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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 Belau 1 , Hagai Baumgarten 1 , Danielle Scweke 1 , Yehuda Haas 1 and Wolfgang Rettig 2.

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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


Pyrrolobenzene (PP) conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

Pyrrolobenzenonitrile (PBN)


Fluorescence of pp in solution

N conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

Fluorescence of PP in solution


Introduction – anomalous emission of PBN conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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).


Introduction – TICT mechanism conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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).


PBN:AN results conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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)


PP clusters with acetonitrile in a supersonic jet conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

L. Belau, Y. Haas and W. Rettig, J. Phys. Chem. A, 108 3916 (2004)


PBN:AN results conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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)


Fluorescence of pp in matrix neat argon matrix
Fluorescence of PP in matrix conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matricesNeat argon matrix

D. Schweke and Y. Haas, J. Phys. Chem. A, 107, 9554 (2003(


Fluorescence of pp in supersonic jet compared with argon matrix
Fluorescence of PP in supersonic jet conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices 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 acetonitrile doped argon matrix
Fluorescence of PP in matrix conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matricesAcetonitrile 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(


Emission of pbn in pure argon matrix
Emission of PBN in pure argon matrix conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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? conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices


N conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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

)


0.4 conduct of Phenylpyrrol (PP) and pyrrolyl-benzonitrile (PBN) in supersonic jets and in cryogenic matrices

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


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 0,0 transition of the LE band –

LE state

Energy

CTstate

Ground state

0

30

90

Torsion


Emission of pbn in an doped argon matrices
Emission of PBN in AN doped argon matrices 0,0 transition of the LE band –

 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 acetonitrile doped argon matrix1
Fluorescence of PP in matrix 0,0 transition of the LE band –Acetonitrile 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

)


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


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


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


The structure of the 1:1 PP:AN cluster in the CT state electronic states of PBNis 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


PBN/AN cluster in an argon matrix electronic states of PBN

CT state

Matrix ‘wall’

PBN in

an argon matrix

Energy

PBN in an AN cluster

Ground state

Torsion (+quinoidization)

0

30

90


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
Comparison of the intermolecular distances clusters with AN in a supersonic jet are explained by the simulations as well.

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


Summary clusters with AN in a supersonic jet are explained by the simulations as well.

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


Thanks clusters with AN in a supersonic jet are explained by the simulations as well.

Leonid Belau,

Danielle Schweke,

Hagai Baumgarten,

Elodie Marxe,

Shmuel Zilberg

The Farkas Center for Light Induced Processes –Minerva

Volkswagen Stiftung

The Israel Science Foundation


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