Synthesis and Photophysical Properties of 2-Donor-7-acceptor-9-silafluorenes: Remarkable Fluorescence Solvatochromism and Highly Efficient Fluorescence in Doped Polymer Films

Abstract

Recently, π-conjugated molecules containing a 9-silafluorene moiety have attracted considerable attention owing to their potential applications in organic electronics. While most studies focus on the oligomers and (co)polymers of 9-silafluorenes, functionalized 9-silafluorenes themselves remain unexplored. In this paper, we describe the preparation, photophysical properties, and theoretical calculations of 2-amino-7-acceptor-9-silafluorenes and we show that these silafluorenes can potentially be used as novel chromophores for functional organic materials. The D-π-A type silafluorenes were prepared by Pd-catalyzed intramolecular coupling of 2-(3-aminophenyldiisopropylsilyl)aryl triflates and the subsequent functional group conversion through reduction of the cyano group to a formyl and Knoevenagel condensation of the formyl group with malonitrile. The UV−visible absorption and fluorescence spectra of the D-π-A type silafluorenes exhibited a red-shift when the electron-withdrawing nature of the acceptor increased. The emission maxima of the fluorescence were highly dependent on the solvent. In the cases of formyl- and dicyanoethenyl-substituted silafluorenes, the emission colors ranged from blue to yellow for the formyl derivatives and from green to red for the dicyanoethenyl derivatives. The noticeable fluorescence solvatochromism suggests the intramolecular charge-transfer character of the excited states. The silafluorenes also exhibited fluorescence in the solid state (e.g., a neat thin film and a doped polymer film), and the emission color was dependent on the polarity of the polymer. In the solid state, the quantum yields of diphenylamino derivatives were generally higher than those of dimethylamino derivatives, presumably because the bulky diphenylamino group prevented chromophores from assembling close to each other. A comparison of the photophysical properties and theoretical calculations of D-π-A type silafluorenes with those of the corresponding fluorenes revealed that the silicon bridge contributed to the extension of the effective conjugation length of the biphenyl moiety when the acceptor was either hydrogen, trifluoromethyl, or a cyano group, whereas there was no contribution of the silicon bridge to the π-extension in silafluorenes substituted by strong electron acceptors such as formyl and dicyanoethenyl groups. White photoluminescence was demonstrated with an excellent quantum yield of 0.81 in the solid state from the poly(methyl methacrylate) (PMMA) film doped with Ph2N/CHO- and Ph2N/CH(CN)2-substituted silafluorenes. This work reveals the potential of D-π-A type silafluorenes as versatile organic emitting materials.

Photophysical properties of heteroaromatic ring-fused (di)benzosiloles

“…Since the structural characteristics of the two molecules are quite similar, only one of the two is shown, for clarity, in Figure 3. As reported for silafluorenes [22,37], the geometry of the silicon atoms of 5 is significantly deformed from that of the standard sp 3 -hybridized silicon atom; the bond angles of C(1)-Si(1)-C(9), C(1)-Si(1)-C(10), C(5)-Si(1)-C(9), C(5)-Si(1)-C(10), and C(9)-Si(1)-C(10) are widened to 113.1°, 113.6°, 115.4°, 111.6°, and 112.2°, respectively, while that of C(1)-Si(1)-C(5) in the fivemembered ring was substantially narrowed to 89.1°. The lengths of the carbon-silicon bonds range from 1.852 Å to isopropyl groups are oriented almost perpendicular to the molecular plane; the silylene bridge adopts an ideal conformation for efficient *-* conjugation.…”

Silicon‐substituted Xanthene Dyes and Their Unique Photophysical Properties for Fluorescent Probes

“…[8] Our group then developedS i-rhodamines ( Figure 2b)a sf ar-red to near-infraredf luorescent cores for fluorescentp robes, [9] and since then,w eand others have vigorously developed variousf ar-red to near-infrared fluorescent probesb ased on Si-rhodamines. [10] Other Si-containing fluorophores,such as 9-silafluorenes [11] and silepins, [12] were previously reported to emit strongf luorescencew ith ar ed-shift due to polarization by intramolecular charget ransfer.B ut, in the case of TMDHS and Si-rhodamines, the bathochromic shift of their absorption and fluorescencew avelengths is thought to be ac onsequence of their relatively low-lying LUMO energy levels,d ue to s*-p*c onjugation resulting from interaction of the s*o rbital of the two exocyclic bonds on the silicon atom with the p*s ystem of the xanthene moiety. [13] We furthera pplied the same silicon-substitution design strategy to aw idely utilized green fluorescent xanthenedye, fluorescein, and developed an ew red fluoresceina nalogue, To kyoMagenta (TM), in which the Oa tom at the 10 position of the xanthene moiety of fluorescein is replaced with Si ( Figure 2c).…”

Silicon-substituted xanthene dyes and their applications in bioimaging

“…1b). 12 Some silicon-substituted fluorophores, such as 9-silafluorenes 13 and silepins, 14 have been reported to emit strong fluorescence, showing a redshift due to polarization by intramolecular charge transfer. In the case of TMDHS and Si-rhodamines, the bathochromic shift of their absorption and emission wavelengths is probably a consequence of their relatively low-lying LUMO energy levels, due to σ*-π* conjugation resulting from interaction of the σ* orbital of the two exocyclic bonds on the silicon atom with the π* system of the xanthene moiety.…”