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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 30, Number 1
BKCSDE 30(1)
January 20, 2009 

Molecular Geometries and Electronic Structures of Methyl Pyropheophorbide-a and (Cationic) Tropolonyl Methyl Pyropheophorbides: DFT Calculation
Na Ri Kim, Sujin Kim, Jin Dong Kim, Do Sung Huh, Young Key Shim, Sang Joon Choe*
Tropolonyl methyl pyropheophorbides, Cationic tropolonyl methyl pyropheophorbides, Photodynamic therapy (PDT), DFT
This study reports on the geometry optimizations and electronic structure calculations for methyl pyropheophorbide (MPPa), tropolonyl methyl pyropheophorbides (TMPPa, ITMPPa), and cationic tropolonyl methyl pyropheophorbides (TMPPa+?BF4-, ITMPPa+?BF4-, TMPPa+, and ITMPPa+) using Local Spin Density Approximation (LSDA/6-31G*) and the Restricted Hatree-Fock (RHF/6-31G*) level theory. From the calculated results, we found that substituted cationic tropolonyl groups have larger structural effects than those of substituted neutral tropolonyl groups. The order of structural change effects is ITMPPa+ > ITMPPa+?BF4 - > ITMPPa, as a result of the isopropyl group. Because it is an electron-releasing group, the substituted isopropyl group electronic effect on a 3-position tropolone increases the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital (HOMO-LUMO) energy gap. It was constituted that the larger the cationic characters of these photosensitizers, the smaller the HOMOLUMO band gaps are. The orbital energies of the cationic systems and the ions are stronger than those of a neutral system because of a strong electrostatic interaction. However, this stabilization of orbital energies are counteracted by the distortion of chlorin macrocycle, which results in a large destabilization of chlorin-based compound HOMOs and smaller destabilization of LUMOs as shown in TMPPa (ITMPPa), TMPPa+?BF4- (ITMPPa+?BF4-), and TMPPa+ (ITMPPa+) of Figure 6 and Table 6-7. These results are in reasonable agreement with normal-coordinate structural decomposition (NSD) results. The HOMO-LUMO gap is an important factor to consider in the development of photodynamic therapy (PDT).
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