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The Laser-excited Emission Spectrum Of Tetraphenylphosphonium Tetrathioperrhenate
Volume 84, number 2
CHEMICAL
PHYSICS
LETTERS
1 December 1981
THE LASER-EXCITED EMISSION SPECl-RUM OF TETRAPHENYLPHOSPHONIUM TETRATHIOPERRHENATE Robert
D. PEACOCK
and Brian STEWART
Departmcrrr of Chemistry, The Utrirersity. Glasgow Cl2 8QQ, UK 30 July
1981
Laser cscitation
into
Received
luminescence
which
the visible
is thought
absorption
to originate
band
of [Ph$j
from viironic
1. Introduction The tetrahedral
osyanions
of the early transition
metals (titanium and the vanadium and chromium groups) are known to luminesce from a triple; state when excited into their lowesi-energy singlet absorption band [ 1 ] _The [Mn04] - and [Re04] - ions, however, have not yet been found to exhibit luminescence [3] nor, so far as we are aware, have any of the tetrahedrally coordinated thio-anions of the transition elements. While investigating the resonance Raman spectrum of [ReS4]as its tetraphenylphosphonium salt [3], we observed a fluorescence of comparable intensity to the resonance scattering when exciting (at 10 K) with laser lines of higher energy the
visible absorption
than the 04
sufficiently
was prepared as described by Miiller appears to decompose slowly in the temperature if exposed to sunlight dark. Provided the temperature is
low (80
K is suffkient
but we have not at-
tempted to determine an upper limit) the tetraphenylphosphonium salt can be irradiated with = 100 mW of visble laser light with no sign of deterioration. The luminescence spectrum shown in fig. lb is of a pressed disc containing = 1 mole % ph4P] [ReS4] in KC1 cooled to 10 K by means of a Displex closed-cycle helium gas refrigerator. The sample was excited with 300
results in a mixture of relaxed and partially relaxed excited state.
60 mW 488.0 run light from an Ar+ laser and the emitted radiation collected in a direction at 180” from that of the incident beam. Emission from above approximately 18700 cm-l is severely attenuated by self-absorption of the sample. An attempt has been made to correct
for this using the formula
I corrected 1 observed
where of the tively. 19540
[5,6]
1 - exp(-A
1 -exp[-(AL
L)
+A)]
’
A and A, are the absorbances
at the frequency emitted radiation and of the laser line respecFor our sample,,4 was estimated to be 7.5 at cm-l.
line of
band (fig. lb).
2. Experimental [Ph,P] [ReS4] et al. 141. The salt solid state at room but is stable in the
[RF&]
levels of the ‘T,
3. Results and discussion The lowest-energy absorption band, which is principally a progression in the totally symmetric stretching vibration yl(A ) (i3; = 460 cm-l) based on an origin at 19073 cm- r’ (fig. la), has been assigned as the lT2 + lA (2e+tl) transition in tetrahedral symmetry [7]. The 1T, excited state is expected to be split into IE and lB components in the S4 site symmetry which the [ReS, I - ion adopts in the tetraphenylphosphonium salt [8]. The component observed 77 cm-l above the O-O band could be assigned to one of these site-split components but such a splitting seems unreasonably large considering the small deviation from tetrahedral geometry [8] and the interval may correspond to a lattice mode. The spectrum also shows progressions in “1 based upon non-totally symmetric fundamentals of the [ReSJ - ion. 0 009-2614/81/0000-0000/J
02.75 0 1981 North-Holland
Volume 84, number 2
CHEMICAL PHYSiCS LETTERS
1 December 1981
a
b
c t : :
/i
/I/j //I/ ,
L
452 mole% [Ph#j [R&&j in Fip. 1. (a) The absorption spectrum at 10 K of a pr&d disc con-g =Z1 mole $6 PbPj dress] in KCL The slit width was 150 spectrum at 10 K of a pressed disccontaking 3 cm-” at the 488.0 nrn exciting like. The dotted line shows the luminescence intensity after correcting the sample. (c) Calculated Fran&-Condon intensity For luminescence originating from the u’ = 0 (dashed
lines) livels
of the ‘%z excited
ratios
state. The pog?ri&ions
of all the vibrbnic
levels have bezn assumed
KCl. (b) The em&s&n pm, giving a reso!ution of for the selfabsorption af (EuU Iine) and u’ = 1.2.3 equal.
