- Email: [email protected]
Kiton red S and rhodamine B. The spectroscopy and laser performance of red laser dyes
I September
CHEMICALPHYSiCS LETfERS
1375
The tasing perforin~nc~ of kiton red S and r~od~rnin~ B (COOH) is shown to bc stron@y influenced b:~ the substlturnt bonded to the 9 carban atom of the main chromophore. The lasing efficiency of these dyes is compared under long and short puke flashlamp excitation. The results are shown to be predictable based on the relative differences in @f, Tf and K+T, for these dyes.
The use of&ton red S as a laser dye was reported first by Gregg et al. [I] _ Subsequent work by Marling and co-workers f&3] examined in greater detail the flashlamp Iasing properties of this dye. This work was done without Ihe benefit of knowing the molecular structure
of kiton
red S.
Kiton red S thou&t to b\t tbc monoazo dye j4,5] shown by structure I, but exhibited lasing properties similar, to the common santhene dye, rl~odan~ne B (structure II). We Ilave determined the structure of kiton red S based on nuclear magnetic resonance nnaiysis (see
iig. I) and cleme~t~l analysis*. The proposed structure is structure Hi.
* Preselrt address: Exxon Nuc!ear Company, Inc., R~SXW~I and Technofogy Center, Richhnd, Wshiagton 99357, USA.
* Elomcntol analysis indicated the coirect amount ofsodium znd su$hur present For the combositicm C27H29NZ07S2Nz.
C2%. C,H/N’
++%W5 N’C 2’Y5
181 ~..a
I
1 September 1975
CHEMICAL PHYSICS LETTERS
Volume 35, number :! I
1 cl80
I
I
420
CH:,CH2
I
I
I
360
300
I
I
I
I
I
160
240
I
I
I
I20
60
I
I
OHz
/ CH2 CHS
:N
‘CH2
CHs
_ NA-WH~
.
-
TMS
‘CHpCns
No
RING
9
PROTONS
1 ,s
CH3
‘CH2 -
CH3
-N
PHEN’IL PROTONS DMSO
I
I // I!
FiiJ. 1.60 MHz nuclear magnetic resonance spectrum of kiton red S in DMSO (d&. The striking similar-i-
bonded
to tile 9 carbon
atom is 11strong
structural determinate to the observed spectroscopy‘ and king of these xirnthene molecuIes. The chemistry of tile 9 carbon atom is found to irzfluence the rate of ~tersyst~m crossing (iCS1_T, ) and the energy separation between t:le electronic states of the molecule (S,,S, and TL). These spectroscopic differences can be used to explain the lasing properties of kiton red S and rhodamine B.
2. Experimental Xitorrred S was obtained Inc.., PlainviBe, New Yo&.
from K&X Laboratories, The material received
contained approximately seventy-five percent inorganic salts and twenty-five percent dyestuff by weight. The dye was purified by column chromatography on silica1 gel (Silica Gel Vioelm for absorption, Activity III) by gradient e&&ion using ~c~ton~/metb~nol. The major eluted fraction was homogeneous by thin chaomatographic analysis (TLC). The pyronin B (C-1752) and rhodamine B (P-4453) were obtained from Eastman Kodak and purified until the material was homogeneous by TLC analysis. The solvent used in all spectroscopic and lash-g measurements was absolute ethanol (with no 2-propa1-101,benzene or other denaturing additives present). AU solutions were deoxygenated either by argon saturation or a liquid nitrogen vacuum freeze/thaw cycle. The concentration of dye, for &I spectroscopic mea~remen~s involving the otservation of reradiated liJlt was G t X 10m6 M to avoid errors resulting from
c This [l-3]:
is the same dye sollice used by preview investigtors
CHEBflCAL
Volume 35, number 2
dye self-absorption,
concentration
que~c~~
PHYSKS
tween these electronic levels three spectroscopic meitsuremenfs were made: SI c- S, absorption, S, +- S1 fluorescence and So +- T, phosphorescence.
