1. Intramolecular charge transfer (ICT) in two phenylpyrrol derivatives: PP and PBN Two similar molecules but a different behavior Danielle Schweke Baumgertan Hagai Yehuda Haas

2. Overview of the talk
Outline of the emission spectra of PP in different environments and in particular rare gas matrix.
What conditions lead to Dual Fluorescence (DF) ?
Emission spectra of PBN in solution, supersonic jet and matrix. Tentative assignment of the emission spectra in the matrix in view of the other spectra.
Comparison between the properties of the two phenylpyrrol derivatives: PP and PBN
Discussion

3. Comparison between the properties of PP and PBN S. Zilberg and Y
Comparison between the properties of PP and PBN S. Zilberg and Y. Haas, J. Phys. Chem. A, 106 (2002)
PBN PP
Benzene derivative
Benzene
 = 29.5 ;  = 4.15 D (3.2 D)
 = 40.5;  = -1.3 D S0
1A1g
 = 0.0 ;  = 4.0 D
Exci. energy (eV): 3.78
 = 0.0 ;  = -0.9 D
Exci. energy (eV): 4.1
1B (Lb) LE
1B2u
Covalent
AQ (perp.)  = 16.2 D (22.4 D)
Exci. energy (eV): 5.07
Q (planar)  = 11.0 D
Exci. energy (eV): 3.98
AQ (perp.)  =10.8 D
Exci. energy (eV): 5.30
Q (planar)  = 0.75 D
Exci. energy (eV): 4.68
2A (La) CT
1B1u
Ionic

4. Why studying these molecules in matrices?
1. Molecules are isolated from one another in the matrix. 2. In the matrix, molecules are kept at low temperatures (10 K). The matrix temperature can be varied to a certain extent. 3. Nuclear motion is restricted in the matrix. The degree of restriction depends mainly on the host molecules (Ar, Xe, CO2…) but also on the guest molecule.

5. Fluorescence of PP in solution
K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003)

6. Fluorescence of PP in matrix Pure argon matrix
• Clear vibrational structure, observed for the first time in a condensed phase.
• GS vibrational levels in agreement with the ones recorded by FTIR
• No structure could be observed in the excitation spectrum

7. Fluorescence of PP in supersonic jet Leonid’s results
Observations:
 The emission spectrum recorded in argon perfectly matches the supersonic jet emission spectrum.
 The argon matrix shifts the emission spectrum by about 445 cm-1.
Conclusions:
1. In the argon matrix, emission arises from the LE state.
2. The matrix stabilizes this state (with respect to the GS) by about 450 cm-1 .

8. Fluorescence of PP in matrix Acetonitrile doped argon matrix
Observations:
• A new band, red-shifted with respect to the LE one, appears in the spectrum as a result of addition of AN.
• The red-shifted band exhibits no vibrational structure.
Conclusions:
The red-shifted emission results from the CT state, which is stabilized by the AN molecules.

9. Arguments for the assignment of the red-shifted band to CT
It is observed in AN-doped matrices and not in ethylene doped ones.
The red-shifted emission observed in the matrix is similar to the“band” observed in liquid AN which was assigned to CT emission (K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003) )
The possibility that the band is due to a stable compound photogenerated by a reaction between PP and AN in an argon matrix has been infirmed by IR experiments.

10. Fluorescence of PP in matrix Acetonitrile doped argon matrix
Observations:
The emission spectrum in AN/argon is strongly dependent on the excitation wavelength.
Explanations:
1. The distribution of PP in argon/AN is inhomogeneous and the absorption cross-section differs for each configuration.
The choice of lex determines the population of molecules that is excited.

11. Fluorescence of PP in matrix Acetonitrile doped argon matrix
2. The red-shifted emission band is assigned to the AQ form while the emission band around 305 nm is due, at least partially, to the Q form.

12. Fluorescence of PP in supersonic jet Leonid’s results
Observations:
No CT band could be observed in the emission spectrum of PP co-expanded with AN in a helium jet.
Conclusions:
The binding of PP to AN is weaker than the binding between two AN.
When a LE cluster is vibrationally excited, it tends to eject one or more AN instead of crossing to the CT state.

