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Studies of the triplet state of coumarin 102 laser dye by pulse radiolysis
Volume 157, number 6
STUDIES
CHEMICAL
26 May 1989
PHYSICS LETTERS
OF THE TIUPLET STATE OF COUMARIN
102 LASER DYE
BY PULSE RADIOLYSIS K.I. PRIYADARSINI,
D.B. NAIK and P.N. MOORTHY
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085. India Received 3 November
1988; in final form 3 February
1989
The absorption spectrum of the triplet state of the laser dye coumarin 102 (C 102) has been recorded in benzene solution employing the pulse radiolysis technique. Triplet formation was confirmed and its extinction coefficient determined by energy transfer from known triplet sensitizers. The triplet energy of C 102 has been estimated to be 57.5 * 1S kcal mol-‘. the C 102 triplet are compared with those of its trifluoro analogue coumarin I53 (C I53).
Properties
of
1. Introduction
Pulse radiolysis is useful for the study of triplet states of organic molecules having low intersystem crossing efficiency, and thus difficult to observe by flash photolysis. This is due to the fact that in pulse radiolysis the triplet is not necessarily formed from the singlet excited state by intersystem crossing but can also be produced by other processes such as energy transfer from solvent triplet and ion recombinalion [ 1 ]_ ‘I-amino coumarin dyes which are known to lase efficiently in the blue/green region are characterised by a high fluorescence quantum yield and low triplet yield [ 21. Knowledge of the properties of triplets of these dyes is important as the triplet absorption (which generally overlaps with the fluorescence spectrum) can reduce the laser output. Radiolysis of benzene is known to generate triplet benzene in high yield [ 31 and hence benzene is a good solvent for studying the triplets of solute molecules which are formed by energy transfer from solvent triplets. In the present paper we report the results of our investigation of the triplet properties of coumarin laser dye C 102 (scheme 1) in benzene and compare this dye its trifluoro analogue C 153.
0 009-2614f 89/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
(NORMAL CR-
CH 3,
(ICT)
STRUCTURE) C102)
Scheme 1. Structures
(R-
CF3,
C153)
of C IO2 and C I53 in normal and ICT
states.
2. Experimental C 102 was synthesised and purified to laser grade quality in our laboratory [ 41. The benzene used was of spectrograde quality (Spectra Chem Pvt Ltd., Bombay) and biphenyl (SISCO Ltd., Bombay) was purified by recrystallisation from ethanol. Scintillation grade naphthalene, Fluka puriss grade anthracene and trans-stilbene were used as supplied. Chrysene (F’luka, pract grade) was purified by recrystallisation from methanol. The solutions were purged with IOLAR grade N2 (Indian Oxygen Ltd., Bombay) for 20 min in a 1 cm x 1 cm suprasil pulse radiolysis cuvette using appropriate pretraps immediately prior to the irradiation and were sealed with a “parafilm”. Details of the pulse radiolysis apparatus have been described elsewhere [ 51. For spectral studies 2 KS B.V.
525
species. At higher doses there was progressive deviation from first-order kinetics. At wavelengths where light absorption by the ground state is much higher compared to transient absorption, a bleaching signal (negative signal) is observed immediately after the pulse the recovery of which also obeys a first-order decay law (at low doses) with about the same half-life as that of the transient absorption signal. The recovery is 100% indicating that the product of the transient decay is the ground state molecule itself in 1: 1 stoichiometry. The transient absorption disappears in oxygenated solutions at all wavelengths, showing the high reactivity of oxygen towards the transient or its precursor. Suppression of transient absorption in the presence of oxygen, its first-order decay and 100% recovery of the ground state when the transient decays completely suggest that it could be a triplet of C 102. Furthermore the spectrum obtained via energy transfer from biphenyl triplet was found to be identical with that obtained directly by pulse radiolysis (fig. 1) confirming this conclusion.
pulses of 7 MeV electrons
were utilized, whereas for evaluation of kinetic parameters 25 ns electron pulses were used. Pulse doses were measured using aqueous thiocyanate dosimeter (0.05 mol dmp3) applying electron density corrections, Typical doses of 2 us and 25 ns pulses were 8x 10” eV cmp3 and 1.6 x 10” eV crne3, respectively.
