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Cao Huaiman, Hou Yuqi, Zhao Jianzhang. Application of continuous wave and pulsed lasers in triplet−triplet annihilation upconversion (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201068. doi: 10.3788/IRLA20201068
Citation: Cao Huaiman, Hou Yuqi, Zhao Jianzhang. Application of continuous wave and pulsed lasers in triplet−triplet annihilation upconversion (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201068. doi: 10.3788/IRLA20201068

Application of continuous wave and pulsed lasers in triplet−triplet annihilation upconversion (Invited)

doi: 10.3788/IRLA20201068
  • Received Date: 2020-09-10
  • Rev Recd Date: 2020-10-28
  • Available Online: 2021-01-14
  • Publish Date: 2020-12-25
  • The triplet−triplet annihilation upconversion is a new technology for photonic upconversion, which has the advantages such as continuous wave excitation, tunable upconversion wavelength and high upconversion quantum yields. In this upconversion process, as the energy donor, photosensitizer absorbs the light excitation, and intersystem crossing occurs, then sensitizes the energy acceptor through the triplet−triplet energy transfer process. Finally, the triplet−triplet annihilation of the energy acceptor in the triplet state generates a single excited state which can produce high−efficiency fluorescence (i.e. upconversion luminescence), thus the low−energy light is converted into higher−energy upconversion luminescence, which provides a feasible method for improving the photoelectric conversion efficiency of solar cells or the efficiency of photocatalysis, etc. It is desired to select appropriate lasers to excite the photosensitizer/energy donor system to study the steady upconversion luminescence and the upconversion kinetics. For instance, continuous wave diode pumped solid state laser (DPSSL) was selected as the light source to excite photosensitizer/acceptor system, upconversion luminescence was observed, and the effect of laser power density on upconversion luminescence can be studied conveniently. Additionally, in order to analyze the kinetic process of upconversion luminescence, with optical parametric oscillator (OPO) tunable pulsed laser as the light source, the lifetime of the triplet state, the kinetic characteristics of intermolecular energy transfer and triplet annihilation of photosensitizers can be studied. The application of continuous wave and pulse lasers in triplet–triplet annihilation upconversion experiments was introduced.
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Application of continuous wave and pulsed lasers in triplet−triplet annihilation upconversion (Invited)

doi: 10.3788/IRLA20201068
  • State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

Abstract: The triplet−triplet annihilation upconversion is a new technology for photonic upconversion, which has the advantages such as continuous wave excitation, tunable upconversion wavelength and high upconversion quantum yields. In this upconversion process, as the energy donor, photosensitizer absorbs the light excitation, and intersystem crossing occurs, then sensitizes the energy acceptor through the triplet−triplet energy transfer process. Finally, the triplet−triplet annihilation of the energy acceptor in the triplet state generates a single excited state which can produce high−efficiency fluorescence (i.e. upconversion luminescence), thus the low−energy light is converted into higher−energy upconversion luminescence, which provides a feasible method for improving the photoelectric conversion efficiency of solar cells or the efficiency of photocatalysis, etc. It is desired to select appropriate lasers to excite the photosensitizer/energy donor system to study the steady upconversion luminescence and the upconversion kinetics. For instance, continuous wave diode pumped solid state laser (DPSSL) was selected as the light source to excite photosensitizer/acceptor system, upconversion luminescence was observed, and the effect of laser power density on upconversion luminescence can be studied conveniently. Additionally, in order to analyze the kinetic process of upconversion luminescence, with optical parametric oscillator (OPO) tunable pulsed laser as the light source, the lifetime of the triplet state, the kinetic characteristics of intermolecular energy transfer and triplet annihilation of photosensitizers can be studied. The application of continuous wave and pulse lasers in triplet–triplet annihilation upconversion experiments was introduced.

    • 光子上转换技术是指将长波长的光转化为短波长光的技术。目前在太阳能电池等领域中,对太阳光的利用主要集中在可见光波长区域,对于长波长的光的利用并不充分,为了能够更加高效地利用太阳光等自然光源来进行光电转化或光催化化学反应,光子上转换技术逐渐得到了人们的关注[1-5]

