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Medium effects on fluorescence quantum yields and lifetimes for coumarin laser dyes
CHEhlICAL
Volume 72, number 2
MEDIUM
EFFECTS
FOR COUMARIN
Gutlford JONES Department
ON FLUORESCENCE
LASER
QUANTUM
LET’lTRS
YIELDS
I June I980
AND LIFETIMES
DYES
II, Wdliam
of CWmlst~.
PHYSICS
R . JACKSON
Boston Unwersq,
Boston. ATassachusetts 02215, USA
and Arthur M. HALPERN Department
of CRemrst~, Norrileastern
Received 15 February
1980,
Unwers~ty, Boston, dlassadusetts
02215. USA
in final form 14 hIarch 1980
Fluorescence quantum yields and hfktunes of coumann dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect IS mterpreted m terms of relazxatron of excited dye from an IN&I planar conformation to a twsted zwtterioluc state.
1. Introduction
The mportance of ammocoumarms as laser dyes for the blue-green spectral region has been recognized for some trme [ 11. Fundamental photophysrcal and photochemical properties for this family of dyes have not been studied in deta& however, although fluorescence quantum yreld [2-51 and lifetime [6,7] data are avadable. Interestmg effects of mednun and structure are apparent m the study of Reynolds and Dre_xhage [2]. We report a survey of emission parameters which further reveals the interplay of dye structure and solvent properties that governs the competition between radiative and non-radratrve decay for coumarm dyes The results confirm the polar nature of the aminocoumann fluorescent state and suggest a model for non-radiative decay which is consrstent wrth recent proposals [8 J concerning exerted species which exhrbrt intramolecular charge transfer.
R
R
1,
R=CH3
3.
R=CW3
2.
R=CF3
4,
R=CF3
““QOJ CF3 5
R‘
R’
ICT
2. Experimental Coumarin dyes 1,3,4, and 5 (fig. 1) were commercially available laser grade materials (Eastman Kodak coumarins 1,102,153, and 15 1, respectively).
TICT Fig
1.
Coumarin 2 was prepared and puritied as previously reported [9], and 6 [3] was a gift from Dr_ RL. A&ins. Solvents were spectrograde or purified by distillation and checked for absorbing or fluorescing impurities. 391
Volume 72. number
2
CHEMICAL
PHYSICS
Emlsslon spectra were recorded on a Perkin-Elmer MPF MA fluorescence spectrophotometer equipped
methods
which are reported
[ 121
3. Results Emrssron maxrma for selected dyes obtamed from corrected fluorescence spectra are shown m table 1 _ A large red-smft of dye fluorescence emrssron m more Table 1 Fluorescence
emrsston ma?rtma for coumarm 1
Solvent
cyclohexane ethyl acetate acetomtnle
ethanol ethanol-water glycerol
a’ hf
m nm,
Table 2 Coumarm
vf
m 1000
fluorescence
cyclohexane ethyl acetate acetonitrde ethanol ethanol-mater glycerol
b)
dyesal 4
2
3
Vf
Xf
Vf
Xf
Vf
hf
uf
hf
“f
395 116 33.t 451 458 163
35 3 210 23 0 22 2 21 8 21 6
433 479 510 509 525 525
23 1
455
22.0
407
24 6
500 521 531 542 517
20 0
440 487
22.7
20 9 196
19 2
506
19 8
458
21 8
188 18.5 18 3
515 528
19.4 18.9
473
21 I
. ethanol-water
cm-’
quantum
19 I 19.1 19 1
20 5
432
23.2
= 50: 50 fVv/v).