3. Results and discussion 3.1. Triplet-triplet spectra Following pulse irradiation of deoxygenated lo-’ mol dm-3 solutions of C 102 in benzene a transient species is produced which absorbs light in the 450750 nm region. The spectrum of this species (fig. 1) consists of a single peak with R,, around 580 nm. At low doses (1.6~ lOL7 eV cm--‘) the transient decays by first-order kinetics with t, ,* = 13.6 ps over the entire region, showing that it consists of a single
t
0.06
-
0.05
-
26 May 1989
CHEMICAL PHYSICS LETTERS
Volume 157, number 6
d z
0.04-
0.03
0
-
.o 430
I
450
I
500
I
I
I
I
550
600
650
700
WAVELENGTH
(nm)
_
Fig. 1.Triplet-triplet absorption spectrum of C 102 triplet obtained by (0) e-beam pulsing of C IO2in benzene and (A 1energytransfer from pulse radiolytically generated biphenyl triplet,
526
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CHEMICAL
Quantitative agreement between the two is just coincidental because of the relative concentrations of biphenyl and C 102 used and the electron pulse dose. The decay of the transient was monitored as a function of the absorbed dose. At very low doses it decayed by good first-order kinetics with a rate constant of 5.1 x lo4 s-l. The decay rate remained unchanged up to a dose of 2 x 10” eV cm- ‘. We consider this to be the intrinsic decay rate of the triplet. However, at higher doses it deviates from first-order behaviour. By computer analysis of the decay curve according to the procedure given in ref. [ 61, it was found that with increasing dosage there was a progressively increasing contribution from a second-order component, presumably due to triplet-triplet annihilation. From an analysis of the decay curves at high doses the rate constant for this process was estimated to be 8.8x 10’ dm3 mol-’ s-l (using a value of 14000 dm3 mol-’ cm-’ for the extinction coefficient of C 102 triplet) and that for the first-order process was found to be 5.9 x 1O4s- ‘. The latter value agrees well with the value determined directly from a first-order fit of decay traces at low doses. Triplettriplet annihilation is a spin-allowed process with rate constants close to the diffusion-controlled limit. Our experimentally determined value is found to be within a factor of three of the diffusion-controlled limit in benzene. 3.2. Extinction coeficient measurements The triplet nature of the species was confirmed by energy transfer from known donor triplets. The energy transfer rate constants for different donor-acceptor couples were determined by following either the pseudo-first-order decay of the donor triplet or the pseudo-first-order formation of acceptor triplet. For this purpose, the experimental conditions were chosen so that initially only donor triplets are formed and these then transfer energy to the C 102 molecules. The observed pseudo-first-order decay rate constant k3 of the donor triplet at acceptor concentration [A] is given by k, =k, +kl
[Al >
(1)
where k, is the first-order decay rate constant of the donor triplet in the absence of acceptor and k2 is the
PHYSICS LETTERS
bimolecular process
26 May 1989
energy
transfer
rate constant
for the
h
3D*+A -
Df
3A*.
(2)
The dependence of the maximum absorbance of the acceptor triplet (OD:) on the acceptor concentration is given by [ 7 ] l/OD;;=(G/G)
(l/OX,,)
(l+Wk,[Al), (3)
where OD:, is the absorbance due to acceptor triplet and OD & is the absorbance due to the donor triplet in the absence of acceptor. e*o and er\ are the extinction coefftcients of the donor and acceptor triplets, respectively. Figs. 2a-2c give oscilloscope traces for the decay of biphenyl triplet at 345 nm in the presence of various concentrations of C 102. The decay rate increases with increasing concentrations of C 102. Figs. 2d and 2e show the build-up of absorbance of C 102 triplet at 580 nm at two different concentrations of the acceptor. Again the build-up increases with increasing concentration of C 102. These results clearly indicate the occurrence of energy transfer between biphenyl triplet and C 102. By monitoring the decay rate of biphenyl triplet as a function of C 102 concentration (eq. (1) ), the energy transfer rate con‘ stant from biphenyl to C 102 was found to be 1 x 10O dm3 mol-’ s-l. Fig. 3b gives a linear plot of 1/ODm,, for C 102 triplet as a function of 1/ [C 102 ] expected on the basis of eq. (3). From the intercept of this linear plot and taking the value e*,=27100dm3mol-‘cm-‘forbiphenyltriplet [8], the extinction coefficient for C 102 triplet was calculated to be 13900 dm3 mol-’ cm-‘. This value is comparable with the value of 14200 cm3 mol- ’ cm-’ determined by measuring the extent of ground state bleaching at 395 nm in 5 x iO-’ mol dm-3 solutions of C 102 in benzene. 3.3. Energy transfer from benzene to C 102 The energy transfer rate constant from benzene to C 102 cannot be determined directly by following the decay of benzene triplet because of its very short lifetime [ I]. It was determined indirectly by making use of eq. (3 ). From transient absorbance measure527
26 May 1989
CHEMICAL PHYSICS LETTERS
Volume 151. number 6
-
I
X i0-3dm3mol
-1
[c 1023
9
100e
2888
3088
46BB
5880
Tine(nanosec.)
