1.4
Photophysical Processes
A Jabłon ´ski diagram (Fig. 1.11) is often used to illustrate photophysical processes,
with the transitions between the different electronic states and their vibrational
levels indicated. Absorption (A) of a photon induces a transition from the initial
electronic state (S 0 ) to a higher-energy one (S N ), while when a photon is emitted the
transition is to a lower state. Such a transition is called fluorescence (F) if there is no
change in spin and for phosphorescence (P) if there is a change in spin. Internal
conversion (IC) is a radiationless deexcitation process, whereby a molecule relaxes
to a lower electronic state with the same spin. IC is faster between S N and S N-1 than
S N-1 and S N-2 since the separation of the levels decreases with increasing N, and thus
the nuclear wavefunctions overlap better. This implies that S 1 is populated quickly,
and that the IC from S 1 to S 0 is the slowest. Hence most photochemistry occurs from
S 1 , and emission also normally occurs from there (Kasha’s rule). While the energy
may initially be localised in one specific vibrational mode, it is redistributed among
all modes through a process known as intramolecular vibrational energy redistribution (IVR) that often occurs on sub-ps to ps timescales. (Notice, IC is sometimes
also used to denote a transition between different vibrational levels.) In a solution,
this excess vibrational energy dissipates to the solvent (i.e., vibrational cooling) on
a time scale that depends on the coupling strength between the solute and the
solvent molecules. Intersystem crossing (ISC) is a radiationless transition to a state
with a different spin. This is possible due to spin–orbit coupling and is particularly
important if the S 1 lifetime is long. ISC back to the ground state also requires a spin
flip, and as the overlap of the nuclear wavefunctions for T 1 and S 0 is low, it is an
even slower process (Fig. 1.12).
Fig. 1.10 In the Roepstorff, Fohlman and Biemann fragmentation nomenclature a fragment is
labelled according to which bond in the repeating backbone series was broken, and whether it is
the amino-terminal (N-terminal) or carboxyl-terminal (C-terminal) part. Cleavage of the C α –C β ,
C β –N (amide or peptide bond), or N–C α bonds result in a, b, or c fragments, and x, y, or z fragments
for the N-terminal and C-terminal sections, respectively. The subscript refers to the number of
amino-acid side-chains present
6
S.B. Nielsen and J.A. Wyer
Photophysical Processes
A Jabłon ´ski diagram (Fig. 1.11) is often used to illustrate photophysical processes,
with the transitions between the different electronic states and their vibrational
levels indicated. Absorption (A) of a photon induces a transition from the initial
electronic state (S 0 ) to a higher-energy one (S N ), while when a photon is emitted the
transition is to a lower state. Such a transition is called fluorescence (F) if there is no
change in spin and for phosphorescence (P) if there is a change in spin. Internal
conversion (IC) is a radiationless deexcitation process, whereby a molecule relaxes
to a lower electronic state with the same spin. IC is faster between S N and S N-1 than
S N-1 and S N-2 since the separation of the levels decreases with increasing N, and thus
the nuclear wavefunctions overlap better. This implies that S 1 is populated quickly,
and that the IC from S 1 to S 0 is the slowest. Hence most photochemistry occurs from
S 1 , and emission also normally occurs from there (Kasha’s rule). While the energy
may initially be localised in one specific vibrational mode, it is redistributed among
all modes through a process known as intramolecular vibrational energy redistribution (IVR) that often occurs on sub-ps to ps timescales. (Notice, IC is sometimes
also used to denote a transition between different vibrational levels.) In a solution,
this excess vibrational energy dissipates to the solvent (i.e., vibrational cooling) on
a time scale that depends on the coupling strength between the solute and the
solvent molecules. Intersystem crossing (ISC) is a radiationless transition to a state
with a different spin. This is possible due to spin–orbit coupling and is particularly
important if the S 1 lifetime is long. ISC back to the ground state also requires a spin
flip, and as the overlap of the nuclear wavefunctions for T 1 and S 0 is low, it is an
even slower process (Fig. 1.12).
Fig. 1.10 In the Roepstorff, Fohlman and Biemann fragmentation nomenclature a fragment is
labelled according to which bond in the repeating backbone series was broken, and whether it is
the amino-terminal (N-terminal) or carboxyl-terminal (C-terminal) part. Cleavage of the C α –C β ,
C β –N (amide or peptide bond), or N–C α bonds result in a, b, or c fragments, and x, y, or z fragments
for the N-terminal and C-terminal sections, respectively. The subscript refers to the number of
amino-acid side-chains present
6
S.B. Nielsen and J.A. Wyer