Acceptor-Type Singlet Fission Material Based on Strong Absorption Tetracyanothienoquinoid Skeleton

The practical efficiency of singlet fission(SF)-based photovoltaic devices is still far from satisfactory due to the limited scope of SF materials suitable for device application and the scarcity of schemes available for triplet utilization.Most SF materials identi-fied to date are typically electron donors while acceptor-type SF materials remain largely unexplored.Basically,the combination of a conventional electron donor and SF-active electron acceptor could circumvent the competitive energy transfer channel and better play the unique advantages of the SF process,which might be an adequate alternative for practical application.In this work,we presented a new acceptor-type SF material based on a tetracyanothienoquinoid skeleton.Such a quinoid skeleton exhibited strong absorption,ultrafast SF process,and excellent stability.Using transient spectroscopy and multireference calculations(XDWCASPT2),the SF dynamics were examined featuring the rapid generation and subsequent annihilation and/or partial dissociation of multiexciton states.Therefore,our results not only provide a robust acceptor-type SF material but also suggest an adequate donor–acceptor alternative for SF-based solar cells,which could pave the way for the practical application of such a potential process.

Improving Ultraviolet Stability of Perovskite Solar Cells via Singlet Fission Down-Conversion

The instability of perovskite materials under continuous ultraviolet(UV)light irradiation and high sensitivity in humid environments remain obstacles to future commercialization.Especially,the photovoltaic performance of perovskite solar cells(PVSCs)is prone to decline under UV light exposure for sustained periods of time.However,in conventional methods,preventing UV light from entering PVSCs usually comes at the expense of reducing short circuit photocurrent(Jsc).Herein,the UV stability of PVSCs is modified by in-troducing a singlet fission down-conversion layer 6,13-bis(triisopropylsilylethynyl)pentacene(TIPS-PEN)via one-step anti-solvent method without sacrificing device efficiency.The introduction of down conversion layer can not only improve the Jsc by converting UV light into multiple excitons,but also enhance the open-circuit voltage(Voc)owing to a better matched energy level alignment at the perovskite/spiro-OMeTAD interface.Consequently,the TIPS-PEN incorporated PVSCs attain the champion power conversion effi-ciency(PCE)up to 22.92%accompanied with dramatically increased UV photostability which can retain 80%of its primitive PCE un-der continuous UV light soaking for 150 h.Moreover,the unencapsulated PVSCs with TIPS-PEN exhibit remarkable moisture stability which can sustain over 80%of the initial value under air conditions(50%relative humidity,25℃)after 1000 h.

Tuning Intermolecular Singlet Fission Efficiencies of Perylenediimide Derivatives through Bay Aromatic Substitution

Singlet fission(SF)has potential applications in high-efficiency photo-energy harvesting applications,but its practical application is hindered by the limited number of materials.In this work,we explored the bay aromatic substitution strategy for the design of new perylenediimide(PDI)based SF materials.A series of PDI derivatives with biphenyl or naphthalene units substituted at the bay posi-tions were designed and synthesized to investigate the effects of aromatic substitutes on their photodynamic behaviours.The bay substitutions do not shift the energy level of the PDI core significantly but give rise to different intermolecular coupling strengths in the thin films and affect the intermolecular SF efficiency.Femtosecond transient absorption(fsTA)spectroscopy reveals that appro-priate spacing configuration from the bay aromatic substitution groups enhances the SF yields by promoting the interaction of neighbouring PDI cores.Triplet exciton yields of up to 183%have been obtained from these new PDI derivatives,making them po-tential candidates in future SF-based optoelectronics.

