Page 664 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Selected Topics 627

An IC is observed when a molecule lying in the excited state relaxes to a lower excited state. This
is a radiationless transition between two different electronic states of the same multiplicity and is
possible when there is a good overlap of the vibrational wave functions (or probabilities) that are
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involved between the two states (beginning and final). IC occurs on a time scale of 10 s, which
is a time scale associated with molecular vibrations. A similar process occurs for an IP when it is
accompanied by a change in multiplicity (e.g., triplet T to S ). Upon nonradiative relaxation heat is
1 0
released. This heat is transferred to the media by collision with neighboring molecules.
Fluorescence is a radiative process in a diamagnetic molecule involving two states (excited and
ground states) of the same multiplicity (i.e., S → S and S → S ). Fluorescence spectra show the
1 0 2 0

intensity of the emitted light versus the wavelength. A fluorescence spectrum is obtained by initial
irradiation of the sample normally at a single wavelength, where the molecule absorbs light. The
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lifetime of fluorescence is typically on the order of 10 –10 s (i.e., ns time scale) for organic mol-

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ecules and faster for metal-containing compounds (10 s or shorter).
In general, the fluorescence band, typically S → S , is a mirror image of the absorption band

1 0
(S → S ). This is particularly true for rigid molecules such as aromatics. Once again, the Franck–
0 1
Condon principle is applicable and hence the presence of vibronic bands is expected in the fl uores-
cence band. However, there are numerous exceptions to this rule, particularly when the molecule
changes its geometry in the excited states. Another observation is that the emission is usually red
shifted in comparison with absorption because the vibronic energy levels involved are lower for the

fluorescence and higher for the absorption processes. The difference in wavelength between the 0–0
absorption and emission band is usually known as the Stokes shift. The magnitude of the Stokes
shift gives an indication of the extent of geometry difference between the ground and excited states
of a molecule as well as the solvent–solute reorganization.
Another nonradiative process that can take place is known as intersystem crossing from a singlet
to a triplet or triplet to a singlet state. This process is very rapid for metal-containing compounds. This
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process can take place on a time scale of ~10 –10 s for an organic molecule while for organometal-
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lics it is ~10 s. This rate enhancement is due to spin-orbit coupling present in the metal-containing
systems, that is, an interaction between the spin angular momentum and the orbital angular momen-
tum, which allows mixing of the spin angular momentum with the orbital angular momentum of S
n
and T states. Thus, these singlet and triplet states are no longer “pure” singlets and triplets and the
n
transition from one state to the other is “less forbidden” by multiplicity rules. A rate increase in inter-
system crossing can also be achieved by the “heavy atom effect” arising from an increased mixing of
spin and orbital quantum number with increased atomic number. This is accomplished either through
introduction of heavy atoms into the molecule via chemical bonding (internal heavy atom effect) or
with the solvent (external heavy atom effect). The spin-orbit interaction energy for atoms grows with
the fourth power of the atomic number Z. In addition to the increase in the intersystem crossing rate,
heavy atoms exert more effects that can be summarized as follows. Their presence acts to (1) decrease
the phosphorescence lifetime due to increase in the nonradiative rates; (2) decrease the fl uorescence
lifetime; and (3) increase the phosphorescence quantum yield. The presence of a heavy atom not only
affects the rate for intersystem crossing but also the energy gap between the singlet and the triplet state,
where the rate for the intersystem crossing increases as the energy gap between S and T decreases.
1 1
Moreover, the nature of the excited state exerts an important effect on the intersystem crossing. For
example, the S (n, π*) → T (π, π∗) (e.g., as in benzophenone) transition occurs almost three orders of
1 2
magnitude faster than the S (π, π*) → T (π, π∗) transition (such as in anthracene).
1 2
Relaxation of triplet state molecules to the ground state can be achieved by either internal conver-
sion (nonradiative IP) or phosphorescence (radiative). Emissions from triplet states (i.e., phosphores-
cence) exhibit longer lifetimes than fluorescence. These long-lived emissions occur on time scale of

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10 s for organic samples and 10 –10 s for metal-containing species. This difference between the
fl uorescence and phosphorescence is associated with the fact that it involves a spin-forbidden elec-
tronic transition. Moreover, the phosphorescence bands are always red shifted in comparison with

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