Fig. 2. Transient signals of pulse radiolytically generated biphenyl triplet at 345 nm (a) in the absence and (b) in the prescncc of 4x lo-’ and (c) 8~10~~ mol dmm3 C 102. (A small negative signal towards the end of the decay in (c) is due to bleaching ofground state C 102; this has been taken into account in evaluating the rate constants.) Build-up of C 102 triplet absorption at 580 nm as a result of energy transfer from biphenyl triplet at C I02 concentrationsof (d) 4x I0-sand (e) 8x IO-’ mol drn-’
ments on C 102 triplet formed in benzene at different concentrations of C 102 (at constant dose) the slope/intercept ratio of a plot of 1/absorbance versus 1/[C 102 ] was found to be 4.9 X 10m3. From this and knowing t,,z of benzene triplet in neat benzene (3.4 ns) the rate constant for energy transfer was calculated to be 4x 10” dm3 molP’ s’.
Oo4 -[c
1 X lO-4 dm3mol-1 1021
Fig. 3. Variation of reciprocal absorbance of C IO2 triplet at 580 urn as a function of 1/ [C 1021 produced hy energy transfer from (a) pulse radiolytically generated benzene triplet and (b) biphcnyl triplct.
Table 1 Energy transfer rate constants from various donors to C 102
3.4. Triplet energy level of C 102 As biphenyl triplet (T, ) with energy ET=65 kcal mol- ’ is able to sensitize C 102 triplet which in turn is quenched by 0, (presumably due to energy transfer), we can infer that the C 102 triplet energy must be between 63 and 25 kcal mol-‘. Table I gives the 528
Donor molecule
Energy level (kcal mol- ’ )
Energy transfer rate constant (dm3 mol-’ s-‘)
benzene biphenyl naphthalene
82 65 6I
4 x 1O’O
I XIO’O 1.4x IO’O
Volume 157, number 6
CHEMICAL PHYSICS LETTERS
energy transfer rate constants for different donor molecules which sensitize C 102 triplet together with their triplet energy levels. Triplet naphthalene was found to efficiently sensitize C 102 triplet. On the other hand C 102 triplet was found to sensitize transstilbene, pyrene, anthracene and benzil triplets (table 2). This would imply that triplet energy level of C 102 is below that of naphthalene (I!&= 61 kcal mol- ’ ) and above that of benzil (& = 54 kcal mol- ’ ). Thus the C 102 triplet energy can be inferred to be 57.5 f 1.5 kcal mol-‘. In the case of the chrysene (ET= 47 kcal mol- ’ )-C 102 pair, there was no energy transfer observable in either direction. 3.5. Comparison ofC 102 triplet properties with those of C 153 triplet C 153 is the trifluoro analogue of C 102 (scheme 1) and as a laser dye has better photostability and a broader tuning range. Earlier we reported the results of our studies on the triplet characteristics of C 153 by pulse radiolysis [ 7 1. The singlet and triplet characteristics of these two dyes are compared in table 3. The S, state in 7-amino coumarin is represented as the fully charge separated state known as the in-
26 May 1989
tramolecular charge transfer (ICT) state [ 91 (scheme I). If any substituent on the coumarin ring can stabilise the ICT state, then the S, energy level is lowered. There is an electron withdrawing CF, group on the pyran ring of C 153, which helps delocalise the negative charge in the ring [lo] and hence the S, level is expected to be stabilised more in C 153 than in C 102. Consequently the absorption and fluorescence maxima are both red-shifted in the former. Our present results reveal that the triplet level of C 102 is higher than that of C 15 3. Thus it is seen that, as for the singlet excited states of these compounds, the energy of the lowest triplet levels also decrease on replacing the 4-CH, group by 4-CF,. Hence we may conclude that the T, states of C 102 and C 153 also have ICT character like the S, states. The energy gap between the singlet and triplet levels is nearly the same for these two dyes (table 3). Hence one can expect the singlet-triplet transition to have nearly the same probability. Our experimentally observed G value for triplet formation at a given concentration of the dye for these two compounds in benzene is nearly the same (table 3). This, however, may not be the consequence solely of a similar S-T energy gap. Considering the reactions k4
Table 2 Rate constants for energy transfer from C 102 to various acceptors Acceptor molecule
Energy level (kcal mol- ’ )
Energy transfer rate constant (dm’ mol-' s-‘)
anthracene ?-stilbene pyrene bcnzil
42 49 50 54
l.lxlo’” 7.4x IO9 4.6X IO4 6.7x 10”
‘B*+C-
B+3C*,
(4)
B+B (blradlcal)’
(5)
k5 3B*+B -
where C and 3C* are respectively the coumarin dye and its triplet, B and %* are respectively benzene and benzene triplet, and T(~B*) is the intrinsic lifetime of the benzene triplet, the G value for C 102 triplet is given by
Table 3 Sin&t and triplet characteristics of C 153 and C 102 in benzene
G(3C*)=G(3B*)
b[Cl kb[C] fk,
[B]+ ~/T(~B*)
(6) Property
c 153”’
c 102 b’
Es (kcal mol-I) & (kcalmol-‘) k, Ltnplet benzene + dye (dm3 mol-’ SK’) G value for dye triplet at IO-’ mol dm-’
72 5Ok2.5
79 57.5? 1.5
a) Ref. [ 71. ‘) Present work.
