Comparison between SiMe₂and CMe₂Spacers as σ-Bridges for Photoinduced Charge Transfer

Abstract

The potential of dimethylsilylene and isopropylidene σ-spacers as bridges for photoinduced charge transfer (CT) in 4-cyano-4′-(dimethylamino)-and 4-cyano-4′-methoxy-substituted diphenyldimethylsilanes and 2,2-diphenylpropanes was studied. Fluorescence solvatochromism and time-resolved microwave conductivity measurements show that upon photoexcitation a charge separated state (D •+ σA •-)* is populated in all compounds. Excited state dipole moments for a given donor-acceptor combination are, irrespective of the bridge, equal. The CT states of the silanes are however lying at lower energies, implying that the presence of silicon thermodynamically facilitates the CT process. Cyclic voltammetry data of model compounds show that this is a consequence of the lowering of the acceptor reduction potential by the silicon bridge. It was however inferred from radiative decay rates that the electronic coupling between the CT and locally excited states as well as the coupling between the ground and CT state is larger for the carbon-bridged compounds. As shown by both solution and solid state electronic spectra and radiative decay rates, the photophysics of the DσA compounds are dominated by intensity borrowing of the CT transitions from transitions localized in the Dσ and σA chromophores.

Electron Delocalization in Cross‐Conjugated p‐Phenylenevinylidene Oligomers

“…Linear p-conjugated systems, of which polyacetylene, poly(p-phenylenevinylene)s, and polythiophenes are well-known examples, have by far received the most attention. However, alternative modes of conjugation, [2,3] such as s conjugation found in polysilanes, [4] s ± p conjugation in oligo(cyclohexylidene)s [5,6] and silylene-arylene polymers, [7,8] and homoconjugation in diphenylpropanes, diphenylsilanes, [9,10] and 7,7-diarylnorbornanes, [11] can also form the basis of interesting electronic features.…”

Calcium‐Catalyzed Friedel–Crafts Alkylation at Room Temperature

“…The transformation of a tertiary allylic alcohol was, to our knowledge, thus far unsuccessful. Arylation of tertiary benzylic [18] and tertiary propargylic [14] alcohols have not yet been realized under mild reaction conditions. A central drawback of Friedel-Crafts-type additions remains the low functional group tolerance, because even modern procedures generally suffer from the necessity of rather harsh reaction conditions.…”

Calcium‐katalysierte Friedel‐Crafts‐Alkylierung bei Raumtemperatur

“…[7,12, 14,15] Palladium- [16] und Molybdän-katalysierte [17] Reaktionen seien hier ausgenommen, da sie nach einem anderen Mechanismus verlaufen. Auch die Arylierung von tertiären Benzylalkoholen [18] und tertiären Propargylalkoholen [14] konnte noch nicht unter milden Reaktionsbedingungen realisiert werden. Eine intermolekulare Arylierung eines tertiären Allylalkohols ist unseres Wissens bislang ebenfalls nicht gelungen.…”