Catalyzing Singlet Fission by Transition Metals:Second versus Third Row Effects

The synthesis and characterization of platinum(II)and palladium(II)complexes bearing two(dimers Pt(L_(pc))_(2)Cl_(2)and Pd(L_(pc))_(2)Cl_(2)),one(monomers Pt(L_(pc))(L_(ref))Cl_(2)and Pd(L_(pc))(L_(ref))-Cl_(2)),or no(reference compounds Pt(L_(ref))_(2)Cl_(2)and Pd(L_(ref))_(2)Cl_(2))pentacene-based pyridyl ligands are presented.Photophysical properties of the dimers are probed by means of steady-state and time-resolved transient absorption measurements in compar-ison to the monomer and model compounds.Our results document that despite enhanced spin−orbit coupling from the presence of heavy atoms,intramolecular singlet fission(iSF)is not challenged by intersystem crossing.iSF thus yields correlated triplet pairs and even uncorrelated triplet excited states upon decoherence.Importantly,significant separation of the two pentacenyl groups facilitates decoupling of the two chromophores.Furthermore,the mechanism of iSF is altered depending on the respective metal center,that is,Pt(II)versus Pd(II).The dimer based on Pt(II),Pt(L_(pc))2Cl_(2),exhibits a direct pathway for the iSF and forms a correlated triplet pair with singlet−quintet spin-mixing within 10 ns in variable solvents.On the other hand,the dimer based on Pd(II),Pd(L_(pc))_(2)Cl_(2),leads to charge transfer mixing during the population of the correlated triplet pair that is dependent on solvent polarity.Moreover,Pd(L_(pc))_(2)Cl_(2)gives rise to a stable equilibrium between singlet and quintet correlated triplet pairs with lifetimes of up to 170 ns.Inherent differences in the size and polarizability,when contrasting platinum(II)with palladium(II),are the most likely rationale for the underlying trends.

Molecular stacking controlling coherent and incoherent singlet fission in polymorph rubrene single crystals Special Issue:Emerging Investigators

Singlet fission(SF)is an appealing process where one photoexcited singlet transforms to two triplets,which can overcome thermalization energy loss and improve solar cell efficiency.However,it remains unclear how intermolecular coupling,which is subject to molecular stacking,controls SF pathways and dynamics.Here,we prepared polymorph rubrene single crystals with different stacking geometries,including orthorhombic(Orth.),triclinic(Tri.),and monoclinic(Mono.)phases.By micro-area ultrafast spectroscopy,we find that Orth.and Tri.phases with closerπ-πstacking exhibit co-existing coherent and incoherent SF channels while loosely stacked Mono.phase shows only incoherent SF.Furthermore,incoherent SF is thermally activated in Orth.but barrierless in Mono.and Tri.phases.Quantum mechanical calculation reveals that different electronic coupling strength in different phases leads to different SF dynamics.This study demonstrates that molecular stacking governs SF dynamics through electronic coupling,providing guidance for designing efficient SF materials via crystal structural engineering.

Molecular Design Strategy for Practical Singlet Fission Materials:The Charm of Donor/Acceptor Decorated Quinoidal Structure

Singlet fission(SF)has attracted much attention on account of its great potential for applications in high efficiency solar energy conversion.The major roadblock to realize this potential is rooted in the limited availability of practical SF material with strong absorption,suitable triplet energy level,an efficient SF process,and good chemical stability.Quinoidal structures feature an innate diradical character,which endows these skeletons with SF potential yet results in some structural instability.

Efficient triplet pair separation from intramolecular singlet fission in dibenzopentalene derivatives

In intramolecular SF(iSF), the strong coupling nature and confinement of diffusional separation of ^1(TT) limits the extraction and harnessing of triplet energy. In order to investigate the possible ^1(TT) separation and the role of molecular parameters on it, a series of iSF-capable dibenzopentalene derivatives(DBPs) have been synthesized and their photoinduced dynamics are monitored. iSF takes place in DBPs, accompanied by consecutive ^1(TT) separation in polycrystalline film with almost ^100% yield. It is suggested the strong intermolecular coupling provided by the closely packing configuration in the film facilitates the disentanglement of correlated ^1(TT). Highly efficient triplet pair separation to yield free triplets makes one step forward for utilizing triplet energy from iSF materials for further optoelectronic applications.