ytelds and hfettmesal
2
4
6
3
Qf
Tf
&f
'f
*f
Tf
Of
7f
Of
0 0 0 0 0 0
28 31 34 3.1 1.4 38
1 04 0 64 0 09 009 0 03 0.14
41 46 0 6Ocl 0 85 0.45 2.7 cl
0 73 0 88 0.43 0 26 0.26 0 17
43 5.4 56 3.4 47 3.5 c)
0.93 0 72 068 065 0.43 0.27
4.2
1 05 0.70 0.59 0 74 0.77
32 93 73 59 22 53
al rf m ns. dye concenrratton = (l-51 X 10-5hl. b150 50 (V/V) mLxture,for 1 tn 20.80 ethanol-water. c) hfenrne corresponds to dommant smglee~ponenttal
39’
6
hf
1
Solvent
1 June 1980
polar solvents was observed as prevrously reported [l ]. Stmilar shrfts in emission wavelength have been recently observed for a number of ammocoumanns and mterpreted u-t terms of solvent polarity-poIanzabrhty, hydrogen bonding, and cohesion parameters [13,14]. Fluorescence quantum yields and hfetimes for selected dyes for a variety of solvents are shown in table 2. Some reductron in fluorescence yrelds in more polar solvents was generally observed, but the effect was most strrkmg for coumarins 1 and 2. Emission hfetune was sharply reduced (< I ns) for 2 as solvent polarity increased. The trend was less pronounced but continuous for 1 in more polar media (note data for a 20 _ 80 ethanol-water mtxture). The effect on emrsston properties of a polar but vrscous solvent was evaluated. In glycerol the usual polar solvent red-shift of fluorescence was observed, but
wrth a spectrum correctton unit and a low-temperature phosphorescence accessory and using quartz cells. Quantum yrelds were deterrmned usmg quinine sulfate m 1 @ N H$SO, (Qt= 0.55 [IO]) as the fluorescence standard wrth refrachve mdes and dtfferentud absorption correcttons. Fiuorescence hfetimes were deternuned using smgle-photon counting apparatus prevrously descrrbed [ 1 l] and employmg analytlcal deconvolution
LETTERS
df= 0 11 and decay: see text
rt= 0.70 ns.
56 5.1 45
Volume 72, number 2
CHEMICALPHYSICS LETTERS
emission yields and hfetimes were similar for all the dyes. Most noticeable was the continuation of reduction m fluorescence yield (ethanol-water versus glycerol) for dyes with more rigid structures (4 and 6) compared to the restoratton of emission yteld and hfetime for dyes with a tlexrble amine substituenr (1 and 2). Comparison was also made of the emission of 2 adn 4 m room temperature fluid solution and in a glassy matrix. In 50 :50 ethanol-methanol relative fluorescence yields (2/4) were 0.15 (room temperature) and 0.90 (77 K). A samphng of data was also obtained for the NH?-substituted coumarin 5 v&h results which lffer from the analogous structure 2. Emission yields and lifetimes for 5 were sustained in polar solvents (acetonitnle, @r = 0.72, or = 5.2 ns; 50 -50 ethanolwater, @t = 0.54, or = 5.3 ns). Special attention was pard to the posslbihty of dual enusslon for the dyes. A contmuous pattern of emrsston red-shift (Jr = 522-534 run), a general broadening of the fluorescence band but no fundamental change m emission, structure or shape and a regular reductton of fluorescence yield (q?q= 0.036-0.023) were observed for coumarin 2 on changmg the composition (20-90%) of water-ethanol solvent mixtures. The ‘-growing m” of a second enussion component [ 151 as a function of medium was not apparent. Lifetune results were obtained from excellent tits of singleexponential decay curves to experimental data over 2-3 decades of photon counting. Small deviations from single-exponential behavior were found 111a few cases when fluorescence decay was monitored at different wavelengths usmg mterference fdters. For example, the deconvoluuon data allowed that the decay of emission of 2 in acetonitrile (500 * 10 run) could be composed of a dominant 0.6 ns decay and as much as 5% of a 3 0 ns component. Sinularly small second components were apparent for 2 and 4 m glycerol (500 or 550 MI filters) but, stgnificantly, all emissions were cleanly single exponential when momtored at 600 run. The temperature dependence of emission parameters for 1,2, and 4 was examined_ Fluorescence yields for 2 m acetonitrile decreased (0.12-0.05) with a moderate increase (4--58O) in temperature, whereas the yield of emssron of 4 was temperature independent in this range. The fluorescence quantum yreld for 1 111glycerol was srgnificantEy reduced (056-0.23) from 20-80”, m parallel wrth a drop m measured emissron lifetime (3.8-l .6 ns).
