Fig.1 Optical properties of the PBI-4Cz pentad. (a) The chemical structure of PBI-4Cz and its optimized ground state and CT state configurations in 2-MeTHF solvent. (b) Diagram of energy levels and excited state pathways following photoexcitation. This diagram includes the singlet ground state (G), singlet excited state (S1) and triplet excited state (T1) of PBI, and the CT state. The states of Cz are not included for simplicity. The key photophysical processes are highlighted with gray arrows. (c) Steady state absorption spectra of PBI-4Cz in 2-MeTHF solvent (pink solid line) and PMMA matrix (blue dashed line) measured at room temperature, showing absorption bands associated with Cz, PBI and CT. The red shaded area is the CT band fitted to a Gaussian peak. The gray curve represents the residual spectrum after subtraction of the optimized CT band, which retains the characteristic vibronic structure of the PBI absorption. (d) Photoluminescence (PL) spectrum acquired in frozen 2-MeTHF (pink solid line, 80 K) and time-gated phosphorescence spectrum in PMMA (blue solid line, 300 K).#

为了明确激发态动力学过程，团队进行了变温飞秒瞬态吸收光谱测量。如图2所示，在2-甲基四氢呋喃溶剂中，500 nm光激发后，少于250飞秒内即完成了从咔唑到苝二酰亚胺的超快电荷分离，生成电荷转移态。随着温度从300 K降低到80 K，电荷转移态的衰减显著减慢，同时苝二酰亚胺三重态吸收信号的强度增强了约6倍，对应三重态产率从300 K下的10%提高到了80 K下的60%。为了验证构象锁定对产率的影响，团队将分子分散于聚甲基丙烯酸甲酯（PMMA）薄膜中。如图3所示，在PMMA固态基质中，分子的电荷重组动力学和三重态产率在80 K到300 K范围内几乎保持恒定，产率稳定在约45%。这表明约束分子的构象运动是实现高效三重态生成的关键。

Fig.2 TA spectra and dynamics of PBI-4Cz in 2-MeTHF solvent at different temperatures under 500 nm excitation. (a, b) TA spectra at indicated pump–probe delays measured at (a) 300 K and (b) 80 K. (c, d) Comparison of the (c) initial (at ∼1 ps) and (d) later (at ∼3 ns) TA spectra at different temperatures, which are dominated by the CT state and the PBI triplet state, respectively. (e, f) Temperature dependent (e) CT state recombination dynamics probed at around 670 nm (the PBI anion) and (f) triplet formation dynamics probed at around 532 nm.#

Fig.3 TA spectra and dynamics of PBI-4Cz in PMMA film at different temperatures under 500 nm excitation. (a, b) TA spectra at indicated pump–probe delays measured at (a) 300 K and (b) 80 K. (c, d) Comparison of the (c) initial (at ∼1 ps) and (d) later (at ∼3 ns) TA spectra at different temperatures, which are dominated by the CT state and the PBI triplet state, respectively. (e, f) Temperature dependent (e) CT state recombination dynamics probed at around 670 nm (the PBI anion) and (f) triplet formation dynamics probed at around 535 nm.#

为了定量厘清竞争机制，团队建立了高度约束的动力学模型并开展了全局拟合分析。如图4所示，实验光谱与拟合光谱高度吻合，准确提取出了电荷转移态向基态的单线态重组速率和向三重态的重组速率。如图5所示，在液体溶剂中，当温度从300 K降至80 K时，单线态重组速率降低了一个数量级以上，而三重态重组速率仅减小约2倍。这表明室温下的热激活分子运动会导致构象发生瞬时偏离，频繁出现单线态重组极快的偏平面构象，导致大量能量通过单线态无辐射衰减损失。而当溶剂冻结或处于PMMA基质中时，分子构象被锁定，单线态重组通道被大幅抑制，从而使三重态生成通道占据主导。

Fig.4 Global fitting analysis of the TA dynamics of PBI-4Cz in 2-MeTHF solvent at (a) 80 K, (b) 300 K and (c) in PMMA at 300 K. The first and second panels display experimental and simulated 2D pseudocolor plots of time-dependent TA spectra, respectively. The third panels show the globally extracted species-associated difference spectra corresponding to the CT species and the PBI triplet species. The lower panels show the global fitted dynamics for CT state recombination and triplet formation (colored lines) in comparison to normalized experimental curves (colored open circles). Insets in the lower panels present the kinetic model with fitted time constants indicated.#

Fig.5 Temperature dependent charge recombination rates and triplet yields. (a, b) Global fitting yielded rate constants k CR,S (blue circles) and k CR,T (pink circles) in (a) 2-MeTHF solvent and (b) in PMMA at varying temperatures. (c) Temperature dependent triplet yields in solvent (2-MeTHF) and in PMMA. Error bars indicate estimated uncertainties (see Supplementary Note 4 for details).#

基于PBI-4Cz兼具强CT吸收与高效三重态生成的独特优势，团队展示了其在光上转换中的应用。如图6所示，以PBI-4Cz为纯有机光敏剂，以TIPS-蒽为湮灭剂，构建了三重态-三重态湮灭光上转换系统。使用637 nm的红光激光器直接激发PBI-4Cz的CT吸收带，能够有效产生敏化剂三重态并转移至TIPS-蒽，进而通过湮灭过程发射出青色荧光。该系统实现了0.70 eV的大反斯托克斯位移，上转换功率阈值低至0.8 W cm⁻²，线性区上转换量子效率达到7.1%。相比传统系统，这种直接激发CT带的策略极大降低了敏化剂内部的能量损耗。

Fig.6 (a) Schematics of the TTA-UC system using PBI-4Cz sensitizers and TIPS-An (9,10-bis[(triisopropylsilyl)ethynyl]anthracene) annihilators. (b) Emission spectra of the mixture solution of PBI-4Cz and TIPS-An under excitation by a 637 nm cw laser at varying excitation power densities (colored lines). The anti-Stokes shift is indicated. Inset is a picture of TTA-UC emission in solution. (c) Integrated TTA-UC intensity as a function of the excitation power density (black circles). The TTA-UC threshold (∼0.8 W cm–2) and quantum yield in the linear regime (Φ′UC) are indicated.#