2.6 Electronic States of Polyatomics and Photoreactivity
65
n → π
∗ transitions have molar extinction coefficients roughly in the range ε max ≈
10–100 mol
−1 L cm
−1 , while for π → π
∗ states one may easily have ε max ≈ 10
4 .
In the absorption spectrum of nonconjugated carbonyl compounds the n → π
∗
band is found at about 300 nm and usually has an extended vibrational structure.
In fact, at variance with S 0 which has a planar structure, in the n → π
∗ state the
carbon atom tends to pyramidalize, with an ibridation going from sp
2 to sp
3 , due to
the antibonding interaction of the π
∗ electron. The π → π
∗ band is usually found
around 190 nm.
In compounds with a C=N or a N=N double bond (imines or azocompounds,
respectively) the ground and the first excited states S 1 and T 1 show a behavior similar to that of Fig. 2.8 with respect to the torsion about the double bond. However,
the energy gap between S 1 and T 1 , which have n → π
∗ character, is smaller, while
S 1 is normally lower in energy than in alkenes. For example, in azomethane the first
absorption band has maximum at about 360 nm. In general, imines and azocompounds are not fluorescent and photoisomerize with a mechanism similar to that of
alkenes.
2.6.6 Excited States of Conjugated Systems
The distinctive feature of a conjugated π system with 2n c centers is that, being the π
orbitals delocalized, the energy gap between two consecutive π orbitals (hence the
HOMO-LUMO gap) decreases with n c , in analogy with the levels of a particle in a
box. The π → π
∗ transitions are therefore displaced at lower energies, as shown in
Fig. 2.9. If present, n → π
∗ transitions show a similar behavior, but to a lesser extent
(the energy of the n orbital being not affected by the extension of the π system).
Therefore, increasing n c , n → π
∗ and π → π
∗ bands tend to approach, with the
n → π
∗ absorption disappearing below the more intense π → π
∗ one for n c large
enough. For that reason in Fig. 2.9 the plot is interrupted at n c = 3 for n → π
∗
bands.
In aromatic compounds the first absorption band is again displaced to shorter
wavelengths, because aromaticity stabilizes the ground state: for instance, the S 1
absorption band in hexatriene peaks at 270 nm, while in benzene it is found at
about 250 nm. Moreover, the easy torsion of the double bonds in linear polyenes
completely obscures the vibrational structure, which is instead quite apparent in the
first absorption band of aromatic compounds. A quite analogous effect is instead the
shift to longer wavelengths which is observed by extending the conjugated systems:
from 250 nm in benzene we go to 270 nm in naphthalene, 345 nm in anthracene,
and 470 nm in tetracene. Polycyclic aromatic hydrocarbons (PAH) are quite rigid,
so the S 1 PES cannot get close to the S 0 one, as it does by double-bond torsion in
linear polyenes. As a result, the S 1 lifetime is long and PAH compounds are often
fluorescent, see Table 2.2.
In general n → π
∗ triplets have larger radiative transition rates with respect
to π → π
∗ triplets. In fact, due to the El-Sayed rules, n → π
∗ triplet states are
65
n → π
∗ transitions have molar extinction coefficients roughly in the range ε max ≈
10–100 mol
−1 L cm
−1 , while for π → π
∗ states one may easily have ε max ≈ 10
4 .
In the absorption spectrum of nonconjugated carbonyl compounds the n → π
∗
band is found at about 300 nm and usually has an extended vibrational structure.
In fact, at variance with S 0 which has a planar structure, in the n → π
∗ state the
carbon atom tends to pyramidalize, with an ibridation going from sp
2 to sp
3 , due to
the antibonding interaction of the π
∗ electron. The π → π
∗ band is usually found
around 190 nm.
In compounds with a C=N or a N=N double bond (imines or azocompounds,
respectively) the ground and the first excited states S 1 and T 1 show a behavior similar to that of Fig. 2.8 with respect to the torsion about the double bond. However,
the energy gap between S 1 and T 1 , which have n → π
∗ character, is smaller, while
S 1 is normally lower in energy than in alkenes. For example, in azomethane the first
absorption band has maximum at about 360 nm. In general, imines and azocompounds are not fluorescent and photoisomerize with a mechanism similar to that of
alkenes.
2.6.6 Excited States of Conjugated Systems
The distinctive feature of a conjugated π system with 2n c centers is that, being the π
orbitals delocalized, the energy gap between two consecutive π orbitals (hence the
HOMO-LUMO gap) decreases with n c , in analogy with the levels of a particle in a
box. The π → π
∗ transitions are therefore displaced at lower energies, as shown in
Fig. 2.9. If present, n → π
∗ transitions show a similar behavior, but to a lesser extent
(the energy of the n orbital being not affected by the extension of the π system).
Therefore, increasing n c , n → π
∗ and π → π
∗ bands tend to approach, with the
n → π
∗ absorption disappearing below the more intense π → π
∗ one for n c large
enough. For that reason in Fig. 2.9 the plot is interrupted at n c = 3 for n → π
∗
bands.
In aromatic compounds the first absorption band is again displaced to shorter
wavelengths, because aromaticity stabilizes the ground state: for instance, the S 1
absorption band in hexatriene peaks at 270 nm, while in benzene it is found at
about 250 nm. Moreover, the easy torsion of the double bonds in linear polyenes
completely obscures the vibrational structure, which is instead quite apparent in the
first absorption band of aromatic compounds. A quite analogous effect is instead the
shift to longer wavelengths which is observed by extending the conjugated systems:
from 250 nm in benzene we go to 270 nm in naphthalene, 345 nm in anthracene,
and 470 nm in tetracene. Polycyclic aromatic hydrocarbons (PAH) are quite rigid,
so the S 1 PES cannot get close to the S 0 one, as it does by double-bond torsion in
linear polyenes. As a result, the S 1 lifetime is long and PAH compounds are often
fluorescent, see Table 2.2.
In general n → π
∗ triplets have larger radiative transition rates with respect
to π → π
∗ triplets. In fact, due to the El-Sayed rules, n → π
∗ triplet states are