6x 10”
4x 10’0
1.22
1.17
The rate constant for biradical formation (eq. ( 5 ) ) is reported to be 1.8X107 dm3 mol-’ s-’ [ll]. G( 3B*) in benzene radiolysis [3] is 4.2 and T(~B*) is 3.4 ns [ 11. Using these values and our experimentally determined k, values for the two dyes C 102 (present work) and C 153 (ref. [ 71) the G values for triplets of these two compounds are calculated to 529
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CHEMICAL
PHYSICS LETTERS
26 May 1989
be 1.95 and 2.37. From eq. (6) it can be seen that the difference in G for the two compounds is due to differences in their k4 values. On the other hand the G values directly determined from the measured absorbance, dose and extinction coefficient are 1.17 and 1.22 respectively for C 102 and C 153. The difference between these and the calculated values is within the limits of the uncertainties involved in the estimation of various quantities used in these calculations.
The authors wish to record their appreciation of the encouragement and support from Dr. J.P. Mittal and Dr. R.M. Iyer.
4. Conclusions
References
Coumarin laser dye C 102 is found to have a longlived triplet state which can be populated by energy transfer from other higher lying triplets. The triplet spectrum with A,,,,, at 600 nm is broad and overlaps the fluorescence maximum at 484 nm. Hence direct population of the triplet by photoexcitation, however small due to low ISC probability, can cause quenching of the fluorescence and hence impair laser performance, unless the triplets are scavenged. Our studies indicate that any triplet quencher with energy levels below 54 kcal mall ’ can efficiently quench the triplets of C 102. Thus in actual laser operation one can choose a suitable triplet quencher. These results are also useful in selecting proper donor-acceptor pairs for energy transfer dye lasers. Thus for example the tram-stilbene/C 102 pair is used by Kotowski et al. [ 121 to produce lasers in which singlet excited trans-stilbene transfers energy to C 102 and lasing occurs from the singlet levels of C 102. Our results show that C 102 triplets can transfer energy very efficiently to trans-stilbene. Thus in this energy transfer dye laser system, trans-stilbene not
530
only acts as donor for C 102 singlet, but alsoBcts as a quencher for C 102 triplets, which could otherwise impair laser operation.
Acknowledgement
[ 1 ] R. Bensasson and E.J. Land, in: Photochemical and photobiological reviews, Vol. 3, ed. KC. Smith (Plenum Press, New York, 197R ) p. 163. [2] K.H. Drexhage, in: Topics in applied physics. Dye Lasers, Vol. 1,ed. F.P. Schafer (Springer,Berlin, 1973 ) p. 16 1. [ 31 J.H. Baxendale and M. Fiti, J. Chem. Sot. Faraday Trans. II68 (1972)218. [ 41 K.N. Rao, P.N. Moonhy, C.V.C. Prasad, J.T. Kunjappu and K.I. Priyadarsini, BARC Report, BARC-1400 (I 988). [5]S.N.Guha, P.N. Moorthy, K. Kishore, D.B. Naikand K.N. Rao, Proc. Indian Acad. Sci. Chcm. Sci. 99 (1987) 261. [6] N.E. Shank, Intern. J. Chem. Kinetics 5 (1973) 577. [ 71 K.I. Priyadarsini, D.B. Naik and P.N. Moorthy, J. Photochem. Photobiol., in press. [ 8] R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 ( 1971) 1904. [ 91 G. Jones II, W.R. Jackson and A.M. Halpcm, Chcm. Phys. Letters 72 ( 1980) 39 1. [IO] R. Steppel, in: Organic dye lasers, CRC handbook of laser science and technology, Vol. 1. Lasers and masers, ed. M.J. Weber (CRC Press, Boca Raton, 1982 ) p. 299. [ 111 H. Gorner and S. Schulte-Frohlinde, J. Phys. Chcm. 85 (1981) 1835. [ 121 T. Kotowski, W. Majewski and W. Skubiszak, Opt. Appl. 14 (1984) 515.
