Photo-induced electron transfer at K: The different conduct of Phenylpyrrol (PP) and pyrrolyl- benzonitrile (PBN) in supersonic jets and in cryogenic matrices Leonid Belau 1, Hagai Baumgarten 1, Danielle Scweke 1, Yehuda Haas 1 and Wolfgang Rettig 2 1 Department of Physical Chemistry and the Farkash Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem, Israel 2 Humboldt University of Berlin, Brook-Taylor-Str. 2, D Berlin, Germany

Pyrrolobenzene (PP) Pyrrolobenzenonitrile (PBN)

Fluorescence of PP in solution N

anomalous 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).

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 C phen -N bond – Twisted Intramolecular Charge Transfer (TICT). * K. Rotkiewicz, K. H. Grellmann, Z. R. Grabowski, Chem. Phys. Let., 19, 315 (1973). GS LE CT Absorption Normal Fluorescence Anomalous Fluorescence  N 

D D LIF spectra of PBN:AN n clusters Solution T=298 0 K LIF TOF L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, (2004)

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

Fluorescence and REMPI-TOF mass spectra in the same beam conditions varying excitation wavelength LIFTOF-MS L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, (2004)

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

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.

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

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).

PBN in argon– one band only?

Relative fluorescence intensity (a.u.) Wavenumber (cm ) Supersonic jet spectrum red-shifted by 445 cm Argon matrix (25K) Supersonic jet (excitation at the 0-0 band) NN CN Supersonic jet spectrum Blue-shifted by 470 cm Relative fluorescence intensity (a.u.) Wavenumber (cm ) Argon matrix (25K) Supersonic jet (excitation at the 0-0 band)

Relative fluorescence intensity Wavenumber (cm ) Relative fluorescence intensity Wavenumber (cm ) 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

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

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

PBN in an argon matrix CTstate Ground state Torsion Energy LE state

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.

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

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

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

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

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 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 Assume that in an argon matrix the geometry is determined by the ground state cluster

03090 Torsion (+quinoidization) Energy PBN/AN cluster in an argon matrix Ground state CT state PBN in an argon matrix PBN in an AN cluster Matrix ‘wall’

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.

Comparison of the intermolecular distances PP - 4 ANPBN - 4 AN Eint(PBN-AN)= kcal/mol Eint(AN-AN)= kcal/mol EintP(PP-AN)= -8.3 kcal/mol Eint(AN-AN)= kcal/mol

Summary With AN NeatWith AN Neat yesno PP yes noPBN In supersonic jetIn 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 CT fluorescence from PP and PBN

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