Light-Induced Electron Spin Polarization of a Weakly Coupled Triplet−Doublet Spin Pair in a Covalently Linked Porphyrin Dimer
Bibliographic record
Abstract
The spin-polarization patterns of a weakly coupled triplet−doublet spin pair are studied in a covalently linked copper (II)−free base porphyrin dimer partially oriented in a liquid crystalline solvent. The triplet−doublet pair is generated in the dimer by either spin−orbit intersystem crossing (ISC) within the free-base moiety or via energy transfer (EnT) from the copper porphyrin. The two pathways result in two different spin-polarization patterns, which can be observed separately by selectively exciting either the free base porphyrin at 640 nm or the copper porphyrin at 540 nm. The time development and orientation dependence of the two spin-polarization patterns is investigated at two microwave frequencies. An analysis of the EPR transients reveals that the polarization associated with the EnT pathway rises monoexponentially, while the ISC polarization rises rapidly and decays biexponentially. The rise of the EnT polarization pattern and the fast decay component of the polarization from ISC are governed by the same characteristic lifetime of 2 μs independent of the molecular orientation. However, the ratio of the two decay components of the ISC polarization is strongly orientation dependent. Separately, a general treatment of the spin-polarized transient EPR spectra of coupled triplet−doublet spin pairs is presented and two possible mechanisms for the kinetic behavior involving spin-selective depopulation or relaxation between the spin sublevels of the triplet−doublet pair are investigated. It is shown that all the observed features of the polarization patterns including orientation and excitation wavelength dependence as well as the kinetics can be explained in terms of the model and are consistent with either of the two mechanisms. In addition to the weakly coupled triplet−doublet pair, narrow signal components are observed near the center of the spectrum. The microwave frequency dependence of the spectra confirms the previous assignment that these components are the quartet and doublet states of a strongly coupled triplet−doublet pair generated in conformations of the dimer in which the two porphyrins are in close proximity.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".