1 rune 1980
4. Discussion Coumarm dyes are readily divided into two famihes on the basis of the response of fluorescence lifetime and quantum yield to increase in solvent pohuity_ For dyes having a substituted amine function which is not restricted by substituent linkage, emission yield and lifetune are sharply reduced in a polar solvent but recover in viscous or glassy media. The effect is more pronounced for a more polar dye (2 versus 2). The family of rigid dye structures (3,4, and 6) responds more moderately to medium polarity and the effect of a viscous polar solvent (glycerol) is to further reduce rather than fortify emission yield. Bate constants for radiative and non-radiative decay are readily calculated from the relationships, k, = @r/rr and k,, = 1 - &;T~. Values for kf and knd shown in table 3 range close to I X logs-r except for the non-radrative decay parameters for 1 and 2 in the most polar solvents_ The >200-fold range of kna for 2 includmg the return to a relatively slow non-radiative decay in glycerol ISmost revealing. The quantum-yieId data alone suggest that coumarin 3 follows the pattern of solvent independence set by the other rigid dyes. The results are readily interpreted in terms of a rotation-dependent non-radiative decay which links excited-state conformations having different requirements for solvent stabilization. Thus, dye excitation is followed by preliminary relaxation to a moderateEy polar, planar intramolecular charge-transfer (ICT) state (represented below by a single resonance form). For dyes wrth less rigid geometries, rotation at the amine function follows, leading to a non-planar conformation, a twisted charge-transfer (TECT) state. Both polar states are stabilized by substituents (electron donation at nitrogen and at the aniline ring and electron wEthdrawal for the lactone ring) and by interaction with solvent dipoles. For the fuUy charged TLCT state, larger requirements for substituent and soIvent charge delocahzation are expected_ The ICT-TECT model is one adopted from the current proposal of Crabowski, Cowley, and Baumann [5] concerning the behavior of a variety of polar excited species. The emission profile for coumarin [CT and TECT states differs from a protome such as p&methylamEnobenzonitrEle [5,15] in that the twisted form is nonemissive or at best very weakly emEssEve_ (The inherent relative strength of ECT emission re393
L June 1980
CHEMICAL F’iiYSfCS LETTERS
Volume 72, number 2
Tabie 3 Rate constants for radlatlrr and non-radlatzve decaya) Solvent
cyclohexane ethyl acetate acetomtie ethanol ethanol-ILater b) glycerol
4
2
1 kf
knd
kf
knd
x-f
knd
kf
bd
11 30 2.2 19 16 I.4
24 0 23 0 79 1.3 56 1.2
2.4 1.4 1.5 11 0 67 0 52
co.10 0.78 15 0 110 22.0 32
1.7 1.6 0.77 0.77 0.55 049
0.63 022 1.0 2.2 16 24
22
0.17
12 13 0.96
057 069 13
a) k-vaiues m lo8 s-t b, 50 50 (V/V) mixture: for 1 m 20 - 80 ethanol-water,
kf= 1.6 X IO* and R,d = 13 X IO8 5-l _