单线态裂分中间态的理论研究进展

单线态裂分是指有机材料在光激发后,从一个单重态激子转变为两个三重态激子的光物理过程。该过程有望提高光电转换效率,因而受到了广泛的关注。研究发现单线态裂分中存在一个关键的中间态,而如何构建中间态的波函数则是重要的挑战。本文着重介绍了双激发态波函数构建的两类理论模型,而后讨论了振动、轨道和自旋相互作用对中间态形成和解离的影响。最后总结了进一步的理论研究将面临的挑战。

利用磁致发光曲线研究Rubrene器件中激子分裂和湮没过程

为了研究Rubrene分子中激发态的能量共振和分子间π-π共轭的特性对有机磁效应的影响,本文制备了基于不同浓度和厚度的Rubrene有机发光器件,并在不同温度下测量了器件的电致发光磁效应(magnetoelectroluminescence,MEL).实验发现,发光层中Rubrene的厚度和浓度均可以对器件中的MEL产生较大的影响,室温下MEL的高场值随Rubrene层厚度的增加而增加,并在30 nm之后逐步趋于饱和;随着Rubrene分子的浓度和测量温度的降低,MEL高场增加的幅度逐渐减小,甚至在低温时出现高场下降.通过对实验曲线进行数值拟合,认为Rubrene分子之间形成的π-π共轭结构有助于双分子相互作用的发生,单重态激子分裂、三重态激子之间的湮没和单一三重态极化子对的系间窜越三种过程在器件中相互竞争导致了所得MEL的变化.本工作有助于加深对有机光电子器件内部机理的认识.

激子的分裂与高效太阳能电池

能将单重态激子分裂为两个三重态激子的化合物,可作为一种新型的敏化剂,较大程度地提高太阳能电池的光-电转换能力.本文扼要地介绍了激子分裂的基本要点和发展状况,并对研究这一现象的实验方法和分裂机制等进行了讨论.在实验方法部分,叙述了通过稳态与瞬态延迟光谱的测定,可对体系中单重态的分裂和三重态重合,进而对激子的分裂进行研究,此外,还对激子的偶合、能级间的能量关系,以及环境效应等作了介绍.文章还对激子分裂过程的可能机制和如何选择这类敏化剂分子的一些基本准则,给予了讨论和说明.

有机发光激子的演化过程研究

在本研究工作中,采用耦合的微分方程组对具有激子裂变特性的红荧烯材料的瞬态发光衰减过程进行理论拟合。其中一组微分方程组来源于传统的两步三状态演化模型,另一组则来源于新确立的三步四状态演化模型。通过方程组的迭代运算可以反映材料中单重态激子裂变过程的动态变化。拟合结果显示,四状态模型对激子裂变过程的描述较之三状态模型更加合理,拟合曲线与实验曲线符合度更好。

红荧烯类似物C38H24S2的单线态分裂过程

单线态分裂可以规避光电转换中的能量损失,从而提高光电转换效率。利用飞秒时间分辨瞬态吸收光谱技术研究了在不同激发条件下溶于甲苯溶液的红荧烯类似物C38H24S2的激发态动力学过程。在550 nm和365 nm光激发下,激发单线态与基态分子发生碰撞,分别在8.2 ps和3 ps时产生了关联的三线态激子对。但在550 nm光激发下未观察到独立的三线态激子信号;而在365 nm光激发下,关联的三线态激子对的诱导吸收信号发生了转移,在探测波长550 nm附近观察到独立的三线态激子的吸收。在460 nm光激发下,利用奇异值分解的方法在瞬态吸收光谱中观察到了单线态分裂过程。

有机材料中激子裂变过程的演化模型

采用耦合的微分方程组对具有激子裂变特性的红荧烯材料的瞬态发光衰减过程进行理论拟合.其中一组微分方程组来源于传统的两步三状态演化模型,另一组则来源于新确立的三步四状态演化模型.通过方程组的迭代运算可以反映材料中单重态激子裂变过程的动态变化.拟合结果显示,四状态模型对激子裂变过程的描述较之三状态模型更加合理,拟合曲线与实验曲线符合度更好.另外,对曲线拟合所用的速率参数随温度的变化进行了指数拟合,进而得到红荧烯材料激子裂变过程的能量差值.拟合结果同样显示,相较于三状态模型,四状态模型拟合得到的能量差更接近准确值.

