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Lasing in some aromatic couples by means of energy transfer
CHEMICAL PHYSICS LETTERS
Volume 22, number 3
LASING IN SOME AROMATIC
1s
October 1973
COUPLES BY MEANS OF ENERGY TRANSFER
1.B. BERLh?AN, M. ROKNI pje Rocoh lnstitlrte of Physics, The Hebrew University, Jerusalem, Israel arid
Rcceivcd 20 August 1973
LOWthreshold Insing of pcrylene and 9, l~dipheny~an thrncene has been achieved by means of energy transfer in binary solute solutions pumped 5y a nitrogen laser. The low absorption of these compounds to the 337 1 A wcitation radiation has been overcome by the mechanism of nonradiative dipolc-dipole energy transfer. In addition, when ;I donor has ;1high molx extinction coefficient at 3371 A and good spectral overlap characteristics with an acceptor, it can Serve as an efficient donor for lasing even though ir has D low Fluorescence quantum yield. Perylene and 9,l~-di~henyl~~ra~ene are among those s~int~latin~ compounds that have ali the proper spectroscopic characteristics of a “dye” laser medium (1, Z] . Nevertheless, it has been difficult to make them lase due to a lack of a suitable pump field [3,4] . In this Ietter we report low threshold IasCngin these materials when excited via singlet-singlet energy transfer IS).
Two solutes, one a donor and the other an acceptor (either perylene or 9,l O-dipbenyl~~racene), were dissolved in a soivent, generalIy benzene. We wish to emphasize that these acceptor compounds were employed because of their known high lasing threshold when pumped directly by a nitrogen laser. The donors were chosen according to two criteria: (1) a high moIar extinction coefficient at the 3371 .& excitation wavelength, and (2) good spectraI overlap characteristics with the acceptors [5], i.e., Ru > 35 8, where R, is the Fijrster reference parameter that indicates the distance at which the probability of nonradiative energy transier in 50%. This value is for molecutes
that are stationary
during
the fluorescence
pro-
cess.
The 3371 A, 7 nsec, pulsed radiation from a modified 50 kW Avco-Eirerst: nitrogen laser was focused
by a cylindrical quartz rod of radius 4 mm, on a 5 cm long cylindrical cell containing the aerated solutions. No mirrors were employed, and lasing in the different solutions was observed in a superradi~t mode. For this excitation geometry an optimum value of the optical density of a solution was computed to be slightly less than 100. Under these conditions, the diameter of the active region in the solution was approximately 0.01 cm and its length was 4 cm. The existence of a laser beam was observed visually while the wavelength of the m~imum intensity of the lasing radiation was measured by a combination of a % ms?ter B & L monochromator, a 1P28 photomultiplier and a 454 Tektronix oscilloscope. The experimental results are summarized in table 1. As expected, the behavior in solutions with low acceptor concentrations is entirely different from those with high acceptor concentrations hecause two different energy transfer mechanisms are operative. At low acceptor concentrations (< low4 M) radiative transfer (6ctriviaI” process) is the main process and the probability
of nonradiative
transfer
(F6rster
type)
is very
At high acdeptor concentration P 10-S M) the nonradiative dipole-dipole transfer mechanism is doubt.
smaU.
Volume 22, number 3
CHEhiICAL PHYSICS LETTERS
Donor
Acceptor
woe)
Cont. (g/P) -___ 0.8 1.6 3. 3.6 2.
PPO PPO PPO ?PO bis-MSB
2. 2. 7_. 7 ;:6
perylcne perylene perylene 9,lCkDPA perylenc
-perylcne perylene 9,1 @DPAc) his-MSBd)
bis-hlSB
Table 1 transfer characteristics
Enera
----
Cont. (g/P)
-
_
-
,Qoa) (A)
-
0.05 0.E 1.6 3. 0.03
36.3 36.3 36.3 34.8 41.9
3.6
pcrylene
0.05
bis-MSB 3.6 znthracenc 5. ~~__.___-__.___-__-._-.
perylene perylene
1.6 41.3 1.6 35.3 -_----------.-.-.--___
a) The values of Ro are from ref. [ 7). b) The laser was operated between the limits of 14-22.5 c) 9,1C-DPA is 9,lCkdiphenylnnthracene. d) bis-MSB isp-bis (o-methylstyryl) benzene. e) PPO is 2,5-diphenyloxazole.
acceptor concentration the transfer processes mentioned above are insufficient to produce of the acceptor.