13. Fluorescence of PBN in solution
K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003)

14. Emission of PBN in pure argon matrix
 The spectrum exhibits a poor vibrational structure
 The measured fluorescence lifetime is 8.0 ns.
 No structure could be observed in the excitation spectrum

15. Fluorescence of PBN in supersonic jet Leonid’s results
Observations:
 The emission spectrum recorded in argon is very different from the supersonic jet emission spectrum.
 The argon matrix shifts the emission spectrum by at least 820 cm-1.
Conclusions:
1. In the argon matrix, emission arises not only from the LE state but also from the CT state.

16. Comparison between the fluorescence of PBN in argon matrix and in cyclohexane

17. Emission of PBN in AN doped argon matrices
Observations:
 A single emission band appears in the spectrum, even after addition of 5% AN to Argon
 The two spectra are very similar except for the lack of vibrational structure in the Argon/AN spectrum.
Explanation:
The CT state can’t relax to the same potential minimum as in AN solution, due to restriction on nuclear motion.

18. Emission of PBN in AN doped argon matrices Influence of the excitation wavelength
Observations:
1. The emission spectrum in argon/AN is slightly dependent on the excitation wavelength.
2. In contrast to the case of PP, the characteristic CT emission isn’t observed, even for low excitation energies.
Explanation:
The repartition of sites in the argon/AN matrix is narrower in the case of PBN than in the case of PP.

19. Emission of PBN in AN doped argon matrices Influence of the dopant concentration
Observations:
As the concentration of AN in the matrix is increased, the emission spectrum extends farther to the red.
Explanations:
The contribution of the CT state to the emission increases with the concentration of AN in the matrix.

20. Fluorescence of PBN in supersonic jet Leonid’s results
Observations:
The characteristic CT emission is observed for PBN co-expanded with AN in a helium jet.
Conclusions:
The binding of PBN to AN is stronger than the binding between two AN.
When a LE cluster is vibrationally excited, it crosses to the CT state, keeping the solvation layer.

21. Fluorescence of PBN in Xenon matrix
Observations:
A single band around 360 nm appears in the spectrum.
The spectrum is independent on the excitation wavelength.
Conclusions:
A Xenon matrix stabilizes the emitting state better than an Argon matrix (even doped by AN).

22. Fluorescence of PBN in AN doped Xenon matrix
The spectrum remains unchanged after addition of 1% AN

23. Fluorescence of PBN in Xenon matrix
Influence of annealing
Influence of cooling

24. Fluorescence of PBN in CO2 amorphous matrix

25. Fluorescence of PBN in CO2 amorphous matrix doped by AN

26. Emission of PBN in various matrices General Comparison

27. Possible interpretation of the results
  Amorphous CO2
 Xenon
Argon
AN added + heating
AN added
Pure
Geometrical relaxation
2.7 A3 
4.0 A3 
 1.6 A3 
Charge stabilization
26500
28300
27800
29950
30500
Emission max (cm-1)
377
353
360
334
328
(nm)

28. Simulation of the trapping sites of PP and PBN in argon matrix
Site frequency
PP,  = 90°
PP,  = 40°
PBN n
22%
25% 5
68%
72.5% 6
56% 7%
23.5% 7 4% 8
The main trapping sites are common for PP and PBN
One of the main trapping sites of PP was found to be a trapping site of “perpendicular” PP (with approximately the same energy).

29. Main simulated trapping site of PP and PBN

30. Open questions  Why does the emission from the CT state appear
- At the same energy as in AN solution for PP
- At higher energies than in AN solution for PBN?
 Why is the behavior of the molecules PP and PBN opposite in the matrix and in the gas phase?
It is probable that the interaction between PBN and AN is different (stronger) from the interaction between PP and AN.
The matrix results support the hypothesis that a geometrical change is necessary for the transition from LE to CT state.

31. Further work
 Further investigate the influence of the CO2 matrix form (amorphous or crystalline) on the emission spectrum of PBN
 Record the emission of PP in a Xenon matrix (to evaluate the CT state stabilization by Xenon) and in a CO2 matrix.
 Compare the possible geometries of the dimers PP / AN and PBN / AN (by quantum chemical calculations).
And evaluate the possible geometrical changes in the matrix