Aromaticity Concerto in Polycyclic Conjugated Hydrocarbons:Fusion Pattern on Combined Aromaticity Strategy Leads to Distinctive Excited State Photophysics of Dinaphthopentalenes

Understanding the structure-property relationships in polycyclic conjugated hydrocarbons(PCHs)is crucial in controlling their electronic properties and developing new optically functional materials.Aromaticity is a fundamentally important and intriguing property of numerous organic chemical structures and has stimulated a myriad of experimental and theoretical investigations.Exploiting aromaticity rules for the rational design of optoelectronic materials with the desired photophysical characteristics is a challenging yet fascinating task.Herein we present an in-depth computational and spectroscopic study on the structure-property relationships of dinaphthopentalenes(DNPs).Results highlight that the different fusion patterns between 4nπand 4n+2πunits endow these PCHs with the tunable aromaticity in the ground state/excited state,which leads to the diverse electronic structures and consequently the distinctive excited state photophysics.Accordingly,we propose a combined aromaticity design strategy for rationally modulating and tailoring electronic and optical properties of PCH skeletons.These outcomes not only present a full picture of the excited state dynamics of the DNP system and afford a new class of efficient singlet fission-active materials but also provide some basic guidelines for exploiting aromaticity rules to design and develop new optical function materials.

利用有机磁电导分析Rubrene发光器件中三重态激子解离和电子散射过程

为了研究红荧烯（5,6,11,12-tetraphenylnaphthacene,Rubrene）器件中三重态激子与电荷相互作用的微观过程,制备了基于Rubrene的有机发光二极管,并测量了室温下器件的磁电导（magneto-conductance,MC）.实验发现,器件MC曲线的幅值非常小且表现出了奇特的变化：即在0-8 m T的磁场范围内,MC随磁场快速增大;当磁场处于8-100 m T时,MC则表现为下降;但当磁场大于100 m T时,MC则表现为缓慢增加.分析发现,Rubrene器件中除了超精细相互作用外,还有空穴对三重态激子的解离作用和三重态激子对电子的散射作用的共存,且它们都受单重态激子分裂（singlet fission）的影响.利用Lorentzian和non-Lorentzian经验函数可以对MC进行较好拟合,进一步证实了上述观点.三重态激子解离和电子散射共存这一发现不仅有助于对Rubrene器件中电荷与激发态间相互作用机制的认识,在优化器件性能方面也有一定的指导意义.

红荧烯掺杂有机薄膜中的单重态激子裂变过程

在室温下测量了红荧烯掺杂有机薄膜光致发光的磁场效应.磁场效应的线型可以归结为外加磁场对掺杂的红荧烯分子间发生单重态激子裂变过程的调控作用.实验发现,光致发光磁场效应的幅度与红荧烯分子间的平均间距之间展现出非线性的对应关系.这种现象说明,当改变掺杂分子间的间距时,掺杂分子间的耦合强度变化可以对激子裂变过程的强度产生重要的影响.在理论上,磁效应幅度与分子间距之间的非线性关系可以采用朗道-齐纳的非绝热跃迁理论进行解释.而实验上,研究分子间耦合强度改变时激子裂变过程的变化,是研究激子裂变过程微观动力学的一种重要方法.本研究工作表明,对于利用单重态激子裂变敏化的有机光伏器件,分子间的耦合强度是一个需要考虑的重要因素.