The acceptor
fiiter for the donor radiation. Yet times be obtained from the clo~ror
the absorption
acts here
41.9
--Lasing wavelength (A)
l-hresholdb) (keV) _--___ 22.5 19.5 21. 14. 16.5 16. 18.5 17.5 15.5 16.5
4730 4730 4350 4200 3600 3800 3600 4730 4130 4350
16.5
41QO
18.5
4470 4200 4470 4730 4530 4730
16.8 18.5 18.5 14.5 16.3
ke.V.
At low
king
15 October 1973
as a loss
lasing can somemolecules when
of the donor’s emission is sufficiently small. In addition, selective absorption of the acceptor often results in a shift in the donor’s lasing wavelength. For example, the laser bands of PPO in a benzene-PPO solution are centered at about 3600 and 3800 A. When a small amount of perylcne is added, the longer wavelength is quenched and only the 3600 A band is detected, as indicated in table 1. In another system the introduction of 0.03 g/Q of perylene in a benzene solution containing 3.6 g/Q of bisMSB the intensity of the 4200 A (bis-MSB) band is reduced and a new one at 4470 A is produced. At intermediate acceptor concentrations iasing at both the donor and acceptor wavelengths was observed concurrently. In general, tie better the spectra! overlap characteristics between the donor and acceptor compounds, the lower the acceptor concentration
needed to observe lasing from the acceptor. For example, 0.05 glQ of perylene in a benzene solution containing 3.6 g/Q of bis-MSB (R. = 41.9 8~) [7] is sufficient to ensure lasing at the 4730 A Sand of perylene as well as at the
4200
and
4470
.k bmds
of bis-
MSB (see table 1). On the other hand, 0.8 gj‘e of perylene has to be added to a benzene sorution containing 2 g/Q of PPO (R. = 36.3 A) [7] to observe king from the accep tar at approximately tie same threshold. At high acceptor concentrations nonradiative energy transfer rate is so large that it competes effectively with aLl other deexcitation channek of the donor’s first excited singlet state. This happens for a high fluorescence quantum yield donor (e.g., PPO) as well as for a low quantum yield donor (e.g., anthracene). Here lasing occurs only at the acceptor’s emission bands and the threshold for lasing is decrcr:sed as the concentration of the acceptor is increased, as shown in table 1. The limited solubitity of perylene in benzene prevented measuremerIts in sohtions with
perylene concentration
much higher than 1.6 g/Q. At 459
Volume 22, number 3
anthracene can
any one aCCeptor concentration, a rise in the threshold was observed for very high donor concentrations. This may be due to excessive diffraction losses resulting from the narrowing of the active region in the solution. and perylene
Anthracene
15 October
CHEMICAL PHYSICS LETTERS
form
the most
glet energy is used
interesting
when
as an acceptor
a suitable
donor.
to achieve
lasing
short
be achieved
transfer,
wavelen,&
This
by means each
process
excitation
compounds
solute
could
in a variety
of singlet-sin-
of these
in a binary
solution
be used
of materials
pulse.
1973
with
Under
with
in general a single
the proper
pair of compoundstreated becauseanthraceneby it-
conditionsthe Fiirster transfermechanismcan be
self does r?ol lase due to its low quantum yieid (0.36 in cyclohexane) [8], large self=rb;orption, and excited state. absorption (both singlet and triplet) [9-l I]. Perylene alone has a high lasing threshold when pumped by a nitrogen laser (see table 1) because of its relatively low absorption coefficient at 3371 A. Nevertheless, a binary solute solution containing anthracene and pr.Tlene has a low threshold for lasing at the perylene wavelength. The fairly good spectraloverlap between the anthracene fluorescence and the absorption of perylene, produces a rate of energy transfer at 1.6 ,aJQof perylcne that is much faster than the internal and external quenching processes in
made
so efficient
cence
quantum
yield
lasing
systems
as well
anthracene,
so that most
of the excitation
energy
is
periment is similar to the classic experiments of Bowen
et al.
[ 12-141
who
demonstrated
that
non-
radiative
energy transfer is a bona fide mechanism between similar pairs of compounds. The compound PPO is known to form excimers in solutions [ 151 , but in a solution with perylene or with 9, IO-diphenylanthracene the energy transfer process is much more efficient than the rate of excimer formation. Other, more favorable compounds, such as his-MSB, have a relatively low threshold when excited separately (e,g,, without acceptor) or when used as a donor in a binary solute solution containing perylene
as the acceptor
In conclusion,
(see
it has been
table
I>.
demonstrated
that
low
threshold lasing from pe_yle;le and 9,10diphenyl-
,.460.- : ‘.