Volume
CHEMICAL
84, number 2
PHYSfCS
i December
LETTERS
1981
that the lifetime of the lutqinescing state is < 100 ns. Although not conclusive, the shortness of this lifetime argues, as does all the other evidence, in favour of a singlet luminescence origin. Even below the luminescence origin there should
The total emission spectrum (luminescence + resonance Raman scattering) is shown in fig. Ib. The sharp resonance Raman features (which will be discussed fully in a subsequent publication) consist of a main progression in the v1 (Al) totaily symmetric stretching vibration of the [ReS4] - ion (i3; = 500 + 1 cm-l) and subsidiary progressions in Ye based on one quantum of a variety of non-totally symmetric fundamentals. The most notable of these in the 488.0 nm excited spectrum are the symmetric and asymmetric bends Q(E) and v4Cr2) (at = 180 cm-‘) which may be Jahn-Teller active in the excited state. No luminescence is seen when the sample (at 10 K) is excited at 530.9 nm (18836 cm-l) but luminescence is observed when 520.8 nm (1920 1 cm-l) radiation is used. This identifies the band at = 19000 cm-l
be a mixture of relaxed and partially relaxed fluorescence at different wavenumbers (Fdu,, = Zoo + u’i3’ - u“ij”) since i?‘+ is”. Fig. 1 c shows the calculated Franck-Condon intensity distribution for emission (~{u’~v”)~~/~~O~O)~~versus5u.U..) originating from vibronic levels of *Tz with u’= 0 (relaxed fluorescence) and u’ = 1,2,3 (partially relaxed fluorescence). The Franck-Condon overlap integrals were calculated by the method of Manneback [lo,1 I] for the vl vibrational coordinate, assuming a difference of 8 pm in the equih~rium Re-S bond length between the ground
in fig. lb as that
and excited
associated
with
the luminescence
112
dates.
No attempt
has been
made
as
origin. The luminescence spectrum consists of a progression in =500 cm-l on either side of this origin al-
yet to estimate the popu~tion d~st~but~on amongst the excited viironic levels of the lT2 state. The
though the emission to higher energy of = 18700 cm-t is highly attenuated due to self-absorption. Although we have attempted to apply an absorption correction in this region (dotted curve in fig. lb) the exact position of the luminescence origin remains somewhat uncertain. Extrapolation of the higher members of the progression, however, suggests a position for the Iuminescence origin which coincides with the ‘T, +- *A, absorption O-O band to within 20 cm-t. This strongly suggests that, in contrast to the oxyanions, the luminescence of [ReS4]- is from the IT2 state. Further
luminescence is obviously complicated by the presence of partly resolved secondary progressions (based on v2 and u4 as in the resonance Raman spectrum) and the presumed site splitting. The calculated intensity profiles, however, appear to be sufficiently good to encourage further exploration along these lines.
for this singlet origin comes from the observation of partially relased luminescence on the blue
Acknowledgement We thank the Royal Society for the purchase Dispiex closed-cycle helium gas refrigerator.
of the
evidence
side of the luminescence O-O band (at 19470 and 19970 cm-t) which must originate From respectively the U*= 1 and u’= 2 vibronic levels of the IT, excited state. The mixture of relaxed and partially relaxed fluorescence implies that the inverse lifetime of the Iuminescing state is comparable to the rate of non-
radiative decay. We have attempted to measure Fluorescence lifetime employing the phase-shift nique
of Imbusch
ed sinusoidally
191 using exciting
radiation
the techmodulat-
at 50 kHz. Using this frequency it should be possible to phase out selectively resonance scattering and luminescence if the iatter has a lifetime of greater than 100 ns. The entire emission spectrum, and the Rayleigh edge, were found to have the same phase reiationship with respect to the reference signal (and so the modulated excited radiation) implying 302
References i1J G. Btasse, Struct. Bonding 42 (1981)
1. Dafhoeven and G. BIasse, Chem. Phys Letters 76 (1980) 27. I.M. Coulter, R.D. Peacock and B. Stewart, to be published. A. Miiller, E. Diemann and V.V. Krishna Rao. Chem. Ber. 103 (1970) 2961.
121G.A.M. I31 [4l (51
D.P. Shriver and J.B.R.
Dunn,
AppL _.
Spectry. _
28 (1974)
319.
VI T.C. Strekas, D.H. Adams, A. Packer and T.G. Spiro, AppL Spectry.
28 (1974)
324.
171 R.H. Petit, B. Briat,A. MWer and E. Diemann, Mol. Phys. 27 (1974) 1373. (81 E. Diemann and A. hfiilier, 2. Naturforsch. 31b (1976) 1287. ISI C.F. Imbusch, in: Luminescence of inorganic solids, ed. B. Di Bartolo
(Plenum Press, New York,
1978)
p. 13.5.
[lOI C. Illumeback, Physica 17 (1951) 1001. IllI R.J.H. Clark and B. Stewart, J. Am. Chem. Sot.. to be published .
