Intramolecular charge transfer (ICT) in two Intramolecular charge transfer (ICT) in two phenylpyrrol derivatives: PP and PBNphenylpyrrol derivatives: PP and PBN

Two similar moleculesTwo similar molecules but a different behavior but a different behavior

Danielle SchwekeDanielle SchwekeBaumgertan HagaiBaumgertan Hagai

Yehuda HaasYehuda Haas

Overview of the talkOverview of the talk

• Outline of the emission spectra of PP in different environments and in particular rare gas matrix.

What conditions lead to Dual Fluorescence (DF) ?

• Emission spectra of PBN in solution, supersonic jet and matrix. Tentative assignment of the emission spectra in the matrix in view of the other spectra.

• Comparison between the properties of the two phenylpyrrol derivatives: PP and PBN

• Discussion

Comparison between the properties of PP and PBNComparison between the properties of PP and PBN S. Zilberg and Y. Haas, J. Phys. Chem. A, 106 (2002)

BenzeneBenzene derivative

PPPBN

1A1gS0 = 40.5; = -1.3 D = 29.5 ; = 4.15 D (3.2 D)

1B2u

Covalent1B (Lb) LE

= 0.0 ; = -0.9 D

Exci. energy (eV): 4.1

= 0.0 ; = 4.0 D

Exci. energy (eV): 3.78

1B1u

Ionic2A (La) CT

AQ (perp.) =10.8 D

Exci. energy (eV): 5.30

Q (planar) = 0.75 D

Exci. energy (eV): 4.68

AQ (perp.) = 16.2 D (22.4 D)

Exci. energy (eV): 5.07

Q (planar) = 11.0 D

Exci. energy (eV): 3.98

1. Molecules are isolated from one another in the matrix.

2. In the matrix, molecules are kept at low temperatures (10 K).

The matrix temperature can be varied to a certain extent.

3. Nuclear motion is restricted in the matrix. The degree of

restriction depends mainly on the host molecules (Ar, Xe,

CO2…) but also on the guest molecule.

Why studying these molecules in matricesWhy studying these molecules in matrices??

Fluorescence of PP in solutionFluorescence of PP in solution

N

20000 25000 30000 35000 40000 45000

PP / Cyclohexane (exc

=260nm) PP / Acetonitrile (

exc=260nm)

PP Excitation Spectrum

Rel

ativ

e fl

uore

scen

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tens

ity

Wavenumber (cm-1)

K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003)

Fluorescence of PP in matrixFluorescence of PP in matrix Pure argon matrix

29000 30000 31000 32000 33000 34000 35000

ex

= 275 nmT= 25 K

1520

717

74

1429

3646

3993

4280

2676

2363

3339

3016

456

2027

1656

1360

1048

0-0

Fluo

resc

ence

inte

nsity

Wavenumber (cm-1)

•Clear vibrational structure, observed for the first time in a condensed phase.

•GS vibrational levels in agreement with the ones recorded by FTIR

•No structure could be observed in the excitation spectrum

Fluorescence of PP in supersonic jetFluorescence of PP in supersonic jetLeonid’s resultsLeonid’s results

29000 30000 31000 32000 33000 34000 35000 36000

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a.u.

)

Wavenumber (cm-1)

Supersonic jet spectrum

translated by 445 cm-1

Argon matrix (25K) Supersonic jet

(excitation at the 0-0 band)

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.

26000 28000 30000 32000 34000

PP in pure Argon matrix PP in Argon + AN (1%) matrix

Excitation at 275 nm

Relat

ive f

luor

esce

nce i

nten

sity

Wavenumber (cm-1)

Fluorescence of PP in matrixFluorescence 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 exhibits no vibrational structure.

Conclusions:

The red-shifted emission results from the CT state, which is stabilized by the AN molecules.

Arguments for the assignment Arguments for the assignment of the red-shifted band to CTof the red-shifted band to CT

1. It is observed in AN-doped matrices and not in ethylene doped ones.

2. The red-shifted emission observed in the matrix is similar to the“band” observed in liquid AN which was assigned to CT emission (K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003) )

3. The possibility that the band is due to a stable compound photogenerated by a reaction between PP and AN in an argon matrix has been infirmed by IR experiments.

Fluorescence of PP in matrixFluorescence of PP in matrix Acetonitrile doped argon matrix

26000 28000 30000 32000 34000

PP in Argon + AN (1%) matrixExcitation wavelengths:

270 nm 275 nm 278 nm 284 nm

Fluo

resc

ence

inte

nsity

Wavenumber (cm-1)

Observations:

The emission spectrum in AN/argon is strongly dependent on the excitation wavelength.

Explanations:

1 .The distribution of PP in argon/AN is inhomogeneous and the absorption cross-section differs for each configuration.