      传统的光子上转换技术包括稀土上转换[6]以及双光子吸收[7]等,但是这些光子上转换技术存在上转换效率低、所需激光功率强的缺点。

      三重态−三重态湮灭上转换(Triplet−triplet annihilation upconversion, TTA UC)[8-12]是一种新的光子上转换技术,近年来逐渐得到了人们的重视。该上转换方法需使用三重态光敏剂和三重态受体两种化合物,具有使用连续波光源激发、上转换波长灵活可调及上转换量子效率高等优点。为了实现三重态光敏剂/三重态受体体系的上转换发光以及对其微观动力学过程进行研究,在实验中可以选用连续波二极管泵浦固体激光器(DPSSL)以及光学参量振荡器(OPO)可调谐纳秒脉冲激光器作为激光光源。连续波DPSS激光器具有光强可调、准直以及方便使用的优点[13-14],可以作为激发光源,研究上转换的稳态发光光谱。OPO可调谐脉冲激光器具有波长可调,波长准确度高、功率高的特点[15-16],可以作为激发光源研究TTA UC过程的瞬态延迟荧光光谱,并获得上转换荧光衰减动力学等信息,这在研究TTA UC的原理中具有重要的意义。

    • 三重态−三重态湮灭上转换体系中包括三重态光敏剂以及三重态受体。三重态光敏剂吸收长波长的光产生单重激发态(1PS*)布居,经系间窜越(intersystem crossing, ISC),产生三重激发态(3PS*),通过分子间三重态能量转移(triplet−triplet energy transfer, TTET),将能量转移给三重态受体,两个处于三重态的受体分子(3A*)经过碰撞发生三重态−三重态湮灭,最终形成受体的单重激发态(1A*)(根据自旋统计规律,此过程的几率是11%[17]),处于单重激发态的三重态受体弛豫回基态(ground state, GS)进而发出短波长的光(见图1)。相比于其他两种上转换方法,三重态−三重态湮灭上转换体系具有吸光能力强、激发和发射波长灵活可调、上转换效率高等优点。

      Figure 1.  Modified Jablonski diagram illustrating the mechanism of TTA upconversion and the related energy transfer process

      为了观测到三重态−三重态湮灭上转换发光,需要使用持续且稳定的单波长激光器进行激发。而二极管泵浦固体激光器具有多种可选择的波长,与上转换体系的匹配性好,且长期连续发射的激光可以保证上转换体系稳定发光[18],便于观测以及拍摄照片。

      对三重态−三重态湮灭上转换微观动力学特征的深入研究,以及对过程的分析,将有利于指导光敏剂的分子结构设计。使用光学参量振荡器(OPO)可调谐纳秒脉冲激光器对上转换体系进行激发,并通过示波器将信号数字化,可以得到上转换体系的纳秒瞬态延迟荧光光谱以及在特定波长检测的上转换荧光衰减动力学曲线。通过光谱分析,可以较为直观地观察到三重态−三重态能量转移的过程(微秒时间尺度),并通过三重态寿命的变化获得相关淬灭常数等信息。

    • 二极管泵浦全固态激光器兼具激光二极管以及传统固体激光器的优点,极大地提高了激光器的性能。连续波DPSS激光器以Nd: YAG为工作物质,泵浦头作为泵浦源,通过泵浦源、光耦合器和输出耦合器,可持续发出稳定波长的激光。侧面泵浦耦合示意图如图2所示。

      Figure 2.  Schematic diagram of side pump coupling

      笔者所在课题组使用连续波DPSS激光器(实验中的常用激光器波长:445、479、510、532、635 nm等)作为光源,以Bodipy类化合物作为三重态光敏剂(见图3),以苝为三重态受体,在氮气气氛下的溶液中实现了三重态−三重态湮灭上转换发光。

      Figure 3.  Molecular structures of typical triplet photosensitizers

      以三重态光敏剂BDP-PXZ-1/苝上转换体系[19]为例,实验选用MGL−III−510 nm−100 mW型的连续波DPSS激光器,可以长期稳定地输出功率为 0~400 mW/cm2的波长为510 nm的绿色激光(功率连续可调)。对于三重态光敏剂BDP-PXZ-1,其最大吸收波长位于504 nm,而其发光波长位于520~800 nm处,在除氧的甲苯溶液中吸收光子后发出黄色的荧光。对于能量受体苝,其最大吸收波长位于430 nm处,而其发光波长位于420~480 nm处,荧光呈蓝色。将光敏剂与受体在除氧的甲苯中混合,使用波长为510 nm的激光器激发BDP-PXZ-1/苝上转换体系,光敏剂BDP-PXZ-1吸收激光中的能量,到达单重激发态,其中一部分单重激发态产生电荷分离态并弛豫回到基态(ground state, GS),发出黄色荧光;而另一部分单重激发态则经过系间窜越过程到达光敏剂的三重激发态,继而经过分子间能量转移过程将能量传递给受体苝的三重态。当处于三重态的苝达到一定浓度时,处于三重激发态的两个受体经过分子间碰撞产生湮灭,形成苝的单重激发态,从而弛豫回到基态,发出蓝色荧光。