CHEMICAL
PHYSICS
LETTERS
1 December 1981
THE LASER-EXCITED EMISSION SPECl-RUM OF TETRAPHENYLPHOSPHONIUM TETRATHIOPERRHENATE Robert
D. PEACOCK
and Brian STEWART
Departmcrrr of Chemistry, The Utrirersity. Glasgow Cl2 8QQ, UK 30 July
1981
Laser cscitation
into
Received
luminescence
which
the visible
is thought
absorption
to originate
band
of [Ph$j
from viironic
1. Introduction The tetrahedral
osyanions
of the early transition
metals (titanium and the vanadium and chromium groups) are known to luminesce from a triple; state when excited into their lowesi-energy singlet absorption band [ 1 ] _The [Mn04] - and [Re04] - ions, however, have not yet been found to exhibit luminescence [3] nor, so far as we are aware, have any of the tetrahedrally coordinated thio-anions of the transition elements. While investigating the resonance Raman spectrum of [ReS4]as its tetraphenylphosphonium salt [3], we observed a fluorescence of comparable intensity to the resonance scattering when exciting (at 10 K) with laser lines of higher energy the
visible absorption
than the 04
sufficiently
was prepared as described by Miiller appears to decompose slowly in the temperature if exposed to sunlight dark. Provided the temperature is
low (80
K is suffkient
but we have not at-
tempted to determine an upper limit) the tetraphenylphosphonium salt can be irradiated with = 100 mW of visble laser light with no sign of deterioration. The luminescence spectrum shown in fig. lb is of a pressed disc containing = 1 mole % ph4P] [ReS4] in KC1 cooled to 10 K by means of a Displex closed-cycle helium gas refrigerator. The sample was excited with 300
results in a mixture of relaxed and partially relaxed excited state.
60 mW 488.0 run light from an Ar+ laser and the emitted radiation collected in a direction at 180” from that of the incident beam. Emission from above approximately 18700 cm-l is severely attenuated by self-absorption of the sample. An attempt has been made to correct
for this using the formula
I corrected 1 observed
where of the tively. 19540
[5,6]
1 - exp(-A
1 -exp[-(AL
L)
+A)]
’
A and A, are the absorbances
at the frequency emitted radiation and of the laser line respecFor our sample,,4 was estimated to be 7.5 at cm-l.
line of
band (fig. lb).
2. Experimental [Ph,P] [ReS4] et al. 141. The salt solid state at room but is stable in the
[RF&]
levels of the ‘T,
3. Results and discussion The lowest-energy absorption band, which is principally a progression in the totally symmetric stretching vibration yl(A ) (i3; = 460 cm-l) based on an origin at 19073 cm- r’ (fig. la), has been assigned as the lT2 + lA (2e+tl) transition in tetrahedral symmetry [7]. The 1T, excited state is expected to be split into IE and lB components in the S4 site symmetry which the [ReS, I - ion adopts in the tetraphenylphosphonium salt [8]. The component observed 77 cm-l above the O-O band could be assigned to one of these site-split components but such a splitting seems unreasonably large considering the small deviation from tetrahedral geometry [8] and the interval may correspond to a lattice mode. The spectrum also shows progressions in “1 based upon non-totally symmetric fundamentals of the [ReSJ - ion. 0 009-2614/81/0000-0000/J
02.75 0 1981 North-Holland
Volume 84, number 2
CHEMICAL PHYSiCS LETTERS
1 December 1981
a
b
c t : :
/i
/I/j //I/ ,
L
452 mole% [Ph#j [R&&j in Fip. 1. (a) The absorption spectrum at 10 K of a pr&d disc con-g =Z1 mole $6 PbPj dress] in KCL The slit width was 150 spectrum at 10 K of a pressed disccontaking 3 cm-” at the 488.0 nrn exciting like. The dotted line shows the luminescence intensity after correcting the sample. (c) Calculated Fran&-Condon intensity For luminescence originating from the u’ = 0 (dashed
lines) livels
of the ‘%z excited
ratios
state. The pog?ri&ions
of all the vibrbnic
levels have bezn assumed
KCl. (b) The em&s&n pm, giving a reso!ution of for the selfabsorption af (EuU Iine) and u’ = 1.2.3 equal.
