Cramariuc Oana, Hukka Terttu I, Rantala Tapio T, Lemmetyinen Helge
Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101, Tampere, Finland.
J Phys Chem A. 2006 Nov 16;110(45):12470-6. doi: 10.1021/jp062834v.
Structure, photoabsorption, and excited states of a covalently bonded porphyrin-fullerene dyad H(2)P-O34-C(60) are studied using DFT and TD-DFT approaches. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the excited-state geometrical relaxation are of special interest. An analysis of differences in the description of these delicate phenomena due to the different exchange-correlation functionals is presented. We compare the results given by LDA, GGA, and hybrid functionals (i.e., SVWN, PBE, B3LYP, and PBE0). The ground-state center-to-center (cc) equilibrium distance between the donor and the acceptor moieties is 6.3, 7.1, and 7.9 Angstrom with SVWN, PBE, and B3LYP, respectively. The associated charge transfer of 0.15, 0.11, and 0.09 electrons is shown to depend on this distance but not directly on the functional itself. The same trend is seen in the HOMO-LUMO difference results, and further, in the lowest excitation energies, except for the hybrid functional calculations that yield the largest HOMO-LUMO gap and the highest energy for the lowest electronic excitation. The hybrid functionals were not found practical for excited-state conformational relaxation with the present computing resources. With LDA, the relaxation increases the cc distance by about 0.2 Angstrom, which is associated with a 0.14 eV decrease in energy. As compared to the ground-state dipole moment of about 4 D, the relaxed excited-state charge-transfer complex dipole moment turns out to become about 20 D. A local excitation of the porphyrin donor is considered, as well, and based on all these results, the nature and interpretation of the photoinduced electron-transfer process is discussed.
采用密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）方法研究了共价键合的卟啉 - 富勒烯二元体系H(2)P - O34 - C(60)的结构、光吸收和激发态。供体（卟啉）到受体（富勒烯）的电荷转移以及激发态几何弛豫是特别令人感兴趣的。本文分析了由于不同的交换关联泛函而导致的这些微妙现象在描述上的差异。我们比较了局域密度近似（LDA）、广义梯度近似（GGA）和杂化泛函（即SVWN、PBE、B3LYP和PBE0）给出的结果。供体和受体部分之间的基态中心到中心（cc）平衡距离，使用SVWN、PBE和B3LYP时分别为$6.3$、$7.1$和$7.9$埃。结果表明，相关的$0.15$、$0.11$和$0.09$个电子的电荷转移取决于这个距离，而不是直接取决于泛函本身。在最高占据分子轨道（HOMO） - 最低未占据分子轨道（LUMO）差值结果中也观察到相同的趋势，并且在最低激发能中也是如此，除了杂化泛函计算给出了最大的HOMO - LUMO能隙和最低电子激发的最高能量。在当前的计算资源下，杂化泛函对于激发态构象弛豫并不实用。使用LDA时，弛豫使cc距离增加约$0.2$埃，这与能量降低$0.14$电子伏特相关。与约$4$德拜的基态偶极矩相比，弛豫后的激发态电荷转移复合物偶极矩变为约$20$德拜。还考虑了卟啉供体分子的局部激发，并基于所有这些结果，讨论了光致电子转移过程的本质和解释。