Rubrene分子间距对有机发光二极管中单重态激子裂变过程的调控

利用5，6，11，12-tetraphenylnaphthacene（Rubrene）中单重态激子分裂的性质（一个单重态激子（S）可以分裂成两个三重态激子（T），S→T＋T），本文制备了Rubrene与Bathocuproine（BCP）共混型有机发光二极管（OLEDs），测量了器件电致发光的磁效应（Magneto-Electroluminescence，MEL）及其瞬态电致发光光谱．实验发现，随着Rubrene分子间距的增大（掺杂浓度减小），单重态激子分裂强度逐渐变弱，即Rubrene分子间距对内部激子分裂有调控作用．该调控作用表现在两个方面：第一，MEL的高场上升幅度（-500mT处对应的值）随着掺杂浓度减小而下降，当浓度为20wt％时高场出现下降；第二，在～20mT处MEL的值随着掺杂浓度的减小从负变到正．此外，器件瞬态电致发光光谱在撤去脉冲电压后的快速下降部分在小掺杂浓度时下降变快．通过分析单重态激子分裂与Rubrene分子间距之间的关系，以及磁场对单重态激子分裂的影响，我们对该现象做了定性的讨论．

有序体系与无序体系中单重态激子的裂变过程及其磁场效应

利用单重态激子的裂变机制,被认为是提高有机光伏器件量子效率的一种可行途径.然而,在非晶态薄膜中有机分子的取向是杂乱的,能否在排列杂乱的分子之间发生有效的激子裂变过程,是制约这一方法的重要问题.本实验中,在室温下测量了红荧烯掺杂有机薄膜光致发光的磁场效应与光致发光的瞬态衰减,由此计算出了薄膜中红荧烯分子间激子裂变速率的磁场效应.基于Merrifield的唯象理论,研究了薄膜中分子排列有序与排列无序两种情况下激子裂变速率磁场效应的差异.将实验数据与理论曲线相对比,显示出掺杂薄膜中的激子裂变过程主要发生在排列无序的红荧烯分子间.这从实验上证明了在非晶态有机固体中可以发生有效的激子裂变过程.而激子裂变过程的速率主要与材料中的载流子迁移率有关.

Dialectics of nature： Temporal and spatial regulation in material sciences

The cooperative interaction distance measure has been proposed as a novel law pertaining to dialectics of nature, and has been extensively carried out in the design of functional nanomaterials. However, the temporal and spatial dimensions are akin to yin and yang, and thus temporal regulation needs to be accounted for when implementing the above-mentioned principle. Here, we summarize recent advances in temporally and spatially regulated materials and devices. We showcase the temporal regulation of organic semiconductors for organic photovoltaics （OPVs） using the example of exciton lifetime manipulation. As an example of spatial regulation, we consider the distribution of charge carriers in core-shell quantum dot （QD） nanocrystals for modulating their optical properties. Long exciton lifetime can in principle increase the exciton diffussion length, which is desiable for high-efficiency large-area OPV devices. Spatially regulated QDs are highly valuable emitters for light-emitting applications. We aim to show that cooperative spatio-temporal regulation of nanomaterils is of vital importance to the development of functional devices.

Structure and Excitation Dynamics of β-Carotene Aggregates in Cetyltrimethylammonium Bromide Micelle

The β-carotene（β-Car） aggregate was prepared by self-assembly in cetyltrimethylammoniuln bromide （CTAB） micelle. The ground state absorption measurement showed that the aggregate has J-type characteristics and resonance Raman spectra gave the intrinsic explanation of molecular interaction in aggregate. Upon excitation at the optical allowed S2 state of aggregate, direct generation of triplet state via singlet fission（SF）mechanism was observed. Excitation dynamics was elucidated by fs-transient absorption spectroscopy and ns-flash photolysis, respectively. The triplet state life time of aggregate was found to be independent of the ambient oxygen molecules.