Platinum Octaethyl Porphyrin (Pt.OEP) is an efficient phosphor under ultraviolet excitation. The phosphorescent triplet state (T(sub 1)) is readily quenched by the oxygen (O2) molecules. This phenomenon is being utilized as the basis for global air pressure measurements in aerodynamic facilities at various laboratories. The exact mechanism by which O2 molecules quench the (T(sub 1) - S(sub 0)) transitions is still unknown. The diamagnetic S(sub n) singlet states, which feed T(sub 1) states via intersystem crossings, would presumably not be affected by O2. It must be the magnetic T(sub 1) states, which can interact with the paramagnetic O2 molecules, that are affected. However, our positron lifetime and Doppler broadening studies suggest the formation of (S(sub n) central dot O2) complexes which can also eventually reduce the population of the T(sub 1) states (i.e. quench phosphorescence). This is possible since higher triplet states in (Pt.OEP) are admixed with the S(sub n) states via spin orbit interactions. The experimental procedures and the results of various measurements are discussed in this paper

Eftekhari, A.

Naidu, S. V. N.

Singh, Jag J.

English

NASA Technical Reports Server

NASA-TM-|I1534
,,'.z. /.-' ,-,.-
Mechanism of Phosphorescence Quenching in
Photomagnetic Molecules Determined By
Positron Annihilation Spectroscopy
Jag J. Singh
A. Eftekhari
S.V.N.Naidu
NASA Langley Research Center
Hampton, VA.
Presented at
61st Meeting of the Southeastern Section
of the American Physical Society
held at
Newport News, VA
Nov. 10- 12, 1994
https://ntrs.nasa.gov/search.jsp?R=19960028542 2020-06-16T03:59:33+00:00Z

