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Time-Resolved Insight into the Photosensitized Generation of Singlet Oxygen in Endoperoxides.

Martínez-Fernández L, González-Vázquez J, González L, Corral I - J Chem Theory Comput (2014)

Bottom Line: Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and O-O homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products.The triplet states do not intervene in this mechanism, as opposed to the O-O homolysis where the exchange of population between the singlet and triplet manifolds is remarkable.In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic ππ* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is O-O homolysis with a quantum yield of 65%.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química, Universidad Autónoma de Madrid , 28049 Cantoblanco, Madrid, Spain.

ABSTRACT

A synergistic approach combining high-level multiconfigurational static calculations and full-dimensional ab initio surface hopping dynamics has been employed to gain insight into the photochemistry of endoperoxides. Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and O-O homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products. Our results reveal that cycloreversion or the rupture of the two C-O bonds occurs via an asynchronous mechanism that can lead to the population of a ground-state intermediate showing a single C-O bond. Furthermore, singlet oxygen is directly generated in its most stable excited electronic state (1)Δg. The triplet states do not intervene in this mechanism, as opposed to the O-O homolysis where the exchange of population between the singlet and triplet manifolds is remarkable. In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic ππ* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is O-O homolysis with a quantum yield of 65%.

No MeSH data available.


Related in: MedlinePlus

Simulated absorption spectra for CHDEPO. Black solid linecorrespondsto the spectrum calculated using SA4-CASSCF(14,12)/ANO-RCC level oftheory, while red solid line represents the results corrected usingthe MS-CASPT2 method. Black dotted line outlines the CASSCF scaledspectrum (scCASSCF).
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fig1: Simulated absorption spectra for CHDEPO. Black solid linecorrespondsto the spectrum calculated using SA4-CASSCF(14,12)/ANO-RCC level oftheory, while red solid line represents the results corrected usingthe MS-CASPT2 method. Black dotted line outlines the CASSCF scaledspectrum (scCASSCF).

Mentions: Figure 1 displays the positionof the CASSCFand CASPT2 vertical excitations (black and red vertical lines below8 and 6 eV, respectively), superimposed to the CASSCF and CASPT2 spectrabased on the 1000 geometries generated to mimic the nuclear wavepacket(solid black and red spectra). Both spectra consist of an intenseband, showing a shoulder at higher energies, preceded by a weakerabsorption. Consistently with what is observed for the vertical spectrum,we find that CASSCF overestimates by a factor of 0.25 the absorptionenergies, taking CASPT2 as a reference; compare the position of theleast (4.8 vs 3.8 eV) and most energetic bands (6.2 vs 4.7 eV) atCASSCF and CASPT2. A decomposition analysis of these bands in termsof the contributing states (see Figures S6 andS7) reveals that the weakest band mainly results from the firstexcited state in both spectra, whereas the S2 and S3 states are the responsible for the principal band, increasingthe S3 its contribution to the main band after includingdynamic correlation. These results are consistent with the verticalpicture obtained in Table 1. The pathologicaloverestimation of excitation energies by CASSCF at the FC region,as compared with CASPT2 calculations, could be detrimental for thedynamics, leading to undesired photoproducts due to the excess ofenergy accumulated by the starting ensemble of initial geometries.However, taking into account that CASPT2 analytical gradients arenot available in MOLCAS and that the use of CASPT2 numerical gradientsis computationally unaffordable for a study as the one suggested here,we opted for a common solution previously adopted by other authorsthat consists in the scaling of the energies and gradients35 (See Supporting Information for more details).


Time-Resolved Insight into the Photosensitized Generation of Singlet Oxygen in Endoperoxides.

Martínez-Fernández L, González-Vázquez J, González L, Corral I - J Chem Theory Comput (2014)

Simulated absorption spectra for CHDEPO. Black solid linecorrespondsto the spectrum calculated using SA4-CASSCF(14,12)/ANO-RCC level oftheory, while red solid line represents the results corrected usingthe MS-CASPT2 method. Black dotted line outlines the CASSCF scaledspectrum (scCASSCF).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4325559&req=5

fig1: Simulated absorption spectra for CHDEPO. Black solid linecorrespondsto the spectrum calculated using SA4-CASSCF(14,12)/ANO-RCC level oftheory, while red solid line represents the results corrected usingthe MS-CASPT2 method. Black dotted line outlines the CASSCF scaledspectrum (scCASSCF).
Mentions: Figure 1 displays the positionof the CASSCFand CASPT2 vertical excitations (black and red vertical lines below8 and 6 eV, respectively), superimposed to the CASSCF and CASPT2 spectrabased on the 1000 geometries generated to mimic the nuclear wavepacket(solid black and red spectra). Both spectra consist of an intenseband, showing a shoulder at higher energies, preceded by a weakerabsorption. Consistently with what is observed for the vertical spectrum,we find that CASSCF overestimates by a factor of 0.25 the absorptionenergies, taking CASPT2 as a reference; compare the position of theleast (4.8 vs 3.8 eV) and most energetic bands (6.2 vs 4.7 eV) atCASSCF and CASPT2. A decomposition analysis of these bands in termsof the contributing states (see Figures S6 andS7) reveals that the weakest band mainly results from the firstexcited state in both spectra, whereas the S2 and S3 states are the responsible for the principal band, increasingthe S3 its contribution to the main band after includingdynamic correlation. These results are consistent with the verticalpicture obtained in Table 1. The pathologicaloverestimation of excitation energies by CASSCF at the FC region,as compared with CASPT2 calculations, could be detrimental for thedynamics, leading to undesired photoproducts due to the excess ofenergy accumulated by the starting ensemble of initial geometries.However, taking into account that CASPT2 analytical gradients arenot available in MOLCAS and that the use of CASPT2 numerical gradientsis computationally unaffordable for a study as the one suggested here,we opted for a common solution previously adopted by other authorsthat consists in the scaling of the energies and gradients35 (See Supporting Information for more details).

Bottom Line: Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and O-O homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products.The triplet states do not intervene in this mechanism, as opposed to the O-O homolysis where the exchange of population between the singlet and triplet manifolds is remarkable.In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic ππ* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is O-O homolysis with a quantum yield of 65%.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química, Universidad Autónoma de Madrid , 28049 Cantoblanco, Madrid, Spain.

ABSTRACT

A synergistic approach combining high-level multiconfigurational static calculations and full-dimensional ab initio surface hopping dynamics has been employed to gain insight into the photochemistry of endoperoxides. Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and O-O homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products. Our results reveal that cycloreversion or the rupture of the two C-O bonds occurs via an asynchronous mechanism that can lead to the population of a ground-state intermediate showing a single C-O bond. Furthermore, singlet oxygen is directly generated in its most stable excited electronic state (1)Δg. The triplet states do not intervene in this mechanism, as opposed to the O-O homolysis where the exchange of population between the singlet and triplet manifolds is remarkable. In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic ππ* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is O-O homolysis with a quantum yield of 65%.

No MeSH data available.


Related in: MedlinePlus