Transcript Slide 1

Charge-Transfer Fluorescence of Phenylpyrrole (PP)
and Pyrrolobenzonitrile (PBN) in Cryogenic Matrices
Hagai Baumgarten, Danielle Schweke and Yehuda Haas
Department of Physical Chemistry and the Farkas center for Light induced Processes
The Hebrew University of Jerusalem, Jerusalem Israel
Objectives
Experimental
We studied the photo-induced Intramolecular Charge Transfer (ICT) of PP
(N-Phenylpyrrol) [1] and PBN (4-(1H-pyrrol-1-yl)benzonitrile) [2] in
cryogenic matrices by spectroscopic research of the Dual Fluorescence (DF)
phenomenon.
We performed fluorescence spectra and time resolved measurements in both
neat and AN-doped Ar matrices. Our results are compared to previous studies
of DF and ICT in solutions [3] and in gas phase [4].
The DF is due to two different excited states: LE (Locally Excited) labeled as
the B state giving the normal emission and the CT (Charge Transfer) labeled
as A state state giving an anomalous red-shifted emission. The ground state is
labeled as X.
PP and PBN belong to a family of para-substituted aromatic systems with
Donor (D) and Acceptor (A) groups. Their fluorescence spectra exhibit a
strong dependence on the environment polarity.
The motivation to study the photo-physical properties of ICT in rigid matrices
stems from their restrictions on the trapped molecules’ degrees of freedom:
translation and rotation, including the torsion mode.
Our goal was to find out whether ICT occurs in PP and PBN in the different
matrices.
The low temperature prevents the occurrence of barrier-dependent relaxation
processes and the appearance of “hot” lines. Therefore the resulting spectra
have a simple structure than in solutions. The low temperature also enables a
long lifetime of the excited states.
Matrix deposition system: planned to enable a delicate
flow of the gas mixture onto the window which is held
under low temperatures (down to 14K) and pressure.
Signal collection setup
1
2
Temp.
Control
He
Cryostat
Scope
PC
Trigger
Pressure gauge
Data
Valve
Signal
Photodiode
Dewar
Gas
Mix.
Needle
Valve
Matrix
Deposition
Movement
control
PMT
Monochromator
Fluorescence
Turbo Rotation
Pump Pump
Sample
LASER
Cold
Window
PBN
Heater
PBN
Host
Gas
AN
Excitation
beam
Prism
array
Gas mixture
Host
Guest
Results
PBN
24
26
28
30
32
34
~/1000 cm-1
36
CT
LE
22
24
26
28
30
~/1000 cm

-1
32
34
36
38
Fluorescence spectra of PP (3) and PBN (4) in neat Ar matrix, Cyclohexane (CH),
Acetonitrile (AN) and jet-cooled spectra of the bare molecules, which allows the assignment of
the LE spectrum, and the location of the 0,0 band.
To the right: life-times measurements of PBN, supporting the assignment of the emission to 2
different excited states: LE and CT.
PP
PBN
Broad CT emission
LE emission
LE vibronic bands
445 cm-1 Red-shifted
Blue-shifted
(Batochromic effect)  mB>mX. (Hypsochromic effect)  mB<mX.
Wavelength (nm)
350
300
270
PP in Ar matrix +1% AN; T=25K
exc=270nm
exc=275nm
exc=278nm
exc=284nm
20
22
24
26
28
30
32
-1
~
/1000 cm
34
290
285
6
Neat Ar matrix
-1
Supersonic Jet blue-shifted by 446cm
-1
Supersonic Jet blue-shifted by 86cm
PBN in Ar matrix
~/1000 cm

500 450
400
350
300
Neat Ar matrix
20
31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5
T=25K; exc=292nm
275
292
nm
CT
emission
Deformation
22
24
26
28
30
32
~/1000 cm

