Electron-transporting naphthalimide-substituted derivatives of fluorene

• Naphthalimide-substituted fluorenes exhibit high thermal stabilities.
• They form glasses with glass transition temperatures ranging from 30 to 76 °C.
• Fluorescence quantum efficiencies of the dilute solutions up to 0.84.
• Solvatochromic behaviour in polar solvents.
• Electron mobilities appproach 1.2 × 10−3 cm2 V−1 s−1 at an eletric field of 6.4 × 105 V cm−1.

Naphthalimide-substituted derivatives of fluorene were synthesized by Suzuki–Miyaura coupling reactions. Most of the synthesized materials were found to constitute glasses with glass transition temperatures ranging from 30 to 76 °C as characterized by differential scanning calorimetry. Their initial weight loss temperatures range from 315 to 466 °C. Dilute solutions of the naphthalimide-substituted derivatives of fluorene in nonpolar solvents were found to emit in the blue region with high quantum yield ranging from 0.47 to 0.69, while in the solid state fluorescence quantum yields were found to be in the range of 0.06–0.25. Fluorenes possessing two naphthalimide moieties exhibited higher values of the fluorescence quantum yield due to enhanced oscillator strength of the transition. The derivatives of fluorene containing naphthalimide moieties exhibited pronounced positive solvatochromic behaviour in polar solvents indicating on charge-transfer character of the lowest excited states. Fluorescence red shift in dimethylsulfoxide is found of 94 nm, while the fluorescence quantum yield is increasing with polarity from 0.47 in cyclohexane to 0.84 in dimethylsulfoxide. Such untypical enhancement of fluorescence quantum yield is accounted to reduced intersystem crossing in polar solvents, which is proved by almost 10 fold increase in the values of nonradiative decay rate for compounds dissolved in polar solvents. Time-of-flight electron drift mobilities of the layer of 2,7-di((N-octyl-1,8-naphthalimide)-4-yl)-9,9-dioctyl-9H-fluorene in air at 25 °C appproach 1.2 × 10−3 cm2 V−1 s−1 at an eletric field of 6.4 × 105 Vcm−1.

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