Mechanism of PhosphorescenceQuenching in
PhotomagneticMoleculesDetermined By
Positron Annihilation Spectroscopy
2
ABSTRACT
Platinum Octaethyl Porphyrin (Pt.OEP) is an efficient phosphor under ultraviolet
excitation. The phosphorescenttriplet state (T_) is readily quenched by the oxygen (Oz)
molecules. This phenomenon is being utilized as the basis for global air pressure
measurementsin aerodynamicfacilities at various laboratories. The exactmechanismby
which Oz moleculesquench the (T_-S0)transitions is still unknown. The diamagnetic So
singlet states, which feed T_ states via intersystem crossings, would presumably not be
affected by 02. It must be the magnetic T_ states, which can interact with the paramagnetic
02 molecules, that are affected. However, our positron lifetime and Doppler broadening
studies suggest the formation of (S_.O2) complexes which can also eventually reduce the
population of the T_ states (i.e. quench phosphorescence). This is possible since higher
triplet states in (Pt-OEP) are admixed with the S, states via spin orbit interactions. The
experimental procedures and the results of various measurements are discussed in this
paper.
INTRODUCTION
The basic ring structure of a porphyrin molecule is characterized by a 4-fold
symmetry, including 4-nitrogen atoms directed towards the center. When the 8-
exohydrogens in it are replaced by CH_CH 2 groups, the molecule becomes octaethyl
porphyrin. If the 2-central hydrogens are also replaced by metal atoms, the molecule
becomes a metalloporphyrin. These metal atoms can have significant effects on the
3electronic and optical properties of parent molecules "j. Amongst the most widely studied
metalloporphyrins are Platinum-Octaethyl Porphyrin molecules. Figure -l(a) shows the
chemical structure of Platinum-Octaethyl Porphyrin molecule. Figure -l(b) shows its
schematic energy level diagram and decay scheme.
Under UV- irradiation, the Pt.OEP molecule is excited to a higher singlet state
which decays very quickly to the lowest excited singlet state. From this singlet state, the
molecule can decay to the ground state by a radiative/non-radiation transition or internally
convert to the lowest excited triplet state. This triplet state can either be re-excited to the
singlet state by thermal excitation or collision with another triplet state molecule or
phosphoresce to the ground state with a lifetime of the order of I00 microseconds. It has
been reported m that the presence of oxygen molecules near the photoexcited Pt.OEP
molecules leads to the quenching of the phosphorescence in them. When an oxygen
molecule comes in contact with a Pt.OEP molecule in its lowest excited triplet state t , P(Tt),
it can lead to one of the following reactions:
1 t
02(3_-g )+P(TI)-_ 02( 7,q )+P(So)
(I)
-" [02"P(TI) ] (2)
-, 02 +P(S o) + (3)(')
(*) This reaction is possible only in
the condensed state of Pt.OEP formulation.
All these reactions result in the reduction of phosphorescence from the P(Tt) state. It is
well recognized that the P(S,)-states of the aromatic molecules are admixed with the higher
triplet states. Thus the paramagnetic oxygen (02) molecule will not necessarily be repelled
%We shall use P(SI or T i) as the symbol for appropriate state of the
metalloporphyrin molecule in this paper.
molecule, it can form a transient
considerably higher in energythan P(Sa)state.
angstroms°_ above the ground state, P(So).
4
[02 • P(S_)]" complex. Such a complex is expected to be
As seen in figure-2, P(SI) occurs at 5310
Because of the competing radiationless
transitions from it, the formation of [O2 • P(Sa)]" complex will also result in a net reduction
in the phosphorescence from the P(T_) state. Currently, it is believed that the [02 • P(T_)]
interaction alone is responsible for the reduction in the P(T_) -- P(So) radiative transitions.
There is no reason why [02 • P(S_)]* complex formation, which can also reduce the
population of the P(T_) state and hence the P(T_) -- P(So) radiative transition, can also not
play a role. It is the purpose of this investigation to determine whether [02. P(S_)]
interaction plays a role in the O2-induced quenching process of P(T_) -- P(So)
phosphorescence in Pt.OEP molecules.
EXPERIMENTAL PROCEDURES AND RESULTS
Two types of metalloporphyrins were used in this study. (1) Platinum Octaethyl
Porphyrin (Pt. OEP), which has a reported ¢3_phosphorescence quantum yield of
approximately 90%. Figure-3 shows the emission spectrum of (Pt. OEP) molecule.
Clearly, the phosphorescence peak at 6445._ dominates the spectrum. (2) Magnesium
Octaethyl Porphyrin (Mg.OEP) whose phosphorescence quantum yield has been
reported a_ to be less than 2%. Figure-4 shows the emission spectrum of (Mg. OEP)
molecule. Clearly, the phosphorescence peak is very weak and the flourescence peak at
5835.A dominates the spectrum. The low phosphorescence yield of Mg.OEP may be due
to the reduced population, or an even lower energy, of the P(Tm) state in it. It may also
be due to the lighter mass of the central substituent magnesium atom. However,
Mg.OEP was selected for this study mainly on the assumption that the excited S = 0
states in it are less admixed with the higher S= 1 states.
The metalloporphyrin (M.OEP) targets were prepared by air-spraying a solution of
5(M.OEP) molecules in trichioroethane (C2H3C!3) on thin aluminum substrates. The C2H3C! 3
molecules evaporate quickly leaving the M.OEP molecules adhered to the aluminum
substrates.
Positron lifetime and Doppler broadening measurements were made in M-OEP
molecular films spray-coated on both sides of 0.010" thick aluminum substrates. The
substrates were shaped into 2" high concentric cylinders of diameter 1.0" and 1.5",
respectively. The aluminum cylinders were enclosed in a 3" high, 3" diameter glass
cylinder, which could be pumped down to 104 Torr. A 50 /_c Na 22 positron source
deposited on a steel needle tip was located at the center of the aluminum cylinders. The
phosphor films were photoexcited with ultraviolet radiation from a Xenon short arc lamp
located above the glass cylinder.
Positron lifetime measurements were made with a standard fast-fast coincidence
system whose resolution time is of the order of 300 picoseconds. The Doppler broadening
