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In these diagrams, energy transitions are indicated by vertical lines. Not all transitions

are possible; allowed transitions are defined by the selection rules of quantum

mechanics. A molecule in its electronic and vibrational ground state (S 0 v 0 ) can absorb

photons matching the energy difference of its various discrete states. The required

photon energy has to be higher than that required to reach the vibrational ground

state of the first electronic excited state (S 1 v 0 ). The excess energy is absorbed as

vibrational energy (v>0), and quickly dissipated as heat by collision with solvent

molecules. The molecule thus returns to the vibrational ground state (S 1 v 0 ). These

relaxation processes are non-radiating transitions from one energetic state to another

with lower energy, and are calledinternal conversion(IC). From the lowest level of

the first electronic excited state, the molecule returns to the ground state (S 0 ) either

by emitting light (fluorescence) or by a non-radiative transition. Upon radiative

transition, the molecule can end up in any of the vibrational states of the electronic

ground state (as per quantum mechanical rules).

If the vibrational levels of the ground state overlap with those of the electronic

excited state, the molecule will not emit fluorescence, but rather revert to the ground

state by non-radiative internal conversion. This is the most common way for excitation

energy to be dissipated and is why fluorescent molecules are rather rare. Most molecules

are flexible and thus have very high vibrational levels in the ground state. Indeed, most

fluorescent molecules possess fairly rigid aromatic rings or ring systems. The fluores-

cent group in a molecule is called afluorophore.

S 2 S 0 S 1 T 1

T 1

S 1

S 0

hva

v 3

v 2

v 1

v 0

v 3

v 2

v 1

v 0

v 3

v 2

v 1

v 0

v 2

v 1

v 0

Absorbtion

(DS=0)

Fluorescence

(DS=0)

Phosphorescence

(DS=0)

hvf hvp

ISC (DS=0)

IC (DS=0)

IC (DS=0)

Total spin S

MultiplicityM

0

1

0

1

1

3

(10–15S)(10–12S)

(10–6–10^2 S)

(10–9S –10–8S)

Fig. 12.8Jablonski diagram. Shown are the electronic ground state (S 0 ), two excited singlet states (S 1 ,S 2 ) and a

triplet state (T 1 ). Vibrational levels (v) are only illustrated exemplarily. Solid vertical lines indicate radiative

transitions, dotted lines show non-radiative transitions. The inset shows the relationship between electron

configurations, total spin numberSand multiplicityM.

494 Spectroscopic techniques: I Photometric techniques