10.2 Photodissociation and bond cleavage

Photodissociation Fundamentals

Photodissociation is the process of breaking chemical bonds using light energy. It's one of the most direct photochemical reactions you'll encounter: a molecule absorbs a photon, and if that photon carries enough energy, a bond breaks. The products you get depend on how the bond breaks and which bond is targeted, both of which you can predict and control.

Bond dissociation energy in photodissociation

Bond dissociation energy (BDE) is the minimum energy required to break a specific chemical bond, measured in kJ/mol or kcal/mol. It's the threshold your photon needs to clear.

The core relationship is straightforward:

$E = h\nu \geq \text{BDE}$

The photon energy ($E = h\nu$) must meet or exceed the BDE for cleavage to occur. If it doesn't, the molecule may reach an excited state but won't dissociate along that bond.

A few things determine BDE values:

• Bond order: Triple bonds have higher BDEs than double bonds, which are higher than single bonds. For example, the C≡C bond in acetylene has a BDE of ~839 kJ/mol, while a C–C single bond in ethane is ~376 kJ/mol.
• Atomic properties: Larger atoms form weaker bonds (C–I is weaker than C–F), and electronegativity differences affect bond strength.
• Molecular context: The same bond type can have different BDEs depending on the surrounding structure. The O–H bond in water (~497 kJ/mol) differs from O–H in methanol (~436 kJ/mol).

This is why BDE is so useful in practice: if you know the BDE of each bond in a molecule, you can choose a wavelength that selectively breaks one bond while leaving others intact.

Homolytic vs heterolytic bond cleavage

Once a bond breaks, the two bonding electrons have to go somewhere. How they're distributed defines the type of cleavage and the products you get.

Homolytic cleavage splits the electron pair evenly. Each fragment keeps one electron, producing two radicals:

$\text{CH}_3\text{–CH}_3 \xrightarrow{h\nu} \text{CH}_3\bullet + \text{CH}_3\bullet$

Radicals are highly reactive species with unpaired electrons. This pathway dominates in the gas phase and in nonpolar solvents, where there's nothing to stabilize charged species.

Heterolytic cleavage gives both electrons to one fragment, producing a cation and an anion:

$\text{CH}_3\text{–Cl} \xrightarrow{h\nu} \text{CH}_3^+ + \text{Cl}^-$

This pathway is favored when:

• The bond is already polar (uneven electron sharing in the ground state)
• The solvent is polar enough to stabilize the resulting ions
• The molecular structure can delocalize or stabilize the charge on one or both fragments

Photodissociation Processes and Factors

Excited states and dissociative pathways

Absorbing a photon doesn't automatically break a bond. The molecule first reaches an electronically excited state, and what happens next depends on the shape of the potential energy surface (PES) for that state.

The Franck-Condon principle governs the initial excitation: because nuclei are much heavier than electrons, the electronic transition happens so fast that the nuclei don't move during absorption. The molecule lands on the excited-state PES at whatever geometry it had in the ground state.

From there, three main dissociative pathways are possible:

1. Direct dissociation — The excited state itself is repulsive (its PES has no energy minimum). The bond stretches immediately and breaks. This is the fastest pathway, often completing in tens of femtoseconds.
2. Predissociation — The molecule is initially excited to a bound state (one with an energy minimum), but that state crosses a repulsive state at some geometry. The molecule can "leak" onto the repulsive surface and dissociate. This is slower than direct dissociation because it depends on the coupling between the two states.
3. Internal conversion followed by dissociation — The molecule relaxes non-radiatively to a lower electronic state (or a vibrationally hot ground state) that has enough energy to break a bond. This is the slowest of the three and competes most directly with other relaxation pathways like fluorescence.

All three pathways operate on ultrafast timescales (femtoseconds to picoseconds) and compete with radiative decay, intersystem crossing, and other deactivation processes.

Factors in photodissociation selectivity

Several variables determine which bonds break, how efficiently, and what products form.

Wavelength is your primary tool for selectivity. Different chromophores in a molecule absorb at different wavelengths, so tuning your light source lets you excite one part of the molecule while leaving the rest alone. This only works if the absorption bands are reasonably well separated.

Light intensity matters in two regimes. At normal intensities, photodissociation is a one-photon process and scales linearly with intensity. At very high intensities (e.g., focused laser pulses), multiphoton dissociation becomes possible, where a molecule absorbs two or more photons sequentially or simultaneously to reach higher-energy dissociative states.

Molecular structure influences dissociation in several ways:

• Weak bonds (low BDE) break preferentially
• Good leaving groups stabilize the departing fragment
• Steric strain can lower the effective barrier to dissociation
• Conjugation or resonance can either stabilize the excited state (slowing dissociation) or stabilize the products (promoting it)

Environmental factors also play a role. In solution, the solvent cage effect can trap radical pairs together, promoting recombination and lowering the apparent dissociation yield. Temperature affects the rate of thermally activated predissociation pathways.

Quantum yield ($\Phi$) quantifies how efficient the overall process is: it's the number of bonds broken per photon absorbed. A quantum yield of 1.0 means every absorbed photon leads to dissociation. Values below 1.0 indicate that competing processes (fluorescence, internal conversion without dissociation, cage recombination) are siphoning away some of the excited-state population.