MODULE 20 (701)

1. IF Increasing [p] l / nm 380 480 MODULE 20 (701) EXCITED STATE-GROUND STATE INTERACTIONS Pyrene (10mM to 1mM) fluorescence spectrum I480 /I380 was a function of [pyrene] Förster (1954)

2. MODULE 20 (701) When the ground state of the chromophore quenches the fluorescence of its own excited state, or otherwise interferes with the normal decay processes, we call it concentration quenching, or ground state quenching. Birks used a pulsed light source and photoelectric recording to investigate fluorescence time profiles in the same system. Both sets of observations are consistent with the hypothesis that the S1 and S0 states of pyrene undergo a bimolecular, diffusive process. The result is a transient species having different fluorescence properties from those of the pyrene S1. It was proposed to be an excited state dimer of S1 and S0 - an excimer.

3. FIGURE 20.2 480 nm 380 nm IRF MODULE 20 (701) Birks’ result: 5 mM pyrene in cyclohexane. fD(t) fM(t)

4. MODULE 20 (701) The kinetic scheme The bonding in 1D* is regarded as arising from an electron exchange process involving the single electron in the antibonding p* orbital (SOMO) on the initially excited molecule, and the vacant p* orbital (LUMO) on the initially ground state molecule. Similar excimer interactions are found in excited noble gases, e.g., He2*and Kr2*.

5. 1MA* 1MB 1MA 1MB* MODULE 20 (701) For pyrene and other polycyclic aromatic hydrocarbons of rigid structure (benzene, naphthalene, etc.), the excimer is regarded as a "double-decker" dimer with the p-planes about 350 pm apart. The excimer PE curve with the inter-ring axis as the bonding coordinate

6. B P.E. hnF(D) hnF(M) • inter-ring distance MODULE 20 (701) PE curve The one-electron bond in the excited state has an energy minimum and associated vibrational structure; the ground state surface is dissociative since there is no opportunity for bonding interaction. The red arrow shows the radiative transition from the excimer state to the dissociated ground state surface and the blue arrow shows that from the excited monomer. The dissociation energy of the excimer (its binding energy) is indicated by B

7. MODULE 20 (701) The PE diagram shows that Where M00 is the energy of v’ = 0 to v = 0 transition in M and Dmax is the energy at the excimer fluorescence maximum. Monomer- Excimer Equilibrium Kinetics At the end of an excitation pulse the photon-induced formation stops and the system changes according to the following scheme:

8. MODULE 20 (701) At the end of a brief excitation pulse, The solution of this pair of coupled differential equations (by Laplace transform or secular equation methods) yields the time dependence of the monomer and dimer concentrations.

9. MODULE 20 (701) The monomer and dimer fluorescence response functions can be defined as Combining these leads to:

10. MODULE 20 (701) WherekFM and kDM are the radiative rate constants for the monomer and the excimer respectively. Also And The l quantities are the multipliers of time in the exponential terms in the response functions. They have dimensions of reciprocal time and are composite rate constants.

11. MODULE 20 (701) The figure shows my simulation of the response functions using l1 = 5 ns, and l2 = 25 ns. Note that the shapes of the plots are reminiscent of the experimental data presented earlier.

12. MODULE 20 (701) The Dynamic Equilibrium Condition Recall the scheme, and suppose that then the rate of establishment of the equilibrium is much more rapid than the intrinsic decays of the excited states. Thus the concentrations of 1M* and 1D* are in equilibrium at all times as they decay.

13. MODULE 20 (701) As soon as one of the excited states decays the equilibrium adjusts to account for it. The equilibrium state as a whole decays as time evolves.

14. MODULE 20 (701) Thus the monomer and excimer states have a common decay lifetime (1/l) and m and d are the mole fractions of monomer and excimer in the equilibrium mixture, respectively Under the dynamic equilibrium conditions, the addition of a solute that will quench one but not the other of the excited states will result in a decreased value of l, irrespective of which state is being quenched. It appears as if there were only a single excited state present. The equilibrium state behaves kinetically as a single entity.