maming m polar solvents could mask the smali contnbutlon expected For fluorescence of the twlsted state [S] _) Two unportant exceptIons to the decay pattern for the coumarin dyes are found m the NH?-5 and 2-tl[llazole [I ,6] substituted systems both of which maintain robust fluorescence m a polar medmm The apparent reluctance for rotation-dependent radlauonless decay m 5 1s consistent with the prediction of a recent theoretical study [16] of N(CH3)?versus NH?-substituted benzomtrlle and related systems. The temperature dependence of emission yield and hfetlme for 1 and 2 IS consistent with the proposed rotatory radiationless decay. The extent of thermal activation for the ICI --f TlCT conversion is reveaied in plots of ln k,, versus If T. Vaiues for 2 in acetonitnle, calculated from quantum-y:eld data and assuming a temperature-mdependent kf, result in Arrhemus parameters. A = 1 X lot1 s-1 and E,= 2.5 kcallmol. A kugher actlvatron barrier for radiationless decay expected for 1 m glycerol 1s obtamed from the temperature dependence of k,, withA = 6 X 10” s-t and E, = 49 kcaijmol * _ The behavior of the coumarins 1s reminiscent of observations concemmg rhodamine laser dyes. Thus, rhodamines such as rhodamine B and 6G W&I nonrigid structures undergo a solvent-polarity- and ~scos~ty-dependent reduction m fluorescence quan* The assiment
of k,d to Ifl-
Tim bond rotation 1%not
stnctly required A fast reversible rotanon foilowed by rate determinmg IICT decay (fluorescence decay sWt smgle exponenti) ISnot ruled out
394
6
turn yield whereas em1sslon is sustamed for the rigid rhodamine 101 in various media [I]. The temperature dependence of fluorescence yield for rhodamme B and 6G 1171 is sunllar to that observed for the coumarms Further studies of the medmm dependence of photophysical properties of coumarm dyes are continuing_
Acknowledgement We thank Drs. Z.R. Grabowskl, P.R Hammond, AN. FIetcher, F.C. de Schryver and RE. Schwerzel for helpful discussions and for provldmg data pnor to publication. We acknowledge the support of the US Office of Naval Research (Boston Univernty) and the National Scrence Foundation (Northeastern Umverslty) and thank also the NSF for an mstrument grant to Boston University.
References [ 11 K.H. Dreuhage, m- Topics III apphed physics, VoL 1. Dye lasers, ed. F P. Schafer Gpnnger, Be&n, 1973)ch.4. [2] G.A ReynoIds and K.H_ Drexhage. Opt. Commun. 13 (1975) 222 [3] RL. Atkms and D.E. Bbss, J. Org. Chem. 43 (1978) 1976. [4] A-N. Fletcher and D E. Bliss, Appl. Phys 16 (1978) 289 IS1 P.R. frond, pnvate commu~ca~on [6] J If. Richardson, L.L. Stehunetz, S-B. Deutschet, W-A Bookless and W L Schmelunger, 2 Naturforsch 33a (1978) 1592. [71 F-C. de Schryver, pnvate communication.
Volume 72, number
2
CHEMICAL
[8] Z R Grabowsla, K. Rotkiewia, A. Slemiarczuk, D J. Cowley and W. Baumann, Nouv J. Chim. 3 (1979) 443. [9] E J. Schimitschek, J A Tnas, M. Taylor and J.E. Celto. IEEE J. Quantum Electron. QE 9 (1973) 781. [IO] R A. Velapoldi. J. Res. NatL Bur. Std. A76 (1972) 641; W.R Dawson and M.W. Wmdsor, J. Phys. Chem 72 (1968) 3251. [ll] A.M.Halpem, J. Am. Chem Sot. 96 (1974) 7655. [ 121 D.K Wong and Ahi. Halpem, Photochem. Photoblol. 21(1976) 609.
PHYSICS LETTERS
I June 1980
[ 13 J L Coosemans. EC. de Schryver aad A van Dormad, Chem. Phys. Letters 65 (1979) 9.5. [ 141 G. Jones II, W-R. Jackson, S. Kanoktanapom and A.M H&em, submitted for publication_ [IS J E M Kosower and H. Dodiuk, J. Am. Chem Sot. 98 (1976) 924. [L6] W. Rettig and V. Bona%-Koutecw, Chew. Phys. Letzrs 62 (1979) 115. [ 171 R.E. Schwerzd and N.E. KJosterrnan, submitted f.r publication.
