Photophysics terminology: What happens during intersystem crossing in a molecule?

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).