INTERSYSTEM CROSSING FLUORESCENCE

A molecule could also relax from S, or T, to S0 by emitting a photon. The radiational transition Sj —> S0 is called fluorescence (Box 18-2), and the radiational transition T, —> S0 is called phosphorescence. The relative rates of internal conversion, intersystem crossing, fluorescence, and phosphorescence depend on the molecule, the solvent, and conditions such as temperature and pressure. The eneigy of phosphorescence is less than the energy of fluorescence, so phosphorescence comes at longer wavelengths than fluorescence (Figure 18-14). 

In the gas phase at low pressure, vibrational and rotational relaxation are generally slow. It is now well known that a molecule in an upper vibronic state may undergo such processes as intersystem crossing, fluorescence, internal conversion, and chemical reaction before the excess vibrational energy can be removed by collisions. The different vibrational (and rotational) levels of the excited state may then have to be taken into account in the mechanism, although in detail determined by the data which are available. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Figure C 1.5.10. Nonnalized fluorescence intensity correlation function for a single terrylene molecule in p-terjDhenyl at 2 K. The solid line is tire tlieoretical curve. Regions of deviation from tire long-time value of unity due to photon antibunching (the finite lifetime of tire excited singlet state), Rabi oscillations (absorjDtion-stimulated emission cycles driven by tire laser field) and photon bunching (dark periods caused by intersystem crossing to tire triplet state) are indicated. Reproduced witli pennission from Plakhotnik et al [66], adapted from [118].

Bernard J, Fleury L, Talon FI and Orrit M 1993 Photon bunching in the fluorescence from single molecules a probe for intersystem crossing J. Phys. Chem 98 850-9... 

VD = vibrational deactivation IC = internal conversion F = fluorescence IX = intersystem crossing and P = phosphorescence. 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

Thus if one starts with one pure isomer of a substance, this isomer can undergo first-order transitions to other forms, and in turn these other forms can undergo transitions among themselves, and eventually an equilibrium mixture of different isomers will be generated. The transitions between atomic and molecular excited states and their ground states are also mostly first-order processes. This holds both for radiative decays, such as fluorescence and phosphorescence, and for nonradiative processes, such as internal conversions and intersystem crossings. We shall look at an example of this later in Chapter 9. 

Figure 9.1. A Jablonski diagram. So and Si are singlet states, Ti is atriplet state. Abs, absorption F, fluorescence P, phosphorescence IC, internal conversion and ISC, intersystem crossing. Radiative transitions are represented by full lines and nonradiative transitions by dashed lines...

Fluorescence intensity detected with a confocal microscope for the small area of diluted solution temporally fluctuates in sync with (i) motions of solute molecules going in/out of the confocal volume, (ii) intersystem crossing in the solute, and (hi) quenching by molecular interactions. The degree of fluctuation is also dependent on the number of dye molecules in the confocal area (concentration) with an increase in the concentration of the dye, the degree of fluctuation decreases. The autocorrelation function (ACF) of the time profile of the fluorescence fluctuation provides quantitative information on the dynamics of molecules. This method of measurement is well known as fluorescence correlation spectroscopy (FCS) [8, 9]. 

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