395
Volume 72, number 2
MEDIUM
EFFECTS
FOR COUMARIN
Gutlford JONES Department
ON FLUORESCENCE
LASER
QUANTUM
LET’lTRS
YIELDS
I June I980
AND LIFETIMES
DYES
II, Wdliam
of CWmlst~.
PHYSICS
R . JACKSON
Boston Unwersq,
Boston. ATassachusetts 02215, USA
and Arthur M. HALPERN Department
of CRemrst~, Norrileastern
Received 15 February
1980,
Unwers~ty, Boston, dlassadusetts
02215. USA
in final form 14 hIarch 1980
Fluorescence quantum yields and hfktunes of coumann dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect IS mterpreted m terms of relazxatron of excited dye from an IN&I planar conformation to a twsted zwtterioluc state.
1. Introduction
The mportance of ammocoumarms as laser dyes for the blue-green spectral region has been recognized for some trme [ 11. Fundamental photophysrcal and photochemical properties for this family of dyes have not been studied in deta& however, although fluorescence quantum yreld [2-51 and lifetime [6,7] data are avadable. Interestmg effects of mednun and structure are apparent m the study of Reynolds and Dre_xhage [2]. We report a survey of emission parameters which further reveals the interplay of dye structure and solvent properties that governs the competition between radiative and non-radratrve decay for coumarm dyes The results confirm the polar nature of the aminocoumann fluorescent state and suggest a model for non-radiative decay which is consrstent wrth recent proposals [8 J concerning exerted species which exhrbrt intramolecular charge transfer.
R
R
1,
R=CH3
3.
R=CW3
2.
R=CF3
4,
R=CF3
““QOJ CF3 5
R‘
R’
ICT
2. Experimental Coumarin dyes 1,3,4, and 5 (fig. 1) were commercially available laser grade materials (Eastman Kodak coumarins 1,102,153, and 15 1, respectively).
TICT Fig
1.
Coumarin 2 was prepared and puritied as previously reported [9], and 6 [3] was a gift from Dr_ RL. A&ins. Solvents were spectrograde or purified by distillation and checked for absorbing or fluorescing impurities. 391
Volume 72. number
2
CHEMICAL
PHYSICS
Emlsslon spectra were recorded on a Perkin-Elmer MPF MA fluorescence spectrophotometer equipped
methods
which are reported
[ 121
3. Results Emrssron maxrma for selected dyes obtamed from corrected fluorescence spectra are shown m table 1 _ A large red-smft of dye fluorescence emrssron m more Table 1 Fluorescence
emrsston ma?rtma for coumarm 1
Solvent
cyclohexane ethyl acetate acetomtnle
ethanol ethanol-water glycerol
a’ hf
m nm,
Table 2 Coumarm
vf
m 1000
fluorescence
cyclohexane ethyl acetate acetonitrde ethanol ethanol-mater glycerol
b)
dyesal 4
2
3
Vf
Xf
Vf
Xf
Vf
hf
uf
hf
“f
395 116 33.t 451 458 163
35 3 210 23 0 22 2 21 8 21 6
433 479 510 509 525 525
23 1
455
22.0
407
24 6
500 521 531 542 517
20 0
440 487
22.7
20 9 196
19 2
506
19 8
458
21 8
188 18.5 18 3
515 528
19.4 18.9
473
21 I
. ethanol-water
cm-’
quantum
19 I 19.1 19 1
20 5
432
23.2
= 50: 50 fVv/v).
