Molecular Luminescence Spectroscopy

1. Molecular Luminescence Spectroscopy Lecture 29

2. Molecular Luminescence Spectroscopy Luminescence spectroscopy is a technique which studies the fluorescence, phosphorescence, and chemiluminescence of chemical systems. The analyte or its reaction product needs to be luminescent. The relative luminescence intensity is related to analyte concentration as will be seen shortly.

3. Singlet and Triplet States Electrons in molecular orbitals are paired, according to Pauli exclusion principle. When an electron absorbs enough energy it will be excited to a higher energy state; but will keep the orientation of its spin. The molecular electronic state in which electrons are paired is called a singlet transition. On the other hand, the molecular electronic state in which the two electrons are unpaired is called a triplet state. The triplet state is achieved when an electron is transferred from a singlet energy level into a triplet energy level, by a process called intersystem crossing; accompanied by a flip in spin

5. In a singlet state, the spins of the two electrons are paired and thus exhibit no magnetic field and called diamagnetic. Diamagnetic molecules, containing paired electron, are neither attracted nor repelled by a magnetic field. On the other hand, molecules in the triplet state have unpaired electrons and are thus paramagnetic which means that they are either repelled or attracted to magnetic fields. The terms singlet and triplet stems from the definition of multiplicity where: Multiplicity = 2S + 1 Where, S is the total spin. The total spin for a singlet state is zero since electrons are paired which gives a multiplicity of one (the term singlet state).

6. Multiplicity = (2 * 0) + 1 =1 In a triplet state, the total spin is one (the two electrons are unpaired) and the multiplicity is three: Multiplicity = (2 * 1) + 1 = 3 It should also be indicated that the probability of a singlet to triplet transition is much lower than a singlet to singlet transition. Therefore, the intensity of the emission from a triplet state to a singlet state is much lower than emission intensities from a singlet to a singlet state.

7. Energy Level Diagram for Photoluminescent Molecules The following diagram represents the main processes taking place in a photoluminescent molecule when it absorbs and emits energy. The different processes will be discussed below:

9. Absorption The absorption of UV-Vis radiation is necessary to excite molecules from the ground state to one of the excited states. Absorption of radiation promotes electrons in chemical bonds to be excited. However, we have seen earlier that not all transitions have the same probability and while certain transitions are practically very important, others are seldom used and are of either no or marginal importance. Therefore, from information we have discussed in Chapter 14 we concluded that there are four different types of electronic transitions which can take place in molecules when they absorb UV-Vis radiation. A s-s* and a n-s* are not useful while the n-p* transition requires low energy but the molar absorptivity for this transition is low and transition energy will increase in presence of polar solvents.

10. The most frequently used transition is the p-p* transition for the following reasons: a.       The molar absorptivity for the p-p* transition is high allowing sensitive determinations. b.      The energy required is moderate, far less than dissociation energy. c.       In presence of the most convenient solvent (water), the energy required for a p-p* transition is usually smaller. Therefore, best molecules that may show absorption are those with p bonds or preferably aromatic nature as discussed earlier. Absorption to higher excited singlet states requires a very short time (in the range of 10-14s).

11. Vibrational Relaxation Absorption of radiation will excite molecules to different vibrational levels of the excited state. This process is usually followed by successive vibrational relaxations (VR) as well as internal conversion to lower excited states. In cases where transitions occur to the first excited state, vibrational relaxation to the main excited electronic level will take place and/or an intersystem crossing (ISC) to the triplet state can occur.

12. Fluorescence After vibrational relaxation to first excited electronic level takes place, a molecule can return to the ground state by emission of a photon, called fluorescence (FL). The fluorescence lifetime is much greater than the absorption time and occurs in the range from 10-7 to 10-9s. As the lifetime in the excited state is increased, the probability of fluorescence will be decreased since radiationless deactivation processes may take place. However, not all excited molecules can show fluorescence by returning to ground state and most return to ground state by losing excitation energy as heat or through collisions with other molecules or solvent.

13. Internal and External Conversion Internal conversion (IC) is a radiationless deactivation process whereby excited molecules return to the ground state without emission of a photon. This process lacks rigid understanding but seems to be the most efficient deactivation process in luminescence spectroscopy, since most molecules do not show fluorescence. However, molecules with close electronic energy levels, to the extent that their vibrational energy levels of ground and excited states are overlapped, are believed to cause efficient internal conversion.

14. Dissociation and predissociation Internal conversion can result in a phenomenon called predissociation (PD) where an electron relaxes from a higher electronic state to an upper vibrational energy of a lower electronic state. When the vibrational energy is large enough and is greater than the bond senergy, bond rupture occurs in a process called predissociation. Dissociation should be differentiated from predissociation where dissociation involves absorption of high energy so that the molecule is directly promoted to a high energy vibrational level where bond rupture directly occurs.

15. External conversion (EC) is a process whereby excited molecules lose their energy due to collisions with other molecules or by transfer of their energy to solvent or other unexcited molecules. Therefore, external conversion is influenced by temperature, solvent viscosity, as well as solvent composition.

16. Intersystem Crossing Electrons present at the first excited electronic level can follow one of three choices including emission of a photon to give fluorescence, radiationless deactivation to ground state, or intersystem crossing (ISC). The process of intersystem crossing involves transfer of the electron from an excited singlet to a triplet state. This process can actually take place since the vibrational levels in the singlet and triplet states overlap. However, crossing of the singlet state to the triplet state involves a flip in electron spin in order to satisfy the triplet state.

17. Intersystem crossing is facilitated by presence of nonbonding electrons as well as heavy atoms. The presence of paramagnetic atoms or species also enhances intersystem crossing. An electron in the triplet state can also cross back to the singlet state and can result in a photon as fluorescence but at a much longer time than regular fluorescence. This process is termed delayed fluorescence and has the same characteristics as direct fluorescence except for the large increase in lifetime.

18. Phosphorescence Electrons crossing the singlet state to the triplet state with a flipped spin can also follow one of three choices including returning to the singlet state (including a flip in spin), relax to ground state by internal or/and external conversion, or lose their energy as a photon (phosphorescence, Ph) and relax to ground state with a second flip in spin to satisfy the singlet ground state. As can be rationalized from the processes involved in collecting phosphorescence photons, this involves an intersystem crossing and two flips in spin. This, in fact, requires a much longer time than fluorescence (10-4s to up to few s). Therefore, the probability of phosphorescence, and hence the intensity of the phosphorescence spectrum, is very low due to high possibility of radiationless deactivation.