STUDIES
CHEMICAL
26 May 1989
PHYSICS LETTERS
OF THE TIUPLET STATE OF COUMARIN
102 LASER DYE
BY PULSE RADIOLYSIS K.I. PRIYADARSINI,
D.B. NAIK and P.N. MOORTHY
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085. India Received 3 November
1988; in final form 3 February
1989
The absorption spectrum of the triplet state of the laser dye coumarin 102 (C 102) has been recorded in benzene solution employing the pulse radiolysis technique. Triplet formation was confirmed and its extinction coefficient determined by energy transfer from known triplet sensitizers. The triplet energy of C 102 has been estimated to be 57.5 * 1S kcal mol-‘. the C 102 triplet are compared with those of its trifluoro analogue coumarin I53 (C I53).
Properties
of
1. Introduction
Pulse radiolysis is useful for the study of triplet states of organic molecules having low intersystem crossing efficiency, and thus difficult to observe by flash photolysis. This is due to the fact that in pulse radiolysis the triplet is not necessarily formed from the singlet excited state by intersystem crossing but can also be produced by other processes such as energy transfer from solvent triplet and ion recombinalion [ 1 ]_ ‘I-amino coumarin dyes which are known to lase efficiently in the blue/green region are characterised by a high fluorescence quantum yield and low triplet yield [ 21. Knowledge of the properties of triplets of these dyes is important as the triplet absorption (which generally overlaps with the fluorescence spectrum) can reduce the laser output. Radiolysis of benzene is known to generate triplet benzene in high yield [ 31 and hence benzene is a good solvent for studying the triplets of solute molecules which are formed by energy transfer from solvent triplets. In the present paper we report the results of our investigation of the triplet properties of coumarin laser dye C 102 (scheme 1) in benzene and compare this dye its trifluoro analogue C 153.
0 009-2614f 89/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
(NORMAL CR-
CH 3,
(ICT)
STRUCTURE) C102)
Scheme 1. Structures
(R-
CF3,
C153)
of C IO2 and C I53 in normal and ICT
states.
2. Experimental C 102 was synthesised and purified to laser grade quality in our laboratory [ 41. The benzene used was of spectrograde quality (Spectra Chem Pvt Ltd., Bombay) and biphenyl (SISCO Ltd., Bombay) was purified by recrystallisation from ethanol. Scintillation grade naphthalene, Fluka puriss grade anthracene and trans-stilbene were used as supplied. Chrysene (F’luka, pract grade) was purified by recrystallisation from methanol. The solutions were purged with IOLAR grade N2 (Indian Oxygen Ltd., Bombay) for 20 min in a 1 cm x 1 cm suprasil pulse radiolysis cuvette using appropriate pretraps immediately prior to the irradiation and were sealed with a “parafilm”. Details of the pulse radiolysis apparatus have been described elsewhere [ 51. For spectral studies 2 KS B.V.
525
species. At higher doses there was progressive deviation from first-order kinetics. At wavelengths where light absorption by the ground state is much higher compared to transient absorption, a bleaching signal (negative signal) is observed immediately after the pulse the recovery of which also obeys a first-order decay law (at low doses) with about the same half-life as that of the transient absorption signal. The recovery is 100% indicating that the product of the transient decay is the ground state molecule itself in 1: 1 stoichiometry. The transient absorption disappears in oxygenated solutions at all wavelengths, showing the high reactivity of oxygen towards the transient or its precursor. Suppression of transient absorption in the presence of oxygen, its first-order decay and 100% recovery of the ground state when the transient decays completely suggest that it could be a triplet of C 102. Furthermore the spectrum obtained via energy transfer from biphenyl triplet was found to be identical with that obtained directly by pulse radiolysis (fig. 1) confirming this conclusion.
pulses of 7 MeV electrons
were utilized, whereas for evaluation of kinetic parameters 25 ns electron pulses were used. Pulse doses were measured using aqueous thiocyanate dosimeter (0.05 mol dmp3) applying electron density corrections, Typical doses of 2 us and 25 ns pulses were 8x 10” eV cmp3 and 1.6 x 10” eV crne3, respectively.