      由荧光发射光谱(见图4(a))可以得知,通过在BDP-PXZ-1/苝的上转换体系中选择性激发三重态光敏剂,最终观察到了苝的荧光峰,其主要发光峰位于420~480 nm范围处。

      Figure 4.  (a) Fluorescence emission spectra of the triplet photosensitizer BDP-PXZ-1/perylene upconversion system excited by a 510 nm cw DPSS laser and (b) CIE diagram of upconversion

      由上转换荧光的照片(见图5)以及相应的CIE色度图(见图4(b)),观察到了在510 nm连续波DPSS激光器的持续激发下形成的蓝白色的上转换发光。而通过滤光片滤掉光敏剂自身发出的荧光之后,可以看到由苝发出的蓝色荧光。

      Figure 5.  (a) Photographs of the fluorescence of BDP-PXZ-1 alone and the BDP-PXZ-1/perylene upconversion system in deaerated toluene excited by 510 nm continuous wave DPSS laser and (b) photographs with band-pass filter (transparent in the range 380-520 nm)

      在连续波DPSS激光器激发的上转换实验中,也可以研究激光器的激光功率对上转换发光强度和效率的影响(见图6)。以三重态光敏剂BDP-TEMPO-2/苝上转换体系为例[20],上转换发光强度与激光器激光功率密度之间呈现出一定的线性规律。在激光功率密度低于171 mW/cm2时,上转换发光强度与激光功率密度之间呈线性关系,斜率为1.78;而在激光功率密度高于171 mW/cm2时,二者之间的斜率为1.06。这说明在低功率辐照下的上转换体系的发光强度与激光功率密度之间近乎呈现二次关系,而随着激光功率密度的增大,其线性关系逐渐趋近于一次方。实验得到了此体系的能量阈值,在阈值前后激光功率密度对上转换发光的影响不同,且在激光功率密度处于阈值时,上转换效率最高[21]。此实验验证了上转换过程中存在的三重态−三重态湮灭过程。

      Figure 6.  (a) Upconverted fluorescence of BDP-TEMPO-2/perylene excited by different laser powers and (b) the emission intensity plotted as a function of laser power

      此外,使用532 nm连续波DPSS激光器持续辐照diiodo BDP/苝上转换体系系[22],可以获得随着激光照射时间的增加,上转换发光光谱的变化(见图7)。可知上转换发光在一定时间内的光谱变化很小,发光稳定。

      Figure 7.  Variety of the upconverted fluorescence intensity of diiodo BDP/perylene under different illumination time of 532 nm cw DPSS laser

      综上所述,由于连续波DPSS激光器具有可以长期连续稳定发射激光的优点,非常适合研究TTA上转换中的发光现象。并且连续波DPSSL在可见光区如红、绿、蓝、紫(670、660、656、627、594、532、520、510、473、457、451、430 nm等)处均已获得激光输出。在1~100 mW的功率范围内输出非常稳定,且使用寿命较长,可作为比较理想的光源,进行上转换实验。

    • 由Nd:YAG激光器输出三倍频脉冲355 nm的泵浦光,通过光学参量振荡器OPO可输出波长覆盖410~2 500 nm的宽带单波长脉冲。而闪光灯泵浦的Nd:YAG激光器的脉宽通常情况下为5~7 ns,适用于纳秒−微秒尺度范围内的瞬态光谱研究。

      光学参量振荡器以准相位匹配技术[23]为基础,在非线性介质中的差频过程中,每湮灭一个高频的光子就可以产生两个低频的光子。泵浦光与信号光同时入射非线性晶体后,每一个泵浦光光子都可以分为一个信号光光子与一个闲频光光子。泵浦光与信号光多次通过非线性晶体,可以得到经过多次放大的信号光。与其他几种诸如量子级联激光器[24]、自由电子激光器[25]以及稀土掺杂固体激光器[26-27]等相比具有体积紧凑、调谐范围宽、不受抽运光波长的限制等优点。

      激光闪光光解技术是应用OPO调谐过的短激光脉冲激发样品,研究激发产生的瞬态物种的一种光谱表征手段[28]。在文中,此项技术被应用于研究三重态−三重态湮灭上转换的动力学过程。

