From Linear to T-Shaped Indan-1,3-dione Push-Pull Molecules: A Comparative Study

Impact of the Acceptor Group on the Properties of Triphenylamine-Donor-Acceptor Dyes: An Experimental and Computational Study

Three triphenylamine-Indane donor-acceptor dyes with different functional groups on the acceptor were studied to investigate how substitution would affect the optical properties. The dyes studied were IndCN, containing two malononitrile groups; InO, with two ketone groups; and InOCN, which features mixed functional groups. A combination of Raman spectroscopy, UV-vis absorption and emission spectroscopy, and density functional theory (DFT) calculations were employed for characterization. The dyes exhibited intramolecular charge transfer transitions with relatively high molar absorptivity (εabs) of ∼50 mM-1 cm-1 at 491-591 nm within the visible spectrum and emissions at 690-840 nm in dichloromethane. The malononitrile-containing dyes showed lower-energy absorption and emissions due to a reduced band gap compared to ketone-containing dyes. The bulkiness of the malononitrile group led to a bent geometry, increasing nonradiative decay and reducing the fluorescence quantum yield. The dyes exhibited fluorescence quantum yields less than 0.25 and lifetimes of ∼5 ns. Resonance Raman spectra and DFT calculations showed that the longer linker group (propen-1-ylidene linker) in these systems reduced the charge-transfer character of the optical transition. The emission intensities of the three dyes were temperature-sensitive, with ketone-containing dyes showing shifts in emission bands as well. This could be due to molecular stacking and intermolecular π-interactions.

ThDione: A Powerful Electron-Withdrawing Moiety for Push-Pull Molecules

A series of new push-pull chromophores based on a combined cyclopenta[c]thiophene-4,6-dione (ThDione) acceptor, N,N-dimethylaniline, N-piperidinylthiophene or ferrocene donors, and ethylene or buta-1,3-dienylene π-linkers has been designed and synthesized. Utilizing one or two ThDione acceptors afforded linear or branched push-pull molecules. Experimental and theoretical study of their fundamental properties revealed thermal robustness up to 260 °C, a electrochemical/optical HOMO-LUMO gap that is tunable within the range of 1.47-2.19/1.99-2.39 eV, and thorough elucidation of structure-property relationships. Compared to currently available portfolio of heterocyclic electron-withdrawing units, ThDione proved to be a powerful and versatile acceptor unit. It imparts significant intramolecular charge transfer and polarizes the π-system, which results in enhanced (non)linear optical response.

Crystal structures, syntheses, and spectroscopic and electrochemical measurements of two push-pull chromophores: 2-[4-(di-methyl-amino)-benzyl-idene]-1H-indene-1,3(2H)-dione and (E)-2-{3-[4-(di-meth-ylamino)-phen-yl]allyl-idene}-1H-indene-1,3(2H)-dione

The title pull-push chromophores, 2-[4-(di-methyl-amino)-benzyl-idene]-1H-indene-1,3(2H)-dione, C18H15NO2 (ID[1]) and (E)-2-{3-[4-(di-methyl-amino)-phen-yl]allyl-idene}-1H-indene-1,3(2H)-dione, C20H17NO2 (ID[2]), have donor-π-bridge-acceptor structures. The mol-ecule with the short π-bridge, ID[1], is almost planar while for the mol-ecule with a longer bridge, ID[2], is less planar. The benzene ring is inclined to the mean plane of the 2,3-di-hydro-1H-indene unit by 3.19 (4)° in ID[1] and 13.06 (8)° in ID[2]. The structures of three polymorphs of compound ID[1] have been reported: the α-polymorph [space group P21/c; Magomedova & Zvonkova (1978 ▸). Kristallografiya, 23, 281-288], the β-polymorph [space group P21/c; Magomedova & Zvonkova (1980 ▸). Kristallografiya, 25 1183-1187] and the γ-polymorph [space group Pna21; Magomedova, Neigauz, Zvonkova & Novakovskaya (1980 ▸). Kristallografiya, 25, 400-402]. The mol-ecular packing in ID[1] studied here is centrosymmetric (space group P21/c) and corresponds to the β-polymorph structure. The mol-ecular packing in ID[2] is non-centrosymmetric (space group P21), which suggests potential NLO properties for this crystalline material. In both compounds, there is short intra-molecular C-H⋯O contact present, enclosing an S(7) ring motif. In the crystal of ID[1], mol-ecules are linked by C-H⋯O hydrogen bonds and C-H⋯π inter-actions, forming layers parallel to the bc plane. In the crystal of ID[2], mol-ecules are liked by C-H⋯O hydrogen bonds to form 21 helices propagating along the b-axis direction. The mol-ecules in the helix are linked by offset π-π inter-actions with, for example, a centroid-centroid distance of 3.9664 (13) Å (= b axis) separating the indene rings, and an offset of 1.869 Å. Spectroscopic and electrochemical measurements show the ability of these compounds to easily transfer electrons through the π-conjugated chain.

Push-Pull Chromophores Based on the Naphthalene Scaffold: Potential Candidates for Optoelectronic Applications

A series of ten push-pull chromophores comprising 1H-cyclopenta[b]naphthalene-1,3(2H)-dione as the electron-withdrawing group have been designed, synthesized, and characterized by UV-visible absorption and fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. The solvatochromic behavior of the different dyes has been examined in 23 solvents and a positive solvatochromism has been found for all dyes using the Kamlet-Taft solvatochromic relationship, demonstrating the polar form to be stabilized in polar solvents. To establish the interest of this polyaromatic electron acceptor only synthesizable in a multistep procedure, a comparison with the analog series based on the benchmark indane-1,3-dione (1H-indene-1,3(2H)-dione) has been done. A significant red-shift of the intramolecular charge transfer band has been found for all dyes, at a comparable electron-donating group. Parallel to the examination of the photophysical properties of the different chromophores, a major improvement of the synthetic procedure giving access to 1H-cyclopenta[b]naphthalene-1,3(2H)-dione has been achieved.