3. Results and discussion 3.1. Triplet-triplet spectra Following pulse irradiation of deoxygenated lo-’ mol dm-3 solutions of C 102 in benzene a transient species is produced which absorbs light in the 450750 nm region. The spectrum of this species (fig. 1) consists of a single peak with R,, around 580 nm. At low doses (1.6~ lOL7 eV cm--‘) the transient decays by first-order kinetics with t, ,* = 13.6 ps over the entire region, showing that it consists of a single
t
0.06
-
0.05
-
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Volume 157, number 6
d z
0.04-
0.03
0
-
.o 430
I
450
I
500
I
I
I
I
550
600
650
700
WAVELENGTH
(nm)
_
Fig. 1.Triplet-triplet absorption spectrum of C 102 triplet obtained by (0) e-beam pulsing of C IO2in benzene and (A 1energytransfer from pulse radiolytically generated biphenyl triplet,
526
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CHEMICAL
Quantitative agreement between the two is just coincidental because of the relative concentrations of biphenyl and C 102 used and the electron pulse dose. The decay of the transient was monitored as a function of the absorbed dose. At very low doses it decayed by good first-order kinetics with a rate constant of 5.1 x lo4 s-l. The decay rate remained unchanged up to a dose of 2 x 10” eV cm- ‘. We consider this to be the intrinsic decay rate of the triplet. However, at higher doses it deviates from first-order behaviour. By computer analysis of the decay curve according to the procedure given in ref. [ 61, it was found that with increasing dosage there was a progressively increasing contribution from a second-order component, presumably due to triplet-triplet annihilation. From an analysis of the decay curves at high doses the rate constant for this process was estimated to be 8.8x 10’ dm3 mol-’ s-l (using a value of 14000 dm3 mol-’ cm-’ for the extinction coefficient of C 102 triplet) and that for the first-order process was found to be 5.9 x 1O4s- ‘. The latter value agrees well with the value determined directly from a first-order fit of decay traces at low doses. Triplettriplet annihilation is a spin-allowed process with rate constants close to the diffusion-controlled limit. Our experimentally determined value is found to be within a factor of three of the diffusion-controlled limit in benzene. 3.2. Extinction coeficient measurements The triplet nature of the species was confirmed by energy transfer from known donor triplets. The energy transfer rate constants for different donor-acceptor couples were determined by following either the pseudo-first-order decay of the donor triplet or the pseudo-first-order formation of acceptor triplet. For this purpose, the experimental conditions were chosen so that initially only donor triplets are formed and these then transfer energy to the C 102 molecules. The observed pseudo-first-order decay rate constant k3 of the donor triplet at acceptor concentration [A] is given by k, =k, +kl
[Al >
(1)
where k, is the first-order decay rate constant of the donor triplet in the absence of acceptor and k2 is the
PHYSICS LETTERS
bimolecular process
26 May 1989
energy
transfer
rate constant
for the
h
3D*+A -
Df
3A*.
(2)
The dependence of the maximum absorbance of the acceptor triplet (OD:) on the acceptor concentration is given by [ 7 ] l/OD;;=(G/G)
(l/OX,,)
(l+Wk,[Al), (3)
where OD:, is the absorbance due to acceptor triplet and OD & is the absorbance due to the donor triplet in the absence of acceptor. e*o and er\ are the extinction coefftcients of the donor and acceptor triplets, respectively. Figs. 2a-2c give oscilloscope traces for the decay of biphenyl triplet at 345 nm in the presence of various concentrations of C 102. The decay rate increases with increasing concentrations of C 102. Figs. 2d and 2e show the build-up of absorbance of C 102 triplet at 580 nm at two different concentrations of the acceptor. Again the build-up increases with increasing concentration of C 102. These results clearly indicate the occurrence of energy transfer between biphenyl triplet and C 102. By monitoring the decay rate of biphenyl triplet as a function of C 102 concentration (eq. (1) ), the energy transfer rate con‘ stant from biphenyl to C 102 was found to be 1 x 10O dm3 mol-’ s-l. Fig. 3b gives a linear plot of 1/ODm,, for C 102 triplet as a function of 1/ [C 102 ] expected on the basis of eq. (3). From the intercept of this linear plot and taking the value e*,=27100dm3mol-‘cm-‘forbiphenyltriplet [8], the extinction coefficient for C 102 triplet was calculated to be 13900 dm3 mol-’ cm-‘. This value is comparable with the value of 14200 cm3 mol- ’ cm-’ determined by measuring the extent of ground state bleaching at 395 nm in 5 x iO-’ mol dm-3 solutions of C 102 in benzene. 3.3. Energy transfer from benzene to C 102 The energy transfer rate constant from benzene to C 102 cannot be determined directly by following the decay of benzene triplet because of its very short lifetime [ I]. It was determined indirectly by making use of eq. (3 ). From transient absorbance measure527
26 May 1989
CHEMICAL PHYSICS LETTERS
Volume 151. number 6
-
I
X i0-3dm3mol
-1
[c 1023
9
100e
2888
3088
46BB
5880
Tine(nanosec.)