measurements were made with a high purity Germanium detector whose full width at half
maximum (FWHM) resolution for Bi 2°7 radiation is 1.28 kev. Both types of measurements
were made with and without ultraviolet excitation of the phosphor molecules in vacuum,
N2, O_, and air media at room temperature. Figure 5 shows the experimental set up for
positron annihilation measurements.
The positron lifetime results in the Pt.OEP phosphor are summarized in tables I and
II. The Doppler broadening results in Pt.OEP and Mg.0EP phosphors are summarized
in Tables HI and IV, respectively. Figure-6 illustrates the Doppler Broadening results.
These results are discussed in the following section.
DISCUSSION
An examination of the data in Tables I and II shows that UV irradiation has little
effect on the positron lifetime spectra in any one of the media surrounding the Pt.OEP-
6coated aluminum cylinders. In particular, even pure oxygen environment does not seem to
affect positron lifetimes. This may be due to the fact that the positronium atom (Ps)
formed at the [02 .P(S_)]" complex will be readily quenched by the paramagnetic O2
molecule released in the process m. This fast positronium decay component will not be
resolvable from the prompt component in the positron lifetime spectrum.
The Doppler broadening data in tables IH and IV exhibit two interesting features
(1) UV-irradiation does not affect the Doppler broadening parameter (S) in Mg.OEP
molecules in any of the surroundings media, measurably; (2) The Doppler broadening
parameter in Pt.OEP molecules under UV irradiation increases steadily as the partial
pressure of oxygen in the surrounding medium increases. These data are consistent with
a lower excitation energy for P(S_) state in Mg.OEP molecules m as seen in figure-7 and even
lower probability for [02 . P(S_)]" complex formation in them. Under these conditions Ps
atoms are not formed at P(S_) sites in Mg.OEP molecules and therefore the Doppler
parameter should not be affected as seen in figure-6(a). The excitation energy of the P(S_)
state and the probability of [02 • P(S_)]" complex formation in Pt.OEP, on the other hand,
are quite adequate for the formation of Ps atoms at P(S_) sites. The formation of Ps atoms
in Pt.OEP molecules should therefore lead to an increase in the Doppler parameter S with
increasing concentration of oxygen under UV irradiation, as observed in figure-6(b).
CONCLUDING REMARKS
As indicated earlier, positron lifetime results by themselves do not appear to be
sensitive indicators of the role of the P(S_) state in O2-induced quenching of the P(Tm)-*P(S0)
transition in the Pt.OEP molecular phosphor. However, concurrent Doppler broadening
measurements clearly indicate a steady increase in the probability of positronium atom
formation at P(Sm) sites as partial pressure of oxygen in the surrounding medium increases.
The formation of [O_.P(S_)]" complexes at P(S_) sites decreases the population of the
7phosphorescingP(T1)states, thus effectively quenching the P(T_)-P(So)phosphorescence.
It is therefore concludedthat oxygenmoleculesquench the phosphorescencein the Pt.OEP
phosphor moleculesby interacting with both the lowest excited singlet states as well as the
lowest excited triplet states. These conclusions are confirmed by positron annihilation
measurements in non-phosphorescing Mg.OEP molecules.
REFERENCES
I. Martin Gouterman: Optical Spectra and Electronic Spectra of Porphyrins
and Related Rings in the Porphyrins, Vol.Ill, Edited by D. Dolphin (Academic
Press, New York)pp. 1-165.
2. M.J. Morris, J.F. Donovan, J.T. Kegelman, S.D. Schwab, R.L. Levy and
R.C. Crites: Aerodynamic Applications of Pressure sensitive Paint: AIAA Paper
# 92-0264 [Presented at the 30 _ Aerospace Sciences Meeting held at Reno, Nev.
on Jan 6-9, 1992.]
3. D.M. Oglesby: Absorption Spectra in Pt.OEP and Mg.OEP Molecules
Dissolved in CH2CI2 (unpublished).
4. J. Kavandi et al. Luminescent Barometry in Wind Tunnels; Rev. Sci. Inst.,
Vol.61(ll), 3340, 1990.
5. C.I. Eom., S.V. Naidu, S.C. Sharma and J.M. Kowalski: Positronium
Quenching via collisions with triplet states of photomagnetic molecules: Phys.
Rev. A. Vol. 43(No.7), 3247, 1991.
8Summary of Positron Lifetime Results
in (Pt.OEP)
Table- 1- In the absenceof UV- irradiation
Surrounding _l(ps)/Ii(%) Tz(ps)/12(% ) _3(ps)/13(% )
Medium
Vacuum 246-1-2/87.0+0.6 781+32/8.7+0.4 3015+129/4.3+0.3
Air 251:t: 1/89.8+0.4 870+44/6.1 +0.4 2940+ 143/4.1 :i: 0.6
Oxygen 251+1/88.3+0.5 907+61/7.2:1:0.4 2977+96/4.4+0.3
Table- 2- In the presence of UV- irradiation
Surrounding _(ps)/I_(%) z2(PS)/I t(%) f3(PS)/I3(%)
Medium
Vacuum 245+2/86.8+0.8 776+44/8.8+0.5 3029+180/4.3+0.5
Air 242+1/88.9+0.4 869+41/7.1+0.3 3023+147/4.0+0.4
Oxygen 249=1:1/88.4+0.5 927+62/7.4+0.4 3026+ 116/4.2=1:0.3
Apparently, UV- irradiation does not affect the lifetime spectrum in any one of the media
surrounding the porphyrin-coated aluminum cylinders.
Summary of the Doppler Broadening Data
in Metallized Porphyrins
9
S =Doppler Parameter =( Area of the Central Section of Peak
Total Area of the Peak
R-- S( With Light )
S( NoLight )
Table- IIl- (Mg-OEP) Phosphor
Surroun
ding
Medium
S
(No Light)
S
(With Light)
R
Vacuum 0.1086-1- 0.0007 0.1078 :t: 0.0007 0.993 + 0.006
Nitrogen 0.1092 + 0.0004 0.1084 + 0.0004 0.993 + 0.004
Air 0.1077+0.0004 0.1086+0.0004 1.008+0.003
Oxygen 0.1075+0.0007 0.1081 +0.0007 1.005+0.006
Ave: (0.1083-1-0.0006) (0.1084+0.0006) (1.000-1- 0.007)
Apparently, UV-irradiation does not affect Doppler parameter (S) in (Mg.OEP) phosphor.
Surrounding
Medium
Table- IV-
S
(No Light)
(Pt-OEP) Phosphor
S
(With Light)
R
Vacuum 0.1086 + 0.0005 0.1103 + 0.0006 1.015-1- 0.005
Nitrogen 0.1099+0.0004 0.1096+0.0004 0.997+0.004
Air 0.1093+0.0006 0.1153:i=0.0006 1.055:t:0.005
Oxygen 0.1093 + 0.0004 0.1171 -I-0.0004 1.072 =l:0.004
It appears that UV-irradiation increases the Doppler parameters (S) in (Pt.OEP) phosphor,
ie.
S =( 0. 110 _0.0006 ) *0.007 X (Oz)
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Mechanism of phosphorescence quenching in photomagnetic molecules determined by positron annihilation spectroscopy

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