Volume
CHEMICAL
84, number 2
PHYSfCS
i December
LETTERS
1981
that the lifetime of the lutqinescing state is < 100 ns. Although not conclusive, the shortness of this lifetime argues, as does all the other evidence, in favour of a singlet luminescence origin. Even below the luminescence origin there should
The total emission spectrum (luminescence + resonance Raman scattering) is shown in fig. Ib. The sharp resonance Raman features (which will be discussed fully in a subsequent publication) consist of a main progression in the v1 (Al) totaily symmetric stretching vibration of the [ReS4] - ion (i3; = 500 + 1 cm-l) and subsidiary progressions in Ye based on one quantum of a variety of non-totally symmetric fundamentals. The most notable of these in the 488.0 nm excited spectrum are the symmetric and asymmetric bends Q(E) and v4Cr2) (at = 180 cm-‘) which may be Jahn-Teller active in the excited state. No luminescence is seen when the sample (at 10 K) is excited at 530.9 nm (18836 cm-l) but luminescence is observed when 520.8 nm (1920 1 cm-l) radiation is used. This identifies the band at = 19000 cm-l
be a mixture of relaxed and partially relaxed fluorescence at different wavenumbers (Fdu,, = Zoo + u’i3’ - u“ij”) since i?‘+ is”. Fig. 1 c shows the calculated Franck-Condon intensity distribution for emission (~{u’~v”)~~/~~O~O)~~versus5u.U..) originating from vibronic levels of *Tz with u’= 0 (relaxed fluorescence) and u’ = 1,2,3 (partially relaxed fluorescence). The Franck-Condon overlap integrals were calculated by the method of Manneback [lo,1 I] for the vl vibrational coordinate, assuming a difference of 8 pm in the equih~rium Re-S bond length between the ground
in fig. lb as that
and excited
associated
with
the luminescence
112
dates.
No attempt
has been
made
as
origin. The luminescence spectrum consists of a progression in =500 cm-l on either side of this origin al-
yet to estimate the popu~tion d~st~but~on amongst the excited viironic levels of the lT2 state. The
though the emission to higher energy of = 18700 cm-t is highly attenuated due to self-absorption. Although we have attempted to apply an absorption correction in this region (dotted curve in fig. lb) the exact position of the luminescence origin remains somewhat uncertain. Extrapolation of the higher members of the progression, however, suggests a position for the Iuminescence origin which coincides with the ‘T, +- *A, absorption O-O band to within 20 cm-t. This strongly suggests that, in contrast to the oxyanions, the luminescence of [ReS4]- is from the IT2 state. Further
luminescence is obviously complicated by the presence of partly resolved secondary progressions (based on v2 and u4 as in the resonance Raman spectrum) and the presumed site splitting. The calculated intensity profiles, however, appear to be sufficiently good to encourage further exploration along these lines.
for this singlet origin comes from the observation of partially relased luminescence on the blue
Acknowledgement We thank the Royal Society for the purchase Dispiex closed-cycle helium gas refrigerator.
of the
evidence
side of the luminescence O-O band (at 19470 and 19970 cm-t) which must originate From respectively the U*= 1 and u’= 2 vibronic levels of the IT, excited state. The mixture of relaxed and partially relaxed fluorescence implies that the inverse lifetime of the Iuminescing state is comparable to the rate of non-
radiative decay. We have attempted to measure Fluorescence lifetime employing the phase-shift nique
of Imbusch
ed sinusoidally
191 using exciting
radiation
the techmodulat-
at 50 kHz. Using this frequency it should be possible to phase out selectively resonance scattering and luminescence if the iatter has a lifetime of greater than 100 ns. The entire emission spectrum, and the Rayleigh edge, were found to have the same phase reiationship with respect to the reference signal (and so the modulated excited radiation) implying 302
References i1J G. Btasse, Struct. Bonding 42 (1981)
1. Dafhoeven and G. BIasse, Chem. Phys Letters 76 (1980) 27. I.M. Coulter, R.D. Peacock and B. Stewart, to be published. A. Miiller, E. Diemann and V.V. Krishna Rao. Chem. Ber. 103 (1970) 2961.
121G.A.M. I31 [4l (51
D.P. Shriver and J.B.R.
Dunn,
AppL _.
Spectry. _
28 (1974)
319.
VI T.C. Strekas, D.H. Adams, A. Packer and T.G. Spiro, AppL Spectry.
28 (1974)
324.
171 R.H. Petit, B. Briat,A. MWer and E. Diemann, Mol. Phys. 27 (1974) 1373. (81 E. Diemann and A. hfiilier, 2. Naturforsch. 31b (1976) 1287. ISI C.F. Imbusch, in: Luminescence of inorganic solids, ed. B. Di Bartolo
(Plenum Press, New York,
1978)
p. 13.5.
[lOI C. Illumeback, Physica 17 (1951) 1001. IllI R.J.H. Clark and B. Stewart, J. Am. Chem. Sot.. to be published .