Fig. 2. Transient signals of pulse radiolytically generated biphenyl triplet at 345 nm (a) in the absence and (b) in the prescncc of 4x lo-’ and (c) 8~10~~ mol dmm3 C 102. (A small negative signal towards the end of the decay in (c) is due to bleaching ofground state C 102; this has been taken into account in evaluating the rate constants.) Build-up of C 102 triplet absorption at 580 nm as a result of energy transfer from biphenyl triplet at C I02 concentrationsof (d) 4x I0-sand (e) 8x IO-’ mol drn-’
ments on C 102 triplet formed in benzene at different concentrations of C 102 (at constant dose) the slope/intercept ratio of a plot of 1/absorbance versus 1/[C 102 ] was found to be 4.9 X 10m3. From this and knowing t,,z of benzene triplet in neat benzene (3.4 ns) the rate constant for energy transfer was calculated to be 4x 10” dm3 molP’ s’.
Oo4 -[c
1 X lO-4 dm3mol-1 1021
Fig. 3. Variation of reciprocal absorbance of C IO2 triplet at 580 urn as a function of 1/ [C 1021 produced hy energy transfer from (a) pulse radiolytically generated benzene triplet and (b) biphcnyl triplct.
Table 1 Energy transfer rate constants from various donors to C 102
3.4. Triplet energy level of C 102 As biphenyl triplet (T, ) with energy ET=65 kcal mol- ’ is able to sensitize C 102 triplet which in turn is quenched by 0, (presumably due to energy transfer), we can infer that the C 102 triplet energy must be between 63 and 25 kcal mol-‘. Table I gives the 528
Donor molecule
Energy level (kcal mol- ’ )
Energy transfer rate constant (dm3 mol-’ s-‘)
benzene biphenyl naphthalene
82 65 6I
4 x 1O’O
I XIO’O 1.4x IO’O
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energy transfer rate constants for different donor molecules which sensitize C 102 triplet together with their triplet energy levels. Triplet naphthalene was found to efficiently sensitize C 102 triplet. On the other hand C 102 triplet was found to sensitize transstilbene, pyrene, anthracene and benzil triplets (table 2). This would imply that triplet energy level of C 102 is below that of naphthalene (I!&= 61 kcal mol- ’ ) and above that of benzil (& = 54 kcal mol- ’ ). Thus the C 102 triplet energy can be inferred to be 57.5 f 1.5 kcal mol-‘. In the case of the chrysene (ET= 47 kcal mol- ’ )-C 102 pair, there was no energy transfer observable in either direction. 3.5. Comparison ofC 102 triplet properties with those of C 153 triplet C 153 is the trifluoro analogue of C 102 (scheme 1) and as a laser dye has better photostability and a broader tuning range. Earlier we reported the results of our studies on the triplet characteristics of C 153 by pulse radiolysis [ 7 1. The singlet and triplet characteristics of these two dyes are compared in table 3. The S, state in 7-amino coumarin is represented as the fully charge separated state known as the in-
26 May 1989
tramolecular charge transfer (ICT) state [ 91 (scheme I). If any substituent on the coumarin ring can stabilise the ICT state, then the S, energy level is lowered. There is an electron withdrawing CF, group on the pyran ring of C 153, which helps delocalise the negative charge in the ring [lo] and hence the S, level is expected to be stabilised more in C 153 than in C 102. Consequently the absorption and fluorescence maxima are both red-shifted in the former. Our present results reveal that the triplet level of C 102 is higher than that of C 15 3. Thus it is seen that, as for the singlet excited states of these compounds, the energy of the lowest triplet levels also decrease on replacing the 4-CH, group by 4-CF,. Hence we may conclude that the T, states of C 102 and C 153 also have ICT character like the S, states. The energy gap between the singlet and triplet levels is nearly the same for these two dyes (table 3). Hence one can expect the singlet-triplet transition to have nearly the same probability. Our experimentally observed G value for triplet formation at a given concentration of the dye for these two compounds in benzene is nearly the same (table 3). This, however, may not be the consequence solely of a similar S-T energy gap. Considering the reactions k4
Table 2 Rate constants for energy transfer from C 102 to various acceptors Acceptor molecule
Energy level (kcal mol- ’ )
Energy transfer rate constant (dm’ mol-' s-‘)
anthracene ?-stilbene pyrene bcnzil
42 49 50 54
l.lxlo’” 7.4x IO9 4.6X IO4 6.7x 10”
‘B*+C-
B+3C*,
(4)
B+B (blradlcal)’
(5)
k5 3B*+B -
where C and 3C* are respectively the coumarin dye and its triplet, B and %* are respectively benzene and benzene triplet, and T(~B*) is the intrinsic lifetime of the benzene triplet, the G value for C 102 triplet is given by
Table 3 Sin&t and triplet characteristics of C 153 and C 102 in benzene
G(3C*)=G(3B*)
b[Cl kb[C] fk,
[B]+ ~/T(~B*)
(6) Property
c 153”’
c 102 b’
Es (kcal mol-I) & (kcalmol-‘) k, Ltnplet benzene + dye (dm3 mol-’ SK’) G value for dye triplet at IO-’ mol dm-’
72 5Ok2.5
79 57.5? 1.5
a) Ref. [ 71. ‘) Present work.