-1
PBN emission in neat Ar matrix is of LE bands (at
high frequencies) superimposed on a broad CT
background. The LE emission includes 2 vibronic
progressions due to 2 different trapping sites.
One progression is blue-shifted by 446 cm-1 relative
to the jet whereas the other is blue-shifted by only
86 cm-1.
The minimal frequency of the 0,0 transition in the
matrix is therefore 34,510 cm-1, which corresponds
to 290 nm.
8
500 450
400
350
300
36
38
270
PBN in Ar matrix
0.7% AN; T=25K
9
exc=285nm
exc=290nm
exc=300nm
34
36
38
-1
Since fluorescence of PBN in neat Ar
matrix is observed upon excitation at
energies lower than the LE 0-0 band (at
the absence of “hot lines”) it is
concluded that the CT state can be
populated directly by light absorption,
and not only via the LE state.
20
22
24
26
28
30
-1
~/1000 cm
32
34
36
38
Fluorescence spectra of PP (5) an PBN (6) in AN doped Ar matrix, in
various excitation wavelengths (exc).
PP
2 bands: normal LE band
and a shifted CT band.
Strong dependence of the
intensities of the 2 bands
Quinoid
Form
Anti-Quinoid
Form
CT; Polar
environment
LE State
LE
Fluorescence
Schematic energy level diagram
of PP and PBN. The diagram
describes the ICT process, which
accounts for the appearance of
dual emission.
The A state surface includes two
forms [5]: Quinoid and AntiQuinoid.
CT
Fluorescence
CT
Absorption
PBN
Ground
State
A slight effect compared to
other polar media.
A Slight shift  narrow
distribution of sites.
0o
30o
60o
90o
Deformation (Torsion, Quinoidization)
Conclusions
DF is observed in both PP and PBN in matrices. CT is therefore possible in
cryogenic temperatures and under the motional restrictions in this rigid
environment.
The different photo-physical behavior of PP and PBN in argon matrices was
explained in terms of the “matrix wall” model (see D. Schweke poster). PBN
emits in AN-doped Ar from a strained adduct, shifted to the blue with respect to
its spectrum in fluid systems, in which large amplitude motions are allowed.
The global minimum of the A state of PBN in Ar matrix is lower than the B state,
and can be directly populated by light absorption.
in addition to its population by a non-radiative process from the B state.
CT State;
Gas phase
Curve
Crossing
Abs.
400
295
Wavelength (nm)
Fluorescence Intensity
Fluorescence Intensity
7
500 450
300
Energy
Fluorescence Intensity
20
38
5
Energy
22
315 310 305
LE
20
500 450
400
350
300
270
Supersonic Jet blue-shifted by 446cm-1; excitation at 0,0 band
Neat Ar matrix exc=266nm; T=25K
AN solution }  =280nm; T
room
CH solution exc
0,0 line
290nm
4
Fluorescence Intensity
3
500 450
400
350
300
270
-1
Supersonic Jet red-shifted by 445cm ; excitation at 0,0 band
Neat Ar matrix exc=275nm; T=25K
AN solution }  =255nm; T
room
CH solution exc
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fluorescence Intensity
Wavelength (nm)
Fluorescence Intensity
PP
Acknowledgements
We thank Prof. B. Dick, Prof. W. Rettig, Dr. W. Fuss and Dr. K. Zachariasse for enlightening
discussions.
This research was supported by the Israel Science Foundation and by The Volkswagen-Stiftung
(I/76 283). The Farkas Center for Light Induced Processes is supported by the Minerva
Gesellschaft mbH.
Literature
1.
2.
3.
4.
5.
D. Schweke, Y. Haas. J. Phys. Chem. A. 107 (2003) 9554.
D. Schweke, H. Baumgarten, Y. Haas, W. Rettig and B. Dick. J. Phys. Chem. A. 109 (2005) 576
T. Yoshihara, V.A. Galiewsky, I.S. Druzhinin, S. Saha, K.A. Zachariasse. Photochem. Photobiol. Sci. 2 (2003) 342.
L. Belau, Y. Haas and W. Rettig. J. Phys. Chem. A. 108, 3916-3925 (2004).
S. Zilberg, Y. Haas.Phys .Chem. A. 106 )2002( 1.