Photophysics terminology: What happens during intersystem crossing in a molecule?
Correct Answer: The spin of an excited electron reverses, changing the state of the molecule (singlet ↔ triplet)
Introduction / Context:After absorbing light, molecules can undergo several relaxation pathways. Distinguishing internal conversion, intersystem crossing, fluorescence, and phosphorescence is essential for interpreting emission spectra and designing fluorescent probes.
Given Data / Assumptions:
- Electronic states are labeled by spin multiplicity: singlet (paired spins) and triplet (unpaired).
- Spin selection rules make some transitions “forbidden,” affecting lifetimes.
- We focus on the definition of intersystem crossing (ISC).
Concept / Approach:ISC is a nonradiative transition between electronic states of different multiplicity (e.g., S1 → T1), enabled by spin–orbit coupling. It involves a change in electron spin orientation, not photon emission, and competes with fluorescence (radiative S1 → S0).
Step-by-Step Solution:Identify ISC as a crossing between states of different spin (singlet to triplet or vice versa).Note that no photon is emitted during ISC; it is nonradiative.Therefore, choose the option explicitly describing spin reversal and state change.
Verification / Alternative check:Systems with heavy atoms exhibit stronger spin–orbit coupling, enhancing ISC and phosphorescence—a common experimental observation.
Why Other Options Are Wrong:
- Photon emission: That is fluorescence or phosphorescence, not ISC.
- Conversion to vibrational energy: That is internal conversion or vibrational relaxation.
- “All of the above”: Bundles mutually exclusive mechanisms.
Common Pitfalls:Confusing ISC (spin change, nonradiative) with phosphorescence (radiative T1 → S0).
Final Answer:The spin of an excited electron reverses, changing the state of the molecule (singlet ↔ triplet).