ytelds and hfettmesal
2
4
6
3
Qf
Tf
&f
'f
*f
Tf
Of
7f
Of
0 0 0 0 0 0
28 31 34 3.1 1.4 38
1 04 0 64 0 09 009 0 03 0.14
41 46 0 6Ocl 0 85 0.45 2.7 cl
0 73 0 88 0.43 0 26 0.26 0 17
43 5.4 56 3.4 47 3.5 c)
0.93 0 72 068 065 0.43 0.27
4.2
1 05 0.70 0.59 0 74 0.77
32 93 73 59 22 53
al rf m ns. dye concenrratton = (l-51 X 10-5hl. b150 50 (V/V) mLxture,for 1 tn 20.80 ethanol-water. c) hfenrne corresponds to dommant smglee~ponenttal
39’
6
hf
1
Solvent
1 June 1980
polar solvents was observed as prevrously reported [l ]. Stmilar shrfts in emission wavelength have been recently observed for a number of ammocoumanns and mterpreted u-t terms of solvent polarity-poIanzabrhty, hydrogen bonding, and cohesion parameters [13,14]. Fluorescence quantum yields and hfetimes for selected dyes for a variety of solvents are shown in table 2. Some reductron in fluorescence yrelds in more polar solvents was generally observed, but the effect was most strrkmg for coumarins 1 and 2. Emission hfetune was sharply reduced (< I ns) for 2 as solvent polarity increased. The trend was less pronounced but continuous for 1 in more polar media (note data for a 20 _ 80 ethanol-water mtxture). The effect on emrsston properties of a polar but vrscous solvent was evaluated. In glycerol the usual polar solvent red-shift of fluorescence was observed, but
wrth a spectrum correctton unit and a low-temperature phosphorescence accessory and using quartz cells. Quantum yrelds were deterrmned usmg quinine sulfate m 1 @ N H$SO, (Qt= 0.55 [IO]) as the fluorescence standard wrth refrachve mdes and dtfferentud absorption correcttons. Fiuorescence hfetimes were deternuned using smgle-photon counting apparatus prevrously descrrbed [ 1 l] and employmg analytlcal deconvolution
LETTERS
df= 0 11 and decay: see text
rt= 0.70 ns.
56 5.1 45
Volume 72, number 2
CHEMICALPHYSICS LETTERS
emission yields and hfetimes were similar for all the dyes. Most noticeable was the continuation of reduction m fluorescence yield (ethanol-water versus glycerol) for dyes with more rigid structures (4 and 6) compared to the restoratton of emission yteld and hfetime for dyes with a tlexrble amine substituenr (1 and 2). Comparison was also made of the emission of 2 adn 4 m room temperature fluid solution and in a glassy matrix. In 50 :50 ethanol-methanol relative fluorescence yields (2/4) were 0.15 (room temperature) and 0.90 (77 K). A samphng of data was also obtained for the NH?-substituted coumarin 5 v&h results which lffer from the analogous structure 2. Emission yields and lifetimes for 5 were sustained in polar solvents (acetonitnle, @r = 0.72, or = 5.2 ns; 50 -50 ethanolwater, @t = 0.54, or = 5.3 ns). Special attention was pard to the posslbihty of dual enusslon for the dyes. A contmuous pattern of emrsston red-shift (Jr = 522-534 run), a general broadening of the fluorescence band but no fundamental change m emission, structure or shape and a regular reductton of fluorescence yield (q?q= 0.036-0.023) were observed for coumarin 2 on changmg the composition (20-90%) of water-ethanol solvent mixtures. The ‘-growing m” of a second enussion component [ 151 as a function of medium was not apparent. Lifetune results were obtained from excellent tits of singleexponential decay curves to experimental data over 2-3 decades of photon counting. Small deviations from single-exponential behavior were found 111a few cases when fluorescence decay was monitored at different wavelengths usmg mterference fdters. For example, the deconvoluuon data allowed that the decay of emission of 2 in acetonitrile (500 * 10 run) could be composed of a dominant 0.6 ns decay and as much as 5% of a 3 0 ns component. Sinularly small second components were apparent for 2 and 4 m glycerol (500 or 550 MI filters) but, stgnificantly, all emissions were cleanly single exponential when momtored at 600 run. The temperature dependence of emission parameters for 1,2, and 4 was examined_ Fluorescence yields for 2 m acetonitrile decreased (0.12-0.05) with a moderate increase (4--58O) in temperature, whereas the yield of emssron of 4 was temperature independent in this range. The fluorescence quantum yreld for 1 111glycerol was srgnificantEy reduced (056-0.23) from 20-80”, m parallel wrth a drop m measured emissron lifetime (3.8-l .6 ns).