The choice of ex determines the population of molecules that is excited.

Fluorescence of PP in matrixFluorescence of PP in matrix Acetonitrile doped argon matrix

26000 28000 30000 32000 34000

PP in Argon + AN (1%) matrixExcitation wavelengths:

270 nm 275 nm 278 nm 284 nm

Fluo

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ence

inte

nsity

Wavenumber (cm-1)

2 .The red-shifted emission band is assigned to the AQ form while the emission band around 305 nm is due, at least partially, to the Q form.

28000 30000 32000 34000 36000

Co-expanded PP and AN in a helium jet Excitation near the 0-0 band

Wavenumber (cm-1)

Fluorescence of PP in supersonic jetFluorescence of PP in supersonic jetLeonid’s resultsLeonid’s results

Observations:

No CT band could be observed in the emission spectrum of PP co-expanded with AN in a helium jet.

Conclusions:

The binding of PP to AN is weaker than the binding between two AN .

When a LE cluster is vibrationally excited, it tends to eject one or more AN instead of crossing to the CT state.

Fluorescence of PBN in solutionFluorescence of PBN in solution

N

C

N

20000 25000 30000 35000 40000 45000

PBN / Cyclohexane (exc

=280nm) PBN / Acetonitrile (

exc=280nm)

PBN Excitation Spectrum

Rel

ativ

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uore

scen

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tens

ity

Wavenumber (cm-1)

K. A. Zachariasse et al, Photochem. Photobiol. Sci., 2 (2003)

Emission of PBN in pure argon matrixEmission of PBN in pure argon matrix

The spectrum exhibits a poor vibrational structure

The measured fluorescence lifetime is 8.0 ns.

No structure could be observed in the excitation spectrum

20000 22000 24000 26000 28000 30000 32000 34000

Deposition and measurement at 25 KExcitation at 286.46 nm

Wavenumber (cm-1)

Fluo

resc

ence

inte

nsity

Fluorescence of PBN in supersonic jetFluorescence of PBN in supersonic jetLeonid’s resultsLeonid’s results

24000 27000 30000 33000

Supersonic jet spectrum

translated by 820 cm-1

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a.u.

)

Wavenumber (cm-1)

Argon matrix (25K) Supersonic jet

(excitation at the 0-0 band)

Observations:

The emission spectrum recorded in argon is very different from the supersonic jet emission spectrum.

The argon matrix shifts the emission spectrum by at least 820 cm-1 .

Conclusions:

1 .In the argon matrix, emission arises not only from the LE state but also from the CT state.

Comparison between the fComparison between the fluorescence of PBN luorescence of PBN in argon matrix and in argon matrix and in in cyclohexanecyclohexane

20000 22000 24000 26000 28000 30000 32000 34000

Cyclohexane solution (T = 298 K) Pure argon matrix (T = 25 K)

Sign

al in

tens

ity

Wavenumber (cm-1)

Emission of PBN in AN doped argon matricesEmission of PBN in AN doped argon matrices

Observations:

A single emission band appears in the spectrum, even after addition of 5% AN to Argon

The two spectra are very similar except for the lack of vibrational structure in the Argon/AN spectrum.

Explanation:

The CT state can’t relax to the same potential minimum as in AN solution, due to restriction on nuclear motion.20000 22000 24000 26000 28000 30000 32000 34000

Wavenumber (cm-1)

Sign

al in

tens

ity

PBN in pure argon (25 K) PBN in Argon + AN (0.7%) matrix

Excitation near 285 nm

Emission of PBN in AN doped argon matricesEmission of PBN in AN doped argon matrices

Influence of the excitation wavelength

20000 22000 24000 26000 28000 30000 32000 34000

Excitation Wavelengths: 285 nm 290 nm 300 nm

PBN in argon + AN matrix (0.7%)

Sign

al in

tens

ity

Wavenumber (cm-1)

Observations:

1 .The emission spectrum in argon/AN is slightly dependent on the excitation wavelength.

2 .In contrast to the case of PP, the characteristic CT emission isn’t observed, even for low excitation energies.

Explanation:

The repartition of sites in the argon/AN matrix is narrower in the case of PBN than in the case of PP.

Emission of PBN in AN doped argon matricesEmission of PBN in AN doped argon matrices

Influence of the dopant concentration

20000 22000 24000 26000 28000 30000 32000 34000

PBN in AN doped Argon matricesExcitation near 300 nm

0.7% AN in Argon 4.7% AN in Argon

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Wavenumber (cm-1)

Observations:

As the concentration of AN in the matrix is increased, the emission spectrum extends

farther to the red .

Explanations:

The contribution of the CT state to the emission increases with the concentration of AN in the matrix.