and dye
aggregation. The spectral data wzs obttined using standard commercial instrumentation. The absorption data was
The molecules &ton red S and rhodamine B have identic& fused ring chromophores. They difier in the type and number of substituents on the pheny! group bonded to the 9 carbon atom. in ethanol the sulphonic acid group in the 2” position (see structure IV) is
taken with a Gary 14. The rluosescence spectra were
recorded using an Aminco43owmart spectrophotofluorimeter equipped with a selected S-20 or 3 c&ibrated S-l spectra! response photomultip~er_
LElTERS
The
&Qsphorescerke was observed using an iiminco phosphorimeter attachment and spectra were taken at 293K and 77K. Tke ~u~resceRce iifetime vfas measured with a TRW Efetime apparatus_ The hsing data ~2s obtained using a 100 joule coaxial lamp and driver (Phase-R, Inc.) 2nd 2 long pulse linear fkshknp pumped system described elsewhere [6’j _ fully dissociated. Kiton red S does not have an ass+ &ted rkon as does rh0damine.B 2nd the carboxylic acid jn the 2’ position of rhodamine B is not fully dissociated in aIco!~~lsolutions. We have made a distinction between the acid and basic form of rltodamiue B. To assure the predominaxe of one form a ~mll amount of HCI or NaOH was added. To understand ehe effect of the 9 carbon substituent
3. Visible spectroscopy The spectroscopic p2mmeters of interest are the energy separation between the ground state (So), the Erst excited singlet state (S1), and the lowest triplet state (T1 > (see fig_ 2). To establish the separation be-
40,000
lO,OOC
ABSORPTION
“i Fig. 2. The.rehtion between the obsened speci~oscopy and
the energy levels of 2 molecule.
183
CHEMICAL: PWSICS
,v01unie 35, nuTn~r 2
,
to be hx same for each molecule
of the 9 carbon substituents
(1) (2)
in
Dye
on the spectroscopy
9 Substitucnt
I
The influence of R’ on A,, might be viewed as a ~ert~rba~~on whose apparent effect is to change L (dist~bu~in~ the x electrons over a larger number of atoms). When R’ exhibits an eiectran withdrawing (--I) force, a net poia~ab~ty in the direction of R’, there is an increase in the effective region over which the n electroils of the main chromophore can be distributed. The result of this small perturbation to the patential of the li electrons is 3 rod-soft-~athoc’~omic skift) in-the absorption maximum. The reverse is observed when R’ is electron donating (4-r), a hypsochromic shift (~I~~-shift) resu!ts.
of the xanthene
Abs.max.
(cm-’
dyes
.%-lift
)
Phosphorescence x max km-“)
--PYRONlFi El
,cy+ Hw4ZCH3n
0
COOH
KITON
RED
-
18115
18060
1975
approximation the separation of So and S, win be dependent upon the conjugated length (L) of the molecu!e and the number of T?electrons (A’),
The property of the substituent (R’) bonded to the 9 carbon atom khich influences thi ener,g of the F electrons of the main ckromophorc, is the direction and magnitude of the inductive effect (I) of R’. The absorption transition S, Z-S, in these dye molecules is a resonance excitation of an electron from the highest filled bonding molecular orbitai to the lowest a~tibo~di~~ molecular orbital fn* C- n). A simple mode1 of the ?r electrons can be made based on the ona dimensional particle in a box problem Table 1 The inffuence
1 September
(ii., free ciectron gas model, Kuhn @3])_To a first
(R’).on the spectroscopy and basing of these dyes we chose the xanthcne series: pyronin B, rhodamine B (COOH); rhodamine f! (COO-) and kiton red S. The molecuIes have identical fused ring chrqtiophores. The substitution on the amino groups are’diethyl for each molecule (this WL\i verified for each dye sample by NMR), ami the i&heRCe Gf the free rOtatiOR Of thb alkylamine groups [‘if &n their relaxation properties is assumed the sertes.