.:.;
.
./
compounds
with
can be employed as those
with
a low fluoresas donors
a high
in
quantum
yield.
References [I] B.B. Snavely, Proc. IEEE 57 (1969) 1374.
[ 21 T.G. Pavlopoulos,IEEE J. Quantum Electron. QE-9 (1973) 510 [3] L.R. Lidholt and W.W. Wlndimiroff, Opto-Election. 2 (1970) 2!. [4] J.A. Myer, C.L. Johnson, E.Kierstead, R.D. Shanna and I. Itzkan,
rapidly transferredto the perylenemolecules.This P_X.
that
IS]
Appl. Phys.
Th. Fijrstcr, Fluoresenz
Lcttcrs
16 (1970)
organischer
3.
Verbindungcn
(Vandcnhoeck and Ruprecht, Giittingen, 1951) p. 83. (61 Th. FGrster, Discussions Faraday Sot. 27 (1959) 7. [7] LB. Berlman, Energy transfer parameters of aromatic compounds (Academic Press, New York, 1973). [a] I.B. Berlman, Handbook of fluorescence spectra of aromatic molecules, 2nd Ed. (Academic Press, New York, 1971) p. 356. [9] D.S. Klieger and AC. Albrecht, J. Cham. Phys. 50 (1969) 4109. [IO] C.R. Goldschmidt, R. Potashnik and hf. Ottolenghi, J. Phys Chem. 75 (1971) 1025. [Ill 1. Wieder, Appl. Phys. Letters 21 (1972) 318. 1121 E.J. Bowen and B. Brocklehurst, Trans. Faraday Sot. 49 (1953) 1131. [I31 E.J. Bowen and R. Livingston, J. Am. Chem. Sot. 76 (1954) 6300. 1141 E.J. Bowen and B. Brocklehurst, Trans. Faraday Sot. 51(1955) 774. [ISI 1.B. Bcrlman, J. Chem. Phyr 34 (19511 1083.
Volume 22, number 3
LASING IN SOME AROMATIC
1s
October 1973
COUPLES BY MEANS OF ENERGY TRANSFER
1.B. BERLh?AN, M. ROKNI pje Rocoh lnstitlrte of Physics, The Hebrew University, Jerusalem, Israel arid
Rcceivcd 20 August 1973
LOWthreshold Insing of pcrylene and 9, l~dipheny~an thrncene has been achieved by means of energy transfer in binary solute solutions pumped 5y a nitrogen laser. The low absorption of these compounds to the 337 1 A wcitation radiation has been overcome by the mechanism of nonradiative dipolc-dipole energy transfer. In addition, when ;I donor has ;1high molx extinction coefficient at 3371 A and good spectral overlap characteristics with an acceptor, it can Serve as an efficient donor for lasing even though ir has D low Fluorescence quantum yield. Perylene and 9,l~-di~henyl~~ra~ene are among those s~int~latin~ compounds that have ali the proper spectroscopic characteristics of a “dye” laser medium (1, Z] . Nevertheless, it has been difficult to make them lase due to a lack of a suitable pump field [3,4] . In this Ietter we report low threshold IasCngin these materials when excited via singlet-singlet energy transfer IS).
Two solutes, one a donor and the other an acceptor (either perylene or 9,l O-dipbenyl~~racene), were dissolved in a soivent, generalIy benzene. We wish to emphasize that these acceptor compounds were employed because of their known high lasing threshold when pumped directly by a nitrogen laser. The donors were chosen according to two criteria: (1) a high moIar extinction coefficient at the 3371 .& excitation wavelength, and (2) good spectraI overlap characteristics with the acceptors [5], i.e., Ru > 35 8, where R, is the Fijrster reference parameter that indicates the distance at which the probability of nonradiative energy transier in 50%. This value is for molecutes
that are stationary
during
the fluorescence
pro-
cess.