      以三重态光敏剂BDP-PXZ-1与苝的上转换体系为例,首先研究了氮气气氛下甲苯溶液中只含有光敏剂的溶液,其纳秒瞬态吸收光谱以及三重态寿命衰减曲线如图8所示。

      Figure 8.  (a) Nanosecond transient absorption spectra of BDP-PXZ-1 excited by a 495 nm OPO tunable pulse laser and (b) the decay curve of triple state at 504 nm

      由纳秒瞬态吸收光谱表明,光敏剂BDP-PXZ-1的基态漂白峰位于504 nm处,与紫外可见稳态吸收光谱的数据相吻合,其激发态吸收峰位于350~700 nm处,这是Bodipy三重态的特征信号,其拟合后的三重态真实寿命为539.0 μs,在氮气甲苯溶液中的三重态量子产率为54%[19]。同时监测了三重态光敏剂BDP-PXZ-1/苝上转换体系的TTA上转换延迟荧光。

      在上转换体系的延迟荧光光谱中(见图9),处于420~600 nm范围内的发光峰可归属为上转换延迟发光。通过监测445 nm处的荧光衰减曲线,可以明显地看到其衰减曲线由一个长达16.0 μs的上升段与71.9 μs的下降段构成。此上升段为三重态受体的三重激发态的激发形成过程(通过分子间三重态能量转移),而下降段则属于三重态−三重态湮灭产生的上转换延迟荧光的衰减过程,其寿命长达71.9 μs。利用OPO可调谐纳秒脉冲激光器激发得到的荧光光谱与荧光衰减曲线提供了上转换发光过程中的动力学数据。

      Figure 9.  (a) Delayed fluorescence spectra of TTA upconversion excited by 510 nm OPO tunable nanosecond pulse laser and (b) decay curves of fluorescence detected at 445 nm

      通过对445 nm处监测的荧光衰减曲线的分析计算可以得到光敏剂的上转换相关参数(见表1)。其中KSV为Stern−Volmer淬灭常数,Kq为双分子淬灭常数,Rq为淬灭剂的分子半径,Dq为能量受体的扩散系数,k0为扩散控制的双分子淬火速率常数,fQ为淬灭效率,Φuc为上转换量子效率。结合对此三重态光敏剂的稳态光谱的研究以及密度泛函理论计算结果,可以得到此三重态光敏剂/能量受体体系在受光激发时的光物理过程的近似雅布隆斯基示意图(见图10)。

      ParameterBDP-PXZ-1
      KSV/105 M−17.3
      Kq /109 M−1s−16.8
      Rq /10−10 m3.74
      Dq /10−6 cm2s−19.74
      k0 /1010 M−1s−11.24
      fQ 56.5%
      Φuc 12.3%

      Table 1.  Upconversion-related parameters of the photosensitizer

      Figure 10.  Modified Jablonski diagram illustrating the process of the triplet photosensitizer BDP-PXZ-1/perylene upconversion

      三重态光敏剂BDP-PXZ-1吸收激光器发出的510 nm激光到达单重激发态(1*Bodipy),通过电荷分离过程(charge separation, CS)产生电荷分离态(charge transfer state, CT state),经过系间窜越过程产生光敏剂的三重态(3*Bodipy);通过三重态−三重态能量转移过程将能量传递给苝的三重态(3*Perylene),然后两分子苝的三重态发生三重态−三重态湮灭过程形成高能级的苝的单重态(1*Perylene (hot)),经内转换过程形成苝的单重态(1*Perylene),弛豫到基态并发出蓝色荧光。

    • 文中分析了在进行三重态–三重态湮灭上转换实验时不同的激光器起到的作用:以连续波DPSS激光器激发上转换体系,由于激光器具有激光输出功率稳定,波长选择方便,功率可调的优点,是用作观察稳态上转换现象、研究不同功率对上转换体系的影响实验的最优选择;以OPO可调谐纳秒脉冲激光器激发上转换体系,此激光器具有波长易于调节,脉冲短等特点,适用于监测上转换过程的相关动力学性质。

      激光器在光谱学表征领域不仅适用于上转换实验,在其他实验中也可起到重要的作用。例如在测试物质的荧光寿命或者磷光寿命时,常用到皮秒脉冲二极管激光器;在测试物质的荧光光谱或者激光光谱时可使用半导体激光器作为光源。但在光谱学表征领域中使用的激光器仍有不足之处,如OPO调谐泵浦光获得的信号光在不同波长处的能量差异较大等。在光谱学研究领域,激光器是不可或缺的激发光源。激光器的革新与改进会带来光物理与光化学领域的进一步发展。

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