6x 10”
4x 10’0
1.22
1.17
The rate constant for biradical formation (eq. ( 5 ) ) is reported to be 1.8X107 dm3 mol-’ s-’ [ll]. G( 3B*) in benzene radiolysis [3] is 4.2 and T(~B*) is 3.4 ns [ 11. Using these values and our experimentally determined k, values for the two dyes C 102 (present work) and C 153 (ref. [ 71) the G values for triplets of these two compounds are calculated to 529
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be 1.95 and 2.37. From eq. (6) it can be seen that the difference in G for the two compounds is due to differences in their k4 values. On the other hand the G values directly determined from the measured absorbance, dose and extinction coefficient are 1.17 and 1.22 respectively for C 102 and C 153. The difference between these and the calculated values is within the limits of the uncertainties involved in the estimation of various quantities used in these calculations.
The authors wish to record their appreciation of the encouragement and support from Dr. J.P. Mittal and Dr. R.M. Iyer.
4. Conclusions
References
Coumarin laser dye C 102 is found to have a longlived triplet state which can be populated by energy transfer from other higher lying triplets. The triplet spectrum with A,,,,, at 600 nm is broad and overlaps the fluorescence maximum at 484 nm. Hence direct population of the triplet by photoexcitation, however small due to low ISC probability, can cause quenching of the fluorescence and hence impair laser performance, unless the triplets are scavenged. Our studies indicate that any triplet quencher with energy levels below 54 kcal mall ’ can efficiently quench the triplets of C 102. Thus in actual laser operation one can choose a suitable triplet quencher. These results are also useful in selecting proper donor-acceptor pairs for energy transfer dye lasers. Thus for example the tram-stilbene/C 102 pair is used by Kotowski et al. [ 121 to produce lasers in which singlet excited trans-stilbene transfers energy to C 102 and lasing occurs from the singlet levels of C 102. Our results show that C 102 triplets can transfer energy very efficiently to trans-stilbene. Thus in this energy transfer dye laser system, trans-stilbene not
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only acts as donor for C 102 singlet, but alsoBcts as a quencher for C 102 triplets, which could otherwise impair laser operation.
Acknowledgement
[ 1 ] R. Bensasson and E.J. Land, in: Photochemical and photobiological reviews, Vol. 3, ed. KC. Smith (Plenum Press, New York, 197R ) p. 163. [2] K.H. Drexhage, in: Topics in applied physics. Dye Lasers, Vol. 1,ed. F.P. Schafer (Springer,Berlin, 1973 ) p. 16 1. [ 31 J.H. Baxendale and M. Fiti, J. Chem. Sot. Faraday Trans. II68 (1972)218. [ 41 K.N. Rao, P.N. Moonhy, C.V.C. Prasad, J.T. Kunjappu and K.I. Priyadarsini, BARC Report, BARC-1400 (I 988). [5]S.N.Guha, P.N. Moorthy, K. Kishore, D.B. Naikand K.N. Rao, Proc. Indian Acad. Sci. Chcm. Sci. 99 (1987) 261. [6] N.E. Shank, Intern. J. Chem. Kinetics 5 (1973) 577. [ 71 K.I. Priyadarsini, D.B. Naik and P.N. Moorthy, J. Photochem. Photobiol., in press. [ 8] R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 ( 1971) 1904. [ 91 G. Jones II, W.R. Jackson and A.M. Halpcm, Chcm. Phys. Letters 72 ( 1980) 39 1. [IO] R. Steppel, in: Organic dye lasers, CRC handbook of laser science and technology, Vol. 1. Lasers and masers, ed. M.J. Weber (CRC Press, Boca Raton, 1982 ) p. 299. [ 111 H. Gorner and S. Schulte-Frohlinde, J. Phys. Chcm. 85 (1981) 1835. [ 121 T. Kotowski, W. Majewski and W. Skubiszak, Opt. Appl. 14 (1984) 515.