The 3371 A, 7 nsec, pulsed radiation from a modified 50 kW Avco-Eirerst: nitrogen laser was focused
by a cylindrical quartz rod of radius 4 mm, on a 5 cm long cylindrical cell containing the aerated solutions. No mirrors were employed, and lasing in the different solutions was observed in a superradi~t mode. For this excitation geometry an optimum value of the optical density of a solution was computed to be slightly less than 100. Under these conditions, the diameter of the active region in the solution was approximately 0.01 cm and its length was 4 cm. The existence of a laser beam was observed visually while the wavelength of the m~imum intensity of the lasing radiation was measured by a combination of a % ms?ter B & L monochromator, a 1P28 photomultiplier and a 454 Tektronix oscilloscope. The experimental results are summarized in table 1. As expected, the behavior in solutions with low acceptor concentrations is entirely different from those with high acceptor concentrations hecause two different energy transfer mechanisms are operative. At low acceptor concentrations (< low4 M) radiative transfer (6ctriviaI” process) is the main process and the probability
of nonradiative
transfer
(F6rster
type)
is very
At high acdeptor concentration P 10-S M) the nonradiative dipole-dipole transfer mechanism is doubt.
smaU.
Volume 22, number 3
CHEhiICAL PHYSICS LETTERS
Donor
Acceptor
woe)
Cont. (g/P) -___ 0.8 1.6 3. 3.6 2.
PPO PPO PPO ?PO bis-MSB
2. 2. 7_. 7 ;:6
perylcne perylene perylene 9,lCkDPA perylenc
-perylcne perylene 9,1 @DPAc) his-MSBd)
bis-hlSB
Table 1 transfer characteristics
Enera
----
Cont. (g/P)
-
_
-
,Qoa) (A)
-
0.05 0.E 1.6 3. 0.03
36.3 36.3 36.3 34.8 41.9
3.6
pcrylene
0.05
bis-MSB 3.6 znthracenc 5. ~~__.___-__.___-__-._-.
perylene perylene
1.6 41.3 1.6 35.3 -_----------.-.-.--___
a) The values of Ro are from ref. [ 7). b) The laser was operated between the limits of 14-22.5 c) 9,1C-DPA is 9,lCkdiphenylnnthracene. d) bis-MSB isp-bis (o-methylstyryl) benzene. e) PPO is 2,5-diphenyloxazole.
acceptor concentration the transfer processes mentioned above are insufficient to produce of the acceptor.
The acceptor
fiiter for the donor radiation. Yet times be obtained from the clo~ror
the absorption
acts here
41.9
--Lasing wavelength (A)
l-hresholdb) (keV) _--___ 22.5 19.5 21. 14. 16.5 16. 18.5 17.5 15.5 16.5
4730 4730 4350 4200 3600 3800 3600 4730 4130 4350
16.5
41QO
18.5
4470 4200 4470 4730 4530 4730
16.8 18.5 18.5 14.5 16.3
ke.V.
At low
king
15 October 1973
as a loss
lasing can somemolecules when
of the donor’s emission is sufficiently small. In addition, selective absorption of the acceptor often results in a shift in the donor’s lasing wavelength. For example, the laser bands of PPO in a benzene-PPO solution are centered at about 3600 and 3800 A. When a small amount of perylcne is added, the longer wavelength is quenched and only the 3600 A band is detected, as indicated in table 1. In another system the introduction of 0.03 g/Q of perylene in a benzene solution containing 3.6 g/Q of bisMSB the intensity of the 4200 A (bis-MSB) band is reduced and a new one at 4470 A is produced. At intermediate acceptor concentrations iasing at both the donor and acceptor wavelengths was observed concurrently. In general, tie better the spectra! overlap characteristics between the donor and acceptor compounds, the lower the acceptor concentration
needed to observe lasing from the acceptor. For example, 0.05 glQ of perylene in a benzene solution containing 3.6 g/Q of bis-MSB (R. = 41.9 8~) [7] is sufficient to ensure lasing at the 4730 A Sand of perylene as well as at the
4200
and
4470
.k bmds
of bis-
MSB (see table 1). On the other hand, 0.8 gj‘e of perylene has to be added to a benzene sorution containing 2 g/Q of PPO (R. = 36.3 A) [7] to observe king from the accep tar at approximately tie same threshold. At high acceptor concentrations nonradiative energy transfer rate is so large that it competes effectively with aLl other deexcitation channek of the donor’s first excited singlet state. This happens for a high fluorescence quantum yield donor (e.g., PPO) as well as for a low quantum yield donor (e.g., anthracene). Here lasing occurs only at the acceptor’s emission bands and the threshold for lasing is decrcr:sed as the concentration of the acceptor is increased, as shown in table 1. The limited solubitity of perylene in benzene prevented measuremerIts in sohtions with
perylene concentration
much higher than 1.6 g/Q. At 459
Volume 22, number 3
anthracene can
any one aCCeptor concentration, a rise in the threshold was observed for very high donor concentrations. This may be due to excessive diffraction losses resulting from the narrowing of the active region in the solution. and perylene
Anthracene
15 October
CHEMICAL PHYSICS LETTERS
form
the most
glet energy is used
interesting
when
as an acceptor
a suitable
donor.