15. MODULE 20 (701) Thermodynamic parameters From measurements of l as a function of [M], kDM and kMD can be extracted. Hence the equilibrium constant for excimer formation: The temperature dependence of Keq leads to thermodynamic parameters for the excimer formation In this way we can find DG0, DH0 and DS0 for the monomer-excimer equilibrium.

16. MODULE 20 (701) From these data you see that bonding is relatively weak, comparable to a hydrogen bond in some cases. Anthracene is not included in the list because it is a special case. Upon photoexcitation at low concentration, anthracene fluoresces with high quantum efficiency. As the concentration is increased, FF decreases but no corresponding excimer fluorescence is observed.

17. anthracene 9 10 9’ 9 MODULE 20 (701) Instead a 9:10 bridged dimer precipitates from solution. The 9 and 10 positions have a high electron density and s bond formation is facile. It is thought that as the component species approach, the excimer geometry is achieved, but 9,9’ and 10,10’ bond formation occurs more rapidly than excimer radiative process and the dimer ground state surface is reached non-radiatively.

18. MODULE 20 (701) However the fleeting existence of an anthracene excimer has been shown through transient absorption spectrometry. 9,10 diphenylanthracene has FF of unity. No excimer or photodimer is formed. The phenyl groups at the 9 and 10 positions create enough steric hindrance to prevent the proximity required for the two p-planes to approach closely enough for interaction.

19. MODULE 20 (701) Exciplex formation Excimer formation is a special case of a general class of weakly bound complexes formed between excited state molecules and ground state molecules. Excimers can be classed as “homo-dimers”, being composed of an identical pair of molecules. However the pair need not be identical. For example, Leonhardt and Weller (1961) observed the fluorescence spectrum of biphenyl in cyclohexane solution in the presence and absence of diethylaniline (DEA).

20. Biphenyl/DEA in cyclohexane l / nm 300 440 MODULE 20 (701) FIGURE 20.6 With increasing [DEA] IF The new fluorescence band at 440 nm arises from an excited state complex (exciplex) between biphenyl (S1) and DEA (S0). When the species involved are good electron donors (D) or acceptors (A).

21. MODULE 20 (701) The 1(AD)* state is regarded as having CT character-- 1(A- D+)* As for excimers, exciplex ground states are dissociative. The binding on the S1 surface is made up of three energy terms: IP is ionization potential, EA is electron affinity, and C is a Coulombic term (bringing ions together)

22. MODULE 20 (701) • Experiments show that the exciplex fluorescence lmax depends on the reduction potentials of both components. • The exciplex fluorescence maxima are affected by solvent polarity • is the exciplex dipole moment, r is the radius of the equivalent sphere, e is the static dielectric constant, and n is the refractive index of the medium. In solvents of high e the exciplex emission disappears and transient absorption studies show the appearance of separated radical cations and radical anions.

23. MODULE 20 (701) Exciplexes are often found between planar aromatic hydrocarbons (naphthalene, anthracene, phenanthrene, etc), and amines. In such cases the amines are the electron donors. Other partners for the aromatics can be strong electron acceptors such as cyanoaromatics. In these cases the aromatics become the electron donors. In molecules such as anthracene and phenanthrene the addition of an electron to the LUMO has about the same energy requirement as removing an electron from the HOMO, so the “direction” of the electron transfer is governed by the nature of the other component of the pair.

24. MODULE 20 (701) Exciplexes are defined as existing in the excited state only; there is no binding interaction in the ground state. Some molecules have very strong tendencies to lose or gain electrons in their ground states and form addition complexes without excitation. These are electron-donor-acceptor (EDA) complexes, also called charge-transfer (CT) complexes. Such complexes also have excited states, but they are not classified as exciplexes, because their ground states are bound.