Sartin Matthew M, Shu Chingfong, Bard Allen J
Chemistry and Biochemistry Department, The University of Texas at Austin, Austin, Texas 78712, USA.
J Am Chem Soc. 2008 Apr 16;130(15):5354-60. doi: 10.1021/ja8000858. Epub 2008 Mar 13.
We report the electrogenerated chemiluminescence (ECL) of 2,2'-bis(10-phenylanthracen-9-yl)-9,9'-spirobifluorene (spiro-FPA), a dichromophoric molecule composed of two phenylanthracenes linked by a spirobifluorene moiety (PA-X-PA). The results are compared to those for 9,10-diphenylanthracene (DPA), a related molecule with a single chromophore. Cyclic voltammetry (CV) of spiro-FPA shows two reversible, closely spaced, one-electron transfers on both reduction and oxidation, occurring at E(o)(1,red) = -2.02, E(o)(2,red) = -2.07 V vs SCE and E(o)(1,ox) = 1.14, E(o)(2,ox) = 1.20 V vs SCE. The potentials for each pair are close enough to appear as a single peak in CV, indicating that the spirobifluorene moiety interrupts conjugation between the redox centers. The potentials observed are similar to those of DPA, which shows E(o)(red) = -2.06 V vs SCE and E(o)(ox) = 1.15 V vs SCE. The absorbance spectrum of spiro-FPA shows lambda(max,abs) = 377 nm, with 377 = 25,700 M(-1) s(-1), while DPA exhibited lambda(max,abs) = 374 nm, with 374 = 13,800 M(-1) s(-1), demonstrating that spiro-FPA has twice the available chromophores as DPA. Photoluminescence (PL) data for spiro-FPA shows lambda(max,PL) = 434 nm, with Phi(PL) = 0.74, while DPA fluoresces at 420 nm with Phi(PL) = 0.91; thus, there is greater solvent or structural relaxation in the spiro-FPA excited state, which may account for the greater internal conversion. Unlike DPA, the ECL spectrum of spiro-FPA exhibits long-wavelength emission not observed in the PL. We attribute this emission to excimers formed during annihilation ECL. Steric hindrance prevents DPA from forming excimers, even in ECL, but spiro-FPA annihilation can occur between pairs of di-ions (PA(-)-X-PA(-) and PA(+)-X-PA(+)), which are electrostatically more strongly attracted to one another than the mono-ions. This greater electrostatic attraction may be sufficient to overcome the steric hindrance to excimer formation. Lowering the electrolyte concentration decreases the electrostatic shielding of the ions from one another; thus, the increase in longer wavelength ECL accompanying a decrease in electrolyte concentration supports the role of the di-ions in excimer formation. Additionally, simulations show, consistent with experiment, a more rapid decrease in excimer concentration than in excited monomer concentration as a function of time after each potential pulse. This is probably due to the greater number of scavenging reactions available for di-ions. The simulations are confirmed experimentally when lower potential pulsing frequencies yield lower relative excimer emission. Since an excited state created by one-electron transfer between two di-ions should be rapidly quenched via electron transfer by the other PA moiety, the existence of excimers suggests the possibility of simultaneous, two-electron transfer to generate the excimer.
我们报道了2,2'-双(10-苯基蒽-9-基)-9,9'-螺二芴（螺芴菲，spiro-FPA）的电化学发光（ECL），它是一种由两个通过螺二芴部分相连的苯基蒽组成的双发色团分子（PA-X-PA）。将结果与9,10-二苯基蒽（DPA）进行比较（DPA是一种具有单个发色团的相关分子）。螺芴菲的循环伏安法（CV）显示在还原和氧化过程中都有两个可逆的、间隔很近的单电子转移，还原过程中发生在E(o)(1,red) = -2.02、E(o)(2,red) = -2.07 V（相对于饱和甘汞电极，SCE）处，氧化过程中发生在E(o)(1,ox) = 1.14、E(o)(2,ox) = 1.20 V（相对于SCE）处。每一对的电位足够接近以至于在循环伏安图中表现为一个单峰，这表明螺二芴部分中断了氧化还原中心之间的共轭。观察到的电位与DPA的电位相似，DPA的E(o)(red) = -2.06 V（相对于SCE），E(o)(ox) = 1.15 V（相对于SCE）。螺芴菲的吸收光谱显示λ(max,abs) = 377 nm，其吸光系数为377 = 25,700 M(-1) s(-1)，而DPA显示λ(max,abs) = 374 nm，其吸光系数为374 = 13,800 M(-1) s(-1)，这表明螺芴菲的发色团数量是DPA的两倍。螺芴菲的光致发光（PL）数据显示λ(max,PL) = 434 nm，其荧光量子产率（Phi(PL)） = 0.74，而DPA在420 nm处发荧光，Phi(PL) = 0.91；因此，在螺芴菲的激发态中有更大的溶剂或结构弛豫，这可能解释了更大的内转换。与DPA不同，螺芴菲的ECL光谱表现出在光致发光中未观察到的长波长发射。我们将这种发射归因于湮灭ECL过程中形成的激基缔合物。空间位阻阻止DPA形成激基缔合物，即使在ECL中也是如此，但螺芴菲的湮灭可以发生在双离子对（PA(-)-X-PA(-)和PA(+)-X-PA(+)）之间，双离子对之间的静电吸引力比单离子对之间更强。这种更大的静电吸引力可能足以克服形成激基缔合物时的空间位阻。降低电解质浓度会降低离子之间的静电屏蔽；因此，随着电解质浓度降低，更长波长的ECL增加，这支持了双离子在激基缔合物形成中的作用。此外，模拟结果与实验一致，显示在每个电位脉冲后，激基缔合物浓度随时间的下降比激发单体浓度下降更快。这可能是由于双离子有更多的清除反应。当较低的电位脉冲频率产生较低的相对激基缔合物发射时，模拟结果得到了实验证实。由于由两个双离子之间的单电子转移产生的激发态应该通过另一个PA部分的电子转移而迅速猝灭，激基缔合物的存在表明了同时进行双电子转移以产生激基缔合物的可能性。
