Pyrrolobenzene (PP)

1. Photo-induced electron transfer at 10-20 K: The different conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices Leonid Belau1, Hagai Baumgarten1, Danielle Scweke1, Yehuda Haas1 and Wolfgang Rettig2 1Department of Physical Chemistry and the FarkashCenter for Light Induced Processes, The HebrewUniversity ofJerusalem, Jerusalem, Israel 2HumboldtUniversity ofBerlin, Brook-Taylor-Str. 2, D-12489Berlin, Germany

2. Pyrrolobenzene (PP) Pyrrolobenzenonitrile (PBN)

3. N Fluorescence of PP in solution

4. Introduction – anomalous emission of PBN 4-Pyrrolobenzonitrile (PBN) and Phenylpyrrole (PP) exhibit anomalous fluorescence: upon increasing solvent polarity emission band shifted to longer wavelengths. This strongly red shifted band was termed “anomalous” emission. Fluorescence and excitation spectra of PBN in different solvents* Wavelength (nm) * C. Cornelissen-Gude, and W. Rettig, J. Phys. Chem., 102, 7754 (1998).

5. Introduction – TICT mechanism N Grabowski et al.*proposed an explanation: the anomalous emission occur from a Charge Transfer (CT) state that is populated by a non-radiative transition from the Locally Excited (LE). The intramolecular charge transfer is accompanied by rotation around the Cphen-N bond – Twisted Intramolecular Charge Transfer (TICT). LE CT Anomalous Fluorescence Absorption Normal Fluorescence GS  00 900  *K. Rotkiewicz, K. H. Grellmann, Z. R. Grabowski, Chem. Phys. Let., 19, 315 (1973).

6. PBN:AN results LIF spectra of PBN:ANn clusters LIF TOF Solution T=2980K D D L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, 108 3916 (2004)

7. PP clusters with acetonitrile in a supersonic jet L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, 108 3916 (2004)

8. PBN:AN results Fluorescence and REMPI-TOF mass spectra in the same beam conditions varying excitation wavelength LIF TOF-MS L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, 108 3916 (2004)

9. Fluorescence of PP in matrixNeat argon matrix D. Schweke and Y. Haas, J. Phys. Chem. A, 107, 9554 (2003(

10. Fluorescence of PP in supersonic jet compared with argon matrix 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 .

11. Fluorescence of PP in matrixAcetonitrile doped argon matrix PP in pure Argon matrix PP in Argon + AN (1%) 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 is devoid of vibrational structure. Conclusions: The red-shifted emission results from the CT state, that is further stabilized by the AN molecules. Excitation at 275 nm Relative fluorescence intensity 26000 28000 30000 32000 34000 -1 Wavenumber (cm ) D. Schweke and Y. Haas, J. Phys. Chem. A, 107, 9554 (2003(

12. Emission of PBN in pure argon matrix Comparison with emission spectrum in cyclohexane in which the CT emission is dominant*: →Most of the intensity is due to a CT state! *T. Yoshihara, V. A. Galiewsky, S. I. Druzhinin, S. Saha and K. A. Zachariasse, Photochem. Photobiol. Sci., 2, 342 (2003) .

13. PBN in argon– one band only?

14. N CN N Argon matrix (25K) Supersonic jet Argon matrix (25K) (excitation at the 0-0 band) Supersonic jet (excitation at the 0-0 band) Relative fluorescence intensity (a.u.) Relative fluorescence intensity (a.u.) Supersonic jet spectrum -1 Blue-shifted by 470 cm Supersonic jet spectrum -1 red-shifted by 445 cm 24000 27000 30000 33000 29000 30000 31000 32000 33000 34000 35000 36000 -1 -1 Wavenumber (cm ) Wavenumber (cm )

15. 0.4 0.4 Relative fluorescence intensity Relative fluorescence intensity 0.2 0.2 -1 -1 Wavenumber (cm Wavenumber (cm ) ) 0.0 0.0 32000 33000 34000 35000 36000 32000 33000 34000 35000 36000 PBN in an argon matrix (black) Compared to Jet-cooled PBN (colored) Two trapping sites in argon Site I blue shifted by 80 cm-1 Site II blue shifted by 470 cm-1

16. Emission at low temp: PBN in argon– excitation at different wavelengths Emission observed upon excitation at 292 nm The 0,0 transition of the LE band is at 286 nm

17. Emission observed upon excitation at a lower energy than the 0,0 transition of the LE band – direct CT-state excitation

18. PBN in an argon matrix LE state Energy CTstate Ground state 0 30 90 Torsion

19. Emission of PBN in AN doped argon matrices  A single emission band appears in the spectrum, even after addition of 1% AN to Argon  The two spectra are very similar (in contrast with the corresponding PP spectrum in AN-doped argon matrix)except for the lack of vibrational structure in the spectrum recorded in the doped matrix.

20. Fluorescence of PP in matrixAcetonitrile doped argon matrix PP in pure Argon matrix PP in Argon + AN (1%) 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 is devoid of vibrational structure. Conclusions: The red-shifted emission results from the CT state, that is further stabilized by the AN molecules. Excitation at 275 nm Relative fluorescence intensity 26000 28000 30000 32000 34000 -1 Wavenumber (cm )

21. Explain different behavior of PP and PBN in an AN-doped argon matrix by assuming 1:1 adducts embedded in argon Cluster structures by atom-atom pair potential functions* * With B. Dick

22. Optimized geometries of PP-AN clusters for different electronic states of PP GS -5.11 kcal/mol CT, AQ min CT, Q min -6.01 kcal/mol -11.17 kcal/mol

23. Optimized geometries of PBN-AN clusters for different electronic states of PBN GS -5.33 kcal/mol CT, AQ min CT, Q min -6.29 kcal/mol -11.90 kcal/mol

24. The structure of the 1:1 PP:AN cluster in the CT state is very similar to the structure in the ground state The structure of the 1:1 PBN:AN cluster in the CT state is very different from the structure in the ground state Assume that in an argon matrix the geometry is determined by the ground state cluster In an argon matrix, large changes in the structure cannot take place, therefore: The PP:AN adduct can reach an optimum geometry upon excitation to the CT state – the system emits from a relaxed configuration The PBN:AN adduct cannot reach an optimum geometry upon excitation to the CT state – the system emits from a strained configuration

25. PBN/AN cluster in an argon matrix CT state Matrix ‘wall’ PBN in an argon matrix Energy PBN in an AN cluster Ground state Torsion (+quinoidization) 0 30 90

26. The different emission spectra observed for PP and PBN clusters with AN in a supersonic jet are explained by the simulations as well. The binding of PP to AN is much weaker than the binding of PBN with AN in a supersonic jet. Therefore a PP:(AN)k cluster tends to dissociate on excitation, while the PBN:(AN)k is stable.

27. Comparison of the intermolecular distances PP - 4 AN PBN - 4 AN EintP(PP-AN)= -8.3 kcal/mol Eint(AN-AN)= -19.4 kcal/mol Eint(PBN-AN)= -13.4 kcal/mol Eint(AN-AN)= -15.4 kcal/mol

28. Summary CT fluorescence from PP and PBN In supersonic jet In argon matrix Direct excitation of CT state of PBN in a jet and argon matrix LE state of PBN less polar than ground state of PP – more polar

29. Thanks Leonid Belau, Danielle Schweke, Hagai Baumgarten, Elodie Marxe, Shmuel Zilberg The Farkas Center for Light Induced Processes –Minerva Volkswagen Stiftung The Israel Science Foundation