1 rune 1980
4. Discussion Coumarm dyes are readily divided into two famihes on the basis of the response of fluorescence lifetime and quantum yield to increase in solvent pohuity_ For dyes having a substituted amine function which is not restricted by substituent linkage, emission yield and lifetune are sharply reduced in a polar solvent but recover in viscous or glassy media. The effect is more pronounced for a more polar dye (2 versus 2). The family of rigid dye structures (3,4, and 6) responds more moderately to medium polarity and the effect of a viscous polar solvent (glycerol) is to further reduce rather than fortify emission yield. Bate constants for radiative and non-radiative decay are readily calculated from the relationships, k, = @r/rr and k,, = 1 - &;T~. Values for kf and knd shown in table 3 range close to I X logs-r except for the non-radrative decay parameters for 1 and 2 in the most polar solvents_ The >200-fold range of kna for 2 includmg the return to a relatively slow non-radiative decay in glycerol ISmost revealing. The quantum-yieId data alone suggest that coumarin 3 follows the pattern of solvent independence set by the other rigid dyes. The results are readily interpreted in terms of a rotation-dependent non-radiative decay which links excited-state conformations having different requirements for solvent stabilization. Thus, dye excitation is followed by preliminary relaxation to a moderateEy polar, planar intramolecular charge-transfer (ICT) state (represented below by a single resonance form). For dyes wrth less rigid geometries, rotation at the amine function follows, leading to a non-planar conformation, a twisted charge-transfer (TECT) state. Both polar states are stabilized by substituents (electron donation at nitrogen and at the aniline ring and electron wEthdrawal for the lactone ring) and by interaction with solvent dipoles. For the fuUy charged TLCT state, larger requirements for substituent and soIvent charge delocahzation are expected_ The ICT-TECT model is one adopted from the current proposal of Crabowski, Cowley, and Baumann [5] concerning the behavior of a variety of polar excited species. The emission profile for coumarin [CT and TECT states differs from a protome such as p&methylamEnobenzonitrEle [5,15] in that the twisted form is nonemissive or at best very weakly emEssEve_ (The inherent relative strength of ECT emission re393
L June 1980
CHEMICAL F’iiYSfCS LETTERS
Volume 72, number 2
Tabie 3 Rate constants for radlatlrr and non-radlatzve decaya) Solvent
cyclohexane ethyl acetate acetomtie ethanol ethanol-ILater b) glycerol
4
2
1 kf
knd
kf
knd
x-f
knd
kf
bd
11 30 2.2 19 16 I.4
24 0 23 0 79 1.3 56 1.2
2.4 1.4 1.5 11 0 67 0 52
co.10 0.78 15 0 110 22.0 32
1.7 1.6 0.77 0.77 0.55 049
0.63 022 1.0 2.2 16 24
22
0.17
12 13 0.96
057 069 13
a) k-vaiues m lo8 s-t b, 50 50 (V/V) mixture: for 1 m 20 - 80 ethanol-water,
kf= 1.6 X IO* and R,d = 13 X IO8 5-l _
