EPR spectroscopy and photophysics of the lowest photoactivated triplet state of a series of highly conjugated (porphinato)Zn arrays
EPR spectroscopy and photophysics of the lowest photoactivated triplet state of a series of highly conjugated (porphinato)Zn arrays
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DOI：
10.1021/ja00155a015
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发表时间：
1995-12-20
影响因子：
15
通讯作者：
Therien, MJ
中科院分区：
文献类型：
--
作者：
Angiolillo, PJ;Lin, VSY;Therien, MJ
通过电子顺磁共振（EPR）、瞬态三重态-三重态吸收和磷光发射研究了高度共轭（卟啉）锌（II）阵列家族的最低光激发亚稳态三重态，其中乙炔或丁二炔单元桥接大环碳骨架，以及其乙炔和丁二炔精心制作的单体卟啉结构单元以及母体（卟啉）锌（II）前体分子。激发的单体前体以及二聚体和三聚体卟啉阵列的所有 EPR 光谱都反映了 4 K 时菱形畸变的对称性（\D\ 大于或等于 3\E\），并表现出电子自旋极化模式，验证了从第一激发单重态的系间跨越主要通过 \T-z> 自旋态子能级发生。击败乙炔基和丁二炔基的（卟啉）锌单体以及共轭卟啉阵列的零场分裂（ZFS）参数在 4-100 K 温度范围内基本上与温度无关。这种行为与大多数单体闭壳金属卟啉三重态中常见的线形状变化形成对比，这表明最低激发三重态具有低于 D-4h 的对称性，因此不易受到 Jahn-Teller 不稳定性的影响。除内消旋至内消旋丁二炔连接的二聚体外，所有化合物的 D 值大小都在单体（卟啉）锌配合物的预期范围内，这表明（1）三重态激发不存在全局离域，（2）在 EPR 探测的时间尺度上，激发可以被认为定位在共轭超分子发色系统中的单体亚基之一上。发色团到发色团连接的类型和模式确实影响共轭（卟啉）锌阵列的低温光激活三重态中的自旋分布和自旋排列。例如，在内消旋至内消旋丁二炔连接的二聚体中，相对于其单体前体，\D\值实际上增加了约30%；这种异常被解释为分子平面中自旋排列的结果。最显着的特征是，在稳态照明条件下，直到接近玻璃化转变温度（T 大约 120 K）的温度下，大多数二聚体和三聚体卟啉阵列中电子自旋极化的持续存在。在一种二聚卟啉系统上进行的渐进功率饱和实验，其中乙炔部分在各自的中间碳位置桥接两个（卟啉）锌发色团，揭示了对饱和参数 P-1/2 的温度依赖性异常弱，并且随着微波功率或温度的增加，ZFS 参数或线宽没有显着变化，表明自旋晶格弛豫相对于在较宽的温度范围内到基态的去激发较长。从主要通过基质无序模式发生的弛豫的角度讨论了这种异常效应。
The lowest photoexcited metastable triplet state of a family of highly conjugated (porphinato)zinc(II) arrays in which ethyne or butadiyne units bridge the macrocycle carbon frameworks, along with their ethyne- and butadiyne-elaborated monomeric porphyrinic building blocks, and parent (porphinato)zinc(II) precursor molecules were studied by electron paramagnetic resonance (EPR), transient triplet-triplet absorption, and phosphorescence emission. All EPR spectra of the excited monomeric precursors and dimeric and trimeric porphyrin arrays reflect symmetries of rhombic distortion (\D\ greater than or equal to 3\E\) at 4 K and exhibit electron spin polarization patterns verifying that intersystem crossing from the first excited singlet state occurs predominantly through the \T-z> spin state sublevel. The zero field splitting (ZFS) parameters of the (porphinato)zinc monomers that beat ethynyl and butadiynyl groups as well as the conjugated porphyrin arrays are essentially temperature independent over a 4-100 K temperature range. This behavior is in contrast to the Line shape changes typically seen in most monomeric closed-shell metalloporphyrin triplet states, signifying that the lowest excited triplet state possesses a symmetry lower than D-4h and hence not susceptible to Jahn-Teller instabilities. The magnitudes of the D values for all compounds except the meso-to-meso butadiyne-linked dimer lie in the range expected for monomeric (porphinato)zinc complexes, indicating that (1) there is no global delocalization of the triplet excitation and (2) the excitation can be considered to be localized on one of the monomeric subunits in the conjugated supramolecular chromophoric systems on the time scale probed by EPR. The type and mode of chromophore-to-chromophore connectivity-does effect both spin distribution and spin alignment in the low temperature photoactivated triplet states of conjugated (porphinato)zinc arrays. For example, in the meso-to-meso butadiyne linked dimer, the \D\ value actually increases approximately 30% with respect to its monomeric precursor; this anomaly is interpreted as resulting from spin alignment in the molecular plane. A most striking feature is the persistence of electron spin polarization in most of the dimeric and trimeric porphyrin arrays under conditions of steady state illumination up to temperatures approaching the glass transition temperature (T approximate to 120 K). Progressive power saturation experiments carried out on one of the dimeric porphyrin systems, in which an ethyne moiety bridges two (porphinato)zinc chromophores at their respective mesocarbon positions, reveals an exceptionally weak temperature dependence on the saturation parameter, P-1/2, along with no significant changes in either the ZFS parameters or line widths with increasing microwave power or temperature, indicating that spin lattice relaxation is long relative to de-excitation to the ground state over a broad temperature range. This anomalous effect is discussed from the standpoint of relaxation occurring primarily through matrix disorder modes.