Barbon Antonio, Bott Eric D, Brustolon Marina, Fabris Marianna, Kahr Bart, Kaminsky Werner, Reid Philip J, Wong Susanna M, Wustholz Kristin L, Zanré Roberto
Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy.
J Am Chem Soc. 2009 Aug 19;131(32):11548-57. doi: 10.1021/ja903284y.
Single-molecule dark states are often attributed to photoexcited triplets with scant evidence of the participation of paramagnetic molecules. The photodynamics of blinking single molecules of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in crystals of potassium hydrogen phthalate (KAP) were compared with the lifetimes of DCM triplet states, likewise in KAP, whose zero-field splitting (ZFS) tensors were fully characterized by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. Luminescent mixed crystals of KAP were grown from solutions containing 10(-4) -10(-9) M DCM, a model optically nonlinear chromophore. The luminescent dye was localized in the {111} crystalline growth sectors. The photoexcited triplets states of DCM in the heavily dyed (10(-4) M) crystals were analyzed by TR-EPR spectroscopy. The photoexcited singlet states of DCM in lightly dyed crystals (10(-9) M) were analyzed by single-molecule microscopy. Large blue shifts in the absorption and emission spectra of DCM in KAP were interpreted as a consequence of protonation at the dimethylamino nitrogen atom, an assignment supported by calculations of the zero-field splitting (ZFS) tensors of molecules in their triplet states. Experimental ZFS tensors with eigenvalues comparable to those of the computed tensors were determined from the angular dependence of the EPR spectra of DCMH(+) triplets within KAP single crystals with respect to the applied magnetic field. Data from individual growth sectors failed to show magnetically equivalent site occupancies, evidence of the kinetic ordering during growth. The intermittent fluorescence of individual chromophores was analyzed. The distributions of on(off) times were characterized by distributed rates fit to power laws. The lifetime of the triplet states was analyzed from the time decay of the EPR signals between 100 and 165 K. The data were well fit with a single time constant for the signal decay, a result wholly inconsistent with the blinking of single molecules with off times commonly of tens of seconds. Triplet decay was extrapolated to approximately 25 micros at room temperature. Therefore, the assumption that single-molecule dark states originate with triplet excited states is not sustainable for single DCM molecules in KAP.
单分子暗态通常被归因于光激发三重态，而顺磁分子参与其中的证据却很少。将邻苯二甲酸氢钾（KAP）晶体中4-二氰基亚甲基-2-甲基-6-对二甲氨基苯乙烯基-4H-吡喃（DCM）单分子的光动力学与同样在KAP中的DCM三重态寿命进行了比较，其零场分裂（ZFS）张量通过时间分辨电子顺磁共振（TR-EPR）光谱进行了全面表征。KAP的发光混合晶体是从含有10⁻⁴ - 10⁻⁹ M DCM（一种光学非线性发色团模型）的溶液中生长出来的。发光染料位于{111}晶体生长扇区中。通过TR-EPR光谱分析了重掺杂（10⁻⁴ M）晶体中DCM的光激发三重态。通过单分子显微镜分析了轻掺杂晶体（10⁻⁹ M）中DCM的光激发单重态。KAP中DCM吸收和发射光谱的大幅蓝移被解释为二甲基氨基氮原子质子化的结果，这一归属得到了三重态分子零场分裂（ZFS）张量计算的支持。通过KAP单晶中DCMH⁺三重态的EPR光谱相对于外加磁场的角度依赖性，确定了与计算张量特征值相当的实验ZFS张量。来自各个生长扇区的数据未能显示出磁等价的位点占有率，这是生长过程中动力学有序性的证据。分析了单个发色团的间歇性荧光。开（关）时间的分布通过拟合幂律的分布速率来表征。从100到165 K之间EPR信号的时间衰减分析了三重态的寿命。数据与信号衰减的单个时间常数拟合良好，这一结果与关时间通常为数十秒的单分子闪烁完全不一致。三重态衰外在室温下外推至约25微秒。因此，对于KAP中的单个DCM分子，单分子暗态起源于三重态激发态的假设是不可持续的。