Fluorescence of PBN in supersonic jetFluorescence of PBN in supersonic jetLeonid’s resultsLeonid’s results

20000 22000 24000 26000 28000 30000 32000 34000

Co-expanded PBN and AN in a helium jet Excitation near the 0-0 band

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a.u.

)

Wavenumber (cm-1)

Observations:

The characteristic CT emission is observed for PBN co-expanded with AN in a helium jet.

Conclusions:

The binding of PBN to AN is stronger than the binding between two AN .

When a LE cluster is vibrationally excited, it crosses to the CT state, keeping the solvation layer.

Fluorescence of PBN in Xenon matrixFluorescence of PBN in Xenon matrix

20000 22000 24000 26000 28000 30000 32000 34000

Tdep

= 40K 60K

exc = 285 nm

Sign

al in

tens

ity

Wavenumber (cm-1)

Observations:

A single band around 360 nm appears in the spectrum.

The spectrum is independent on the excitation wavelength .

Conclusions:

A Xenon matrix stabilizes the emitting state better than an Argon

matrix (even doped by AN) .

20000 22000 24000 26000 28000 30000 32000 34000

0.4

0.6

0.8

1.0

1.2

1.4

1.6

PBN in Xenon + AN (1%) PBN in pure Xenon

exc

= 290 nm

Sign

al in

tens

ity

Wavenumber (cm-1)

Fluorescence of PBN in AN doped Xenon Fluorescence of PBN in AN doped Xenon matrixmatrix

The spectrum remains unchanged after addition of 1% AN

Fluorescence of PBN in Xenon matrixFluorescence of PBN in Xenon matrix

20000 22000 24000 26000 28000 30000 32000 34000

Tdep

= 40KT

meas = 40K

65K

exc = 285 nm

Sign

al in

tens

ity

Wavenumber (cm-1) 20000 22000 24000 26000 28000 30000 32000 34000

Tdep

= 60 KT

meas = 63K

13K

exc = 290 nm

Sign

al in

tens

ity

Wavenumber (cm-1)

Influence of coolingInfluence of annealing

Fluorescence of PBN in COFluorescence of PBN in CO22 amorphous matrix amorphous matrix

20000 22000 24000 26000 28000 30000 32000 34000

Sign

al in

tens

ity

Wavenumber (cm-1)

PBN in a CO2 amorphous matrix (T

dep = 20 K)

Excitation at 285 nmMeasurement temperature:

20 K 65 K

20000 22000 24000 26000 28000 30000 32000 34000

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Amorphous CO2 matrix with 2% AN

Excitation at 285 nm 20 K 75 K 100 K

Fluorescence of PBN in COFluorescence of PBN in CO22 amorphous matrix amorphous matrix

doped by ANdoped by AN

Emission of PBN in various matricesEmission of PBN in various matricesGeneral ComparisonGeneral Comparison

20000 22000 24000 26000 28000 30000 32000 34000

Argon, 25K Xenon, 40K Amorphous CO

2, 20K

Amorphous CO2 + AN (2%), 100K

Excitation at 285 nm

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Possible interpretation of the resultsPossible interpretation of the results

Argon  Xenon

   Amorphous

CO2 

PureAN addedPureAN addedAN added + heatingPure

AN added

AN added + heating

Geometrical relaxation        

Charge stabilization

 1.6 A3  

4.0 A3   

2.7 A3    

Emission max (cm-1)3050029950278002780027800283002830026500

)nm(328334360360360353353377

The main trapping sites are common for PP and PBN One of the main trapping sites of PP was found to be a trapping site of “perpendicular” PP (with approximately the same energy).

Site frequency

nPBNPP, = 40°PP, = 90°525%22%

672.5%68%22%

723.5%7%56%

84%

Simulation of the trapping sites of PP and PBN Simulation of the trapping sites of PP and PBN in argon matrixin argon matrix

Main simulated trapping site of PP and PBNMain simulated trapping site of PP and PBN

Open questionsOpen questions

Why does the emission from the CT state appear

-At the same energy as in AN solution for PP

-At higher energies than in AN solution for PBN?

Why is the behavior of the molecules PP and PBN opposite in the matrix and in the gas phase ?

It is probable that the interaction between PBN and AN is different (stronger) from the interaction between PP and AN.

The matrix results support the hypothesis that a geometrical change is necessary for the transition from LE to CT state.

Further workFurther work

Further investigate the influence of the CO2 matrix form (amorphous or crystalline) on the emission spectrum of PBN

Record the emission of PP in a Xenon matrix (to evaluate the CT state stabilization by Xenon) and in a CO2 matrix.

Compare the possible geometries of the dimers PP / AN and PBN / AN (by quantum chemical calculations).

And evaluate the possible geometrical changes in the matrix