to achieve
lasing
short
be achieved
transfer,
wavelen,&
This
by means each
process
excitation
compounds
solute
could
in a variety
of singlet-sin-
of these
in a binary
solution
be used
of materials
pulse.
1973
with
Under
with
in general a single
the proper
pair of compoundstreated becauseanthraceneby it-
conditionsthe Fiirster transfermechanismcan be
self does r?ol lase due to its low quantum yieid (0.36 in cyclohexane) [8], large self=rb;orption, and excited state. absorption (both singlet and triplet) [9-l I]. Perylene alone has a high lasing threshold when pumped by a nitrogen laser (see table 1) because of its relatively low absorption coefficient at 3371 A. Nevertheless, a binary solute solution containing anthracene and pr.Tlene has a low threshold for lasing at the perylene wavelength. The fairly good spectraloverlap between the anthracene fluorescence and the absorption of perylene, produces a rate of energy transfer at 1.6 ,aJQof perylcne that is much faster than the internal and external quenching processes in
made
so efficient
cence
quantum
yield
lasing
systems
as well
anthracene,
so that most
of the excitation
energy
is
periment is similar to the classic experiments of Bowen
et al.
[ 12-141
who
demonstrated
that
non-
radiative
energy transfer is a bona fide mechanism between similar pairs of compounds. The compound PPO is known to form excimers in solutions [ 151 , but in a solution with perylene or with 9, IO-diphenylanthracene the energy transfer process is much more efficient than the rate of excimer formation. Other, more favorable compounds, such as his-MSB, have a relatively low threshold when excited separately (e,g,, without acceptor) or when used as a donor in a binary solute solution containing perylene
as the acceptor
In conclusion,
(see
it has been
table
I>.
demonstrated
that
low
threshold lasing from pe_yle;le and 9,10diphenyl-
,.460.- : ‘.
.:.;
.
./
compounds
with
can be employed as those
with
a low fluoresas donors
a high
in
quantum
yield.
References [I] B.B. Snavely, Proc. IEEE 57 (1969) 1374.
[ 21 T.G. Pavlopoulos,IEEE J. Quantum Electron. QE-9 (1973) 510 [3] L.R. Lidholt and W.W. Wlndimiroff, Opto-Election. 2 (1970) 2!. [4] J.A. Myer, C.L. Johnson, E.Kierstead, R.D. Shanna and I. Itzkan,
rapidly transferredto the perylenemolecules.This P_X.
that
IS]
Appl. Phys.
Th. Fijrstcr, Fluoresenz
Lcttcrs
16 (1970)
organischer
3.
Verbindungcn
(Vandcnhoeck and Ruprecht, Giittingen, 1951) p. 83. (61 Th. FGrster, Discussions Faraday Sot. 27 (1959) 7. [7] LB. Berlman, Energy transfer parameters of aromatic compounds (Academic Press, New York, 1973). [a] I.B. Berlman, Handbook of fluorescence spectra of aromatic molecules, 2nd Ed. (Academic Press, New York, 1971) p. 356. [9] D.S. Klieger and AC. Albrecht, J. Cham. Phys. 50 (1969) 4109. [IO] C.R. Goldschmidt, R. Potashnik and hf. Ottolenghi, J. Phys Chem. 75 (1971) 1025. [Ill 1. Wieder, Appl. Phys. Letters 21 (1972) 318. 1121 E.J. Bowen and B. Brocklehurst, Trans. Faraday Sot. 49 (1953) 1131. [I31 E.J. Bowen and R. Livingston, J. Am. Chem. Sot. 76 (1954) 6300. 1141 E.J. Bowen and B. Brocklehurst, Trans. Faraday Sot. 51(1955) 774. [ISI 1.B. Bcrlman, J. Chem. Phyr 34 (19511 1083.