Charge-Transfer Fluorescence of Phenylpyrrole (PP) and Pyrrolobenzonitrile (PBN) in Cryogenic Matrices

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 ProcessesThe Hebrew
University of Jerusalem, Jerusalem Israel
Results
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.
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.
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.
PBN PP
Broad CT emission LE vibronic bands LE emission
Blue-shifted(Hypsochromic effect) ? mBltmX. 445 cm-1 Red-shifted (Batochromic effect) ? mBgtmX.
Emittingstates
LE shift relative to jet
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
Anti-Quinoid.
Fluorescence spectra of PP (5) an PBN (6) in AN
doped Ar matrix, in various excitation
wavelengths (lexc).
PBN PP
A slight effect compared to other polar media. 2 bands normal LE band and a shifted CT band.
A Slight shift ? narrow distribution of sites. Strong dependence of the intensities of the 2 bands
Doping effect
lexc