Substituent Effects in Molecular Electronic Relaxation Dynamics via Time-Resolved Photoelectron Spectroscopy: ππ* States in Benzenes
Bibliographic record
Abstract
We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: C C (styrene), C O (benzaldehyde), and C⋮C (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (α-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ππ* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the second ππ* state, a slower decaying component to the first ππ* state. Very fast electronic relaxation constants (<100 fs) for the second ππ* states were observed for all molecules studied and are explained by relaxation to the first ππ* via a conical intersection near the planar minimum. Although a “floppy” methyl substitution (α-methylstyrene, acetophenone) leads as expected to even faster second ππ* decay rates, a rigid ring substitution (indene) has no discernible effect. The much slower electronic relaxation constants of the first ππ* states for styrene and phenylacetylene are very similar to those of benzene in its first ππ* state, at the same amount of vibrational energy. By contrast, the lifetime of the first ππ* state of indene was much longer, attributed to its rigid structure. The second ππ* state of benzaldehyde has a short lifetime, similar to the other derivatives. However, the relaxation of its first ππ* state is orders of magnitude faster than that of the non-carbonyl compounds, due to the well-known presence of a lower lying nπ* state. Methylation (acetophenone) leads to still faster first ππ* state relaxation rates. These results fit very well with the current understanding of aromatic photophysics, demonstrating that time-resolved photoelectron spectroscopy provides for a facile, accurate and direct means of studying electronic relaxation dynamics in a wide range of molecular systems.
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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".