LETTERS
-
3190
14130
3133
bathochromic
14040
3230
bathochromic
13940
3385
-
S
Volume 35, number 2
CHEhlICAL PKYSICS LETTERS
A suitable reference, to establish the magnitude of the inductive effect of R’ on spectroscopy of tile xanthene dyes, is pyronin B. The inductive effect of 2 hydrogen atom is zero. The results shown in table 1 verify the predic’tible shifts in the absorption maximum of this series of dyes. The smallest energy gap (AEsQ_s, ) is associated with the strong electron withdrawmg properties of the sulphonic acid group of kiton red S and the largest energy gap results from
the electron donating property of the dissociated carboxylic acid group of rhodamine B. The inductive
propetiies
of R’ also influence
the
energy of the first excited triplet state (T1 ) of :he xanthene dyes. An argument based on the electrostatic interaction of electrons (the Fermi correlation effects are expected
to increase
as the box gets
smaller (L) 191) can be used to interpret the changes in the ener,T separation between the ground state (So) and T, (AE,,_T~). The phosphorescence m=imum for these dyes (see table 1) show the same characteristic dependence on the inductive properties of R’ as did the absorption maximum. The energy separation between S, and T, (AESI_TI) is also given in table 1. These results may be used to predict the relative magnitude of the rate of intersystem crossing (KS, _TI) and the relative triplet state lifetime (TV) for rhodamine B(COOH) and kiton red S. The relative and rhodamine
triplet
state !ifetime
of kiton
red S
B may be estimated using the equa-
tion [lo]
Table
1 September 1975
2
Spectroscopy of kiton red S md rho&mine B (C30H) 3j Kiton red S Rho&mine 6 (COOH) ES 0X0) km:‘) ET (o,o) (cm-‘)
17770 14385
gs,-T, (cm-‘) d Vs (cm-t)
3385 670 i8020 1 L9000 17330 2.8 + 0.3
17300 2.3 2 OS b)
0.83 c_0.02
0.72 k 0.07, c)
Am,lxS1 (cm-‘) lm3:: (Q Xi-' cm-'j FI.max. (cm-‘) Tf (ns) Of
17680 !4430 3250 750 18060 100000
“)Es(O.O)= e 1c c tronic energy of S, ET (0,O) = elecrronic energy of T, @S, -TV = energy separation of St and T,,
I sndt, .. Ama& = absorption maximum for S1 + SQ transitions, fmex = extinction coefficient at FL max. = fluorescence maximum, Tf = fILOieSAmxSt, cencc lifetime in ethanol at a dye conceniration < 1 x IO-’ M, @f= fluorescence quantum yield for a dye conccntiation c 1 x 10-6M. b)Kef. [ 161: c) ref. [17]. A Vs = Stokes
and indlrectly verify the importance of R’ on the relative rate of intersystem crossing [eq. (a)] .
4.
Iaiag
properties
The spectroscopic properties of a dye are related to their potential performance in 2 laser through an expression of the net gain (G). The net gain for, a round trip in the laser cavity is G(X) = 2(go -cY&,
where tlw, =;:3000 cm-l, y = 1. The relative rate of intersystem cross’flg may be determined from the equaticn [ 111 _
(5)
where the gain (go) and losses ((Yo), for a single pass through an active !ength of gain media (I,): are given by (6)
a0
The spectroscopy of these two molecules are summarized in table 2. It is of interest to noie that the fluorescence quantum yield of kiton red S is larger than rhodamine B. This is consistent with the result of eq. (4). The spectroscopic data may be used to explain the difference ir. the lasing behavior of these two dyes
=~o~&)_~,(~) +~pTI_TpPLJ’
(7)
respectively. The electronic state populations for So, S1 and T, are represented by ~\b, _Nl 2nd iv,, respectively (see fig. 1). The stimulated emission cross section (De) is oe = X4E(X>/8r; In2 rr and normalized
to the fluorescence
(8)
quantum yield (cP~) 185
Volume 35,pumber
1
CHEMICAL PHYSICS LETTERS
?.
Qr = j-E@) dX,
September
i97.5
(9)
where rf is the fluorescence lifetime md q is the refractive index of the media. The self-absorption cross section (u~,__~,) and triplet--triplet absorption cross section (oT,_T,).multiplied by the appropriate state population constitute the losses associated with the dye and 0 represerAs those cavity losses controlled by mirror transmhsion and scattering by the media G-q.
To fully characterize the flashlamp pumped !ssing properties of kiton red S and rhodamine B two temporal pumping regimes were studied. The short pulse regime where the exciting pulse (‘T’,) is short cu_mpared to the rate of intersystem crossing (Ks,_T,)-l. The long pulse regime where ‘TP > (KS1 _-., )- 1. In the short pulse regme eq. (7) reduces to cyO=N 0 u&-S,(X)
(10)
+fl.
These losses are cold cavity losses (present when the gain media is not pumped) and may be determined with little difficulty. The function u~~_~,(A) is shotin
Fig. 4. Stimulated absolute ethanol.
for both
pