Theoretical Study on the Correlation between Band Gap, Bandwidth, and Oscillator Strength in Fluorene-Based Donor−Acceptor Conjugated Copolymers

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• 作者：Ying-Chieh Hung ; Jyh-Chiang Jiang ; Chi-Yang Chao ; Wei-Fang Su ; Shiang-Tai Lin
• 刊名：Journal of Physical Chemistry B
• 出版年：2009
• 出版时间：June 18, 2009
• 年：2009
• 卷：113
• 期：24
• 页码：8268-8277
• 全文大小：334K
• 年卷期：v.113,no.24(June 18, 2009)
• ISSN：1520-5207

文摘

There is a growing interest in developing low-band gap conjugated polymers via synthesis of copolymers containing alternating units of different π-electron-donating/accepting capabilities. In this study, electronic and optical properties of conjugated copolymers containing fluorene and thiophene/cyclopentadithiophene derivatives are determined using density function theory and semiempirical ZINDO calculations. A remarkable linear correlation is found between the amount of charge transfer between the donor−acceptor pair, the band gap, the bandwidth, and the oscillator strength of S₀→S₁ electronic transition (ground state to first excited state) of the copolymers. Strong π-electron withdrawing substituents, such as dicyanoethenyl and carbonyl groups, on the thiophene moiety effectively reduce the band gap of the copolymers. However, the reduction of band gap is frequently accompanied by a linear reduction in bandwidths and in the oscillator strength of S₀→S₁ transition. For very strong π-electron withdrawing thiophene derivatives, the occurrence of maximum oscillator strength may even shift from S₀→S₁ to S₀→S_n>1 (ground state to a higher excited state), giving a blue shift in maxima absorption peak and a red shoulder in the UV−vis spectra as reported in recent experimental measurements. Therefore, the achievement of low band gap for conjugated polymers with alternating arrangement of π-electron-donating/accepting moieties may be achieved at a cost of lowering electron mobility and optical efficiency and sometimes a blue-shift in the major optical (UV−vis) absorption.