Excited States Reverse Proton Affinities
Imagine if a molecule could completely change its chemical personality the moment it catches a beam of light. This isn't science fiction—it's the fascinating world of excited-state chemistry, where fundamental properties like proton and hydride affinities can dramatically reverse, opening new pathways for chemical reactions impossible in normal conditions.
At the heart of this phenomenon lies a quantum mechanical property called aromaticity—a special stability that certain ring-shaped molecules with specific numbers of electrons possess. For decades, chemists have understood this stability through Hückel's Rule, which states that organic compounds with (4n+2) π-electrons are aromatic and exceptionally stable, while those with 4n π-electrons are antiaromatic and unstable 1 6 .
But in 1972, chemist Baird made a revolutionary discovery: this rule flips in excited states 6 . Where Hückel's rule applies to ground states, Baird's rule dictates that in the lowest ππ* excited states—particularly the triplet state (T₁)—molecules with 4n π-electrons become aromatic, while those with (4n+2) π-electrons become antiaromatic 1 6 .
This reversal in aromaticity has profound implications for chemical reactivity. Since aromatic compounds are more stable, they behave differently in acid-base reactions compared to their antiaromatic counterparts. The key insight is that proton and hydride affinities—measuring how strongly a molecule attracts protons or hydride ions—depend directly on this aromatic character 1 .
Ground state aromaticity: (4n+2) π-electrons = aromatic, 4n π-electrons = antiaromatic
Excited state aromaticity: 4n π-electrons = aromatic, (4n+2) π-electrons = antiaromatic
When researchers quantum chemically calculated proton and hydride affinities across different states, they discovered systematic reversals that aligned perfectly with Baird's rule 1 .
Proton affinity (PA) measures how readily a molecule accepts a proton. Higher values indicate stronger bases. The research revealed:
| State | A-Character (Aromatic) | AA-Character (Antiaromatic) |
|---|---|---|
| S₀ (Ground) | 1447 kJ/mol | 1521 kJ/mol |
| T₁ (Triplet) | 1365 kJ/mol | 1493 kJ/mol |
In the ground state (S₀), aromatic (A-character) anions have lower proton affinities than antiaromatic (AA-character) ones, making them weaker bases. But in the excited triplet state (T₁), this relationship reverses—aromatic anions in the excited state become even weaker bases with significantly lower proton affinities 1 .
Hydride affinity (HA) measures how strongly a cation attracts a hydride ion. Higher values indicate stronger Lewis acids. Similar reversals occur:
| State | A-Character (Aromatic) | AA-Character (Antiaromatic) |
|---|---|---|
| S₀ (Ground) | 826 kJ/mol | 996 kJ/mol |
| T₁ (Triplet) | 790 kJ/mol | 879 kJ/mol |
Again, the pattern is clear: aromatic cations in the ground state have lower hydride affinities than antiaromatic ones, but this difference diminishes or reverses in the excited state 1 .
To verify their hypothesis that aromaticity reversals drive affinity changes, researchers conducted sophisticated quantum chemical calculations on eight different proton addition and eight hydride addition reactions of annulenyl and benzannulenyl anions and cations 1 .
Researchers carefully selected annulenyl and benzannulenyl ions—cyclic organic molecules and their benzene-fused derivatives—that would clearly demonstrate the aromaticity switch between states 1 .
They employed the G3(MP2)//(U)B3LYP/6-311+G(d,p) level theory, a high-accuracy quantum chemical method that reliably predicts molecular energies and properties 1 .
For each molecule, they calculated properties in both the lowest singlet ground state (S₀) and the lowest ππ* excited triplet state (T₁), carefully identifying the aromatic character in each state according to Baird's rule 1 .
Proton and hydride affinities were calculated as energy differences between reactants and products:
Proton affinity = Energy(HA) - Energy(A⁻)
Hydride affinity = Energy(BH⁺) - Energy(B) 2
The calculations revealed unambiguous trends: in both proton and hydride affinities, molecules with aromatic character consistently showed lower affinities than those with antiaromatic character, regardless of whether they were in the ground or excited state 1 .
This confirmed that the aromaticity drives affinity—aromatic stabilization makes ions less likely to accept additional protons or hydrides since adding these would disrupt their favorable electronic structure.
Most importantly, the systematic reversal of which molecules were aromatic versus antiaromatic between S₀ and T₁ states directly caused corresponding reversals in their relative affinities, exactly as predicted by Baird's rule 1 .
Studying excited-state proton and hydride transfer requires specialized computational and experimental approaches:
| Tool/Method | Function | Application Example |
|---|---|---|
| G3(MP2)//B3LYP Calculations | High-accuracy energy computation | Predicting proton/hydride affinities in different states 1 |
| Ion Cyclotron Resonance | Measuring excited-state proton affinities | Determining vertical proton affinities of substituted benzenes 3 |
| Time-Resolved IR Spectroscopy | Tracking proton transfer dynamics | Studying hydrogen transfer in phenol-ammonia clusters |
| Mass-Selected Multiphoton Ionization | Cluster-specific reactivity studies | Identifying proton transfer thresholds in molecular clusters |
| Baird's Rule Framework | Predicting aromaticity in excited states | Explaining affinity reversals in annulenyl ions 1 6 |
Advanced quantum chemical calculations for predicting molecular properties in excited states.
Time-resolved methods for tracking ultrafast proton transfer processes.
Ion cyclotron resonance for precise measurement of excited-state properties.
The discovery of reversible proton and hydride affinities opens exciting possibilities in synthetic chemistry. By using light to manipulate these fundamental properties, chemists could potentially trigger specific reactions that are impossible in ground-state chemistry 1 6 .
This understanding also helps explain previously puzzling photochemical behaviors. For instance, benzene—highly stable and unreactive in its ground state—readily undergoes addition reactions when photoexcited, forming complex three-dimensional structures like bicyclo[3.1.0]hexenes 6 . This dramatic change in reactivity stems from benzene's transition from aromatic in S₀ to antiaromatic in S₁/T₁ states, increasing its proton affinity and making it more susceptible to electrophilic attack 6 .
As research continues, scientists are exploring how to harness these affinity reversals for practical applications—from designing light-controllable catalysts to developing novel phototherapeutic agents that activate only under specific wavelengths.
The takeaway is clear: in the quantum world of excited states, chemistry textbooks need rewiring. Molecules that shun protons in their ground state might eagerly embrace them when touched by light, turning chemical intuition upside down and opening new frontiers for molecular manipulation.