maming m polar solvents could mask the smali contnbutlon expected For fluorescence of the twlsted state [S] _) Two unportant exceptIons to the decay pattern for the coumarin dyes are found m the NH?-5 and 2-tl[llazole [I ,6] substituted systems both of which maintain robust fluorescence m a polar medmm The apparent reluctance for rotation-dependent radlauonless decay m 5 1s consistent with the prediction of a recent theoretical study [16] of N(CH3)?versus NH?-substituted benzomtrlle and related systems. The temperature dependence of emission yield and hfetlme for 1 and 2 IS consistent with the proposed rotatory radiationless decay. The extent of thermal activation for the ICI --f TlCT conversion is reveaied in plots of ln k,, versus If T. Vaiues for 2 in acetonitnle, calculated from quantum-y:eld data and assuming a temperature-mdependent kf, result in Arrhemus parameters. A = 1 X lot1 s-1 and E,= 2.5 kcallmol. A kugher actlvatron barrier for radiationless decay expected for 1 m glycerol 1s obtamed from the temperature dependence of k,, withA = 6 X 10” s-t and E, = 49 kcaijmol * _ The behavior of the coumarins 1s reminiscent of observations concemmg rhodamine laser dyes. Thus, rhodamines such as rhodamine B and 6G W&I nonrigid structures undergo a solvent-polarity- and ~scos~ty-dependent reduction m fluorescence quan* The assiment
of k,d to Ifl-
Tim bond rotation 1%not
stnctly required A fast reversible rotanon foilowed by rate determinmg IICT decay (fluorescence decay sWt smgle exponenti) ISnot ruled out
394
6
turn yield whereas em1sslon is sustamed for the rigid rhodamine 101 in various media [I]. The temperature dependence of fluorescence yield for rhodamme B and 6G 1171 is sunllar to that observed for the coumarms Further studies of the medmm dependence of photophysical properties of coumarm dyes are continuing_
Acknowledgement We thank Drs. Z.R. Grabowskl, P.R Hammond, AN. FIetcher, F.C. de Schryver and RE. Schwerzel for helpful discussions and for provldmg data pnor to publication. We acknowledge the support of the US Office of Naval Research (Boston Univernty) and the National Scrence Foundation (Northeastern Umverslty) and thank also the NSF for an mstrument grant to Boston University.
References [ 11 K.H. Dreuhage, m- Topics III apphed physics, VoL 1. Dye lasers, ed. F P. Schafer Gpnnger, Be&n, 1973)ch.4. [2] G.A ReynoIds and K.H_ Drexhage. Opt. Commun. 13 (1975) 222 [3] RL. Atkms and D.E. Bbss, J. Org. Chem. 43 (1978) 1976. [4] A-N. Fletcher and D E. Bliss, Appl. Phys 16 (1978) 289 IS1 P.R. frond, pnvate commu~ca~on [6] J If. Richardson, L.L. Stehunetz, S-B. Deutschet, W-A Bookless and W L Schmelunger, 2 Naturforsch 33a (1978) 1592. [71 F-C. de Schryver, pnvate communication.
Volume 72, number
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CHEMICAL
[8] Z R Grabowsla, K. Rotkiewia, A. Slemiarczuk, D J. Cowley and W. Baumann, Nouv J. Chim. 3 (1979) 443. [9] E J. Schimitschek, J A Tnas, M. Taylor and J.E. Celto. IEEE J. Quantum Electron. QE 9 (1973) 781. [IO] R A. Velapoldi. J. Res. NatL Bur. Std. A76 (1972) 641; W.R Dawson and M.W. Wmdsor, J. Phys. Chem 72 (1968) 3251. [ll] A.M.Halpem, J. Am. Chem Sot. 96 (1974) 7655. [ 121 D.K Wong and Ahi. Halpem, Photochem. Photoblol. 21(1976) 609.
PHYSICS LETTERS
I June 1980
[ 13 J L Coosemans. EC. de Schryver aad A van Dormad, Chem. Phys. Letters 65 (1979) 9.5. [ 141 G. Jones II, W-R. Jackson, S. Kanoktanapom and A.M H&em, submitted for publication_ [IS J E M Kosower and H. Dodiuk, J. Am. Chem Sot. 98 (1976) 924. [L6] W. Rettig and V. Bona%-Koutecw, Chew. Phys. Letzrs 62 (1979) 115. [ 171 R.E. Schwerzd and N.E. KJosterrnan, submitted f.r publication.
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