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International audience; New photochromic systems MY2X-HI (M = Si, Ge) were designed (at the DFT level of theory) by replacing the hydrogen atom in the nitrogen of the pyrrole ring of hemi-indigo, HI, with silyl- or germyl-substituents. The E isomers of MY2X-HI contain a dative bond O -> M (pentacoordinated atom M) both in the ground S-0 and in the excited S-1 states. This leads to a significant difference in the absorption and fluorescence maxima (lambda(fl) - lambda(abs)) of the Z and E forms of hypervalent hemi-indigos; this difference is more than twice that of the parent HI. There is a trend to increasing separation of first pi ->pi* absorption bands of the E and Z isomers of MY2X-HI with the increase in their relative stability. (C) 2021 Published by Elsevier B.V.

A linear donor-π-acceptor-π-donor (D-π-A-π-D) type 4-methoxyphenyl 1 and 4-ferrocenylphenyl 2 conjugated borasiloxanes were synthesized and characterized by various spectroscopic techniques (FT-IR, NMR and HRMS). Further, the molecular structure of 4-methoxyphenyl borasiloxane 1 was confirmed by the single-crystal X-ray diffraction method, shows a monoclinic crystal system in the centrosymmetric space group (P21/c) with various non-covalent interactions in the crystal packing. The intramolecular charge transfer (ICT) process of both the borasiloxanes were studied using the solvatochromic technique, which shows negative solvatochromism for 1 and 2, due to the high ground state dipole moment than the excited states. The second-order nonlinear optical (NLO) properties of the borasiloxanes were studied using Kurtz and Perry powder method. Compound 1 was crystallized in a centrosymmetric crystal system, and it exhibits second-order NLO response, because of the distorted silicon environment in an eight-membered borasiloxane ring with various C–H...π interactions in the crystal packing. The 4-ferrocenylphenyl borasiloxane 2 shows 1.5 times higher SHG efficiency than 1, due to the ferrocene moiety is conjugated in the para position of the borasiloxane ring, which involves an effective one-electron transfer process (FeII ↔ FeIII). Furthermore, the experimental results of optical and nonlinear optical properties of the borasiloxanes 1 and 2 were supported by the DFT and TD-DFT calculations using the B3LYP/6-31+G** level of theory.

Nine cycloplatinated(II) complexes (Pt1-Pt9) bearing 2-phenylpyridine (ppyH) derivatives and phenyl β-diketone (L1H = 1-phenyl-1,3-butanedione; L2H = 1,3-diphenyl-1,3-propanedione; L3H = 3-phenyl-2,4-pentanedione) ligands were designed and synthesized. All complexes except Pt2 exhibited reversible mechanoluminescence property. In addition, the complexes (Pt3, Pt6, and Pt9) containing L3 ligand also displayed concentration-dependent luminescence switching property in CH2Cl2 solution. With the concentration of CH2Cl2 solution of such complexes increasing, their luminescence changed from original cyan or cyan-green to the broad red luminescence with the emission redshifts exceeding 230 nm. Systematic studies on crystal structure, luminescence spectra and PXRD demonstrate that the structures of phenyl β-diketone ligands and the planarity of complexes seriously affect the luminescent switching properties of the complexes. The reversible mechanoluminescence property and the concentration-dependent luminescence switching property are most likely due to the change of the energy of the metal-to-ligand charge transfer (MLCT) transition and/or conversion of the lowest energy excited states from MLCT to metal-metal-to-ligand charge transfer (MMLCT) transitions caused by the formation of aromatic π-π stacking interaction and/or intermolecular Pt···Pt interaction.