of 2D sp2 carbon-linked ionic covalent organic nanosheets (iCON-(1–4)) based on the D-π-A structure are designed and synthesized. iCONs with an ionic nature and ultrathin layered structures greatly promote their hydrophilicity and dispersivity as photocatalysts in catalytic systems and offer numerous interfaces as reaction sites for electron–hole separation. Furthermore, the ultrathin nanosheets possess the advantage of being beneficial to Pt loading apart from facilitating the utilization of surface-active sites. Only 1.5 wt % Pt was added to achieve maximum hydrogen release. Subsequently, triphenylamine was employed as a strong donor and N+ can act as a strong acceptor, and iCON-4 synthesized by the strong–strong union method exhibits a hydrogen production efficiency of 9519 μmol g–1 h–1, which is the highest among iCON-(1–4). Interestingly, the extremely wide visible-light absorption range of iCON-(3–4) extends to about 700 nm and exceeds most photocatalytic semiconductor materials and provides a prerequisite for water decomposition. This work highlights a design concept for PHE in terms of hydrophilicity, dispersivity, charge-carrier separation, and band regulation. It is worth mentioning that iCONs are fabricated by the one-pot method and have not been employed to PHE before

Visible Absorption and Fluorescence Spectroscopy of Conformationally Constrained, Annulated BODIPY Dyes

Six conformationally restricted BODIPY dyes with fused carbocycles were synthesized to study the effect of conformational mobility on their visible electronic absorption and fluorescence properties. The symmetrically disubstituted compounds (<b>2</b>, <b>6</b>) have bathochromically shifted absorption and fluorescence spectral maxima compared to those of the respective asymmetrically monosubstituted dyes (<b>1</b>, <b>5</b>). Fusion of conjugation extending rings to the α,β-positions of the BODIPY core is an especially effective method for the construction of boron dipyrromethene dyes absorbing and emitting at longer wavelengths. The fluorescence quantum yields Φ of dyes <b>1</b>–<b>6</b> are high (0.7 ≤ Φ ≤ 1.0). The experimental results are backed up by quantum chemical calculations of the lowest electronic excitations in <b>1</b>, <b>2</b>, <b>5</b>, <b>6</b>, and corresponding dyes of related chemical structure but without conformational restriction. The effect of the molecular structure on the visible absorption and fluorescence emission properties of <b>1</b>–<b>6</b> has been examined as a function of solvent by means of the recent, generalized treatment of the solvent effect, proposed by Catalán (<i>J. Phys. Chem. B</i> <b>2009</b>, <i>113</i>, 5951–5960). Solvent polarizability is the primary factor responsible for the small solvent-dependent shifts of the visible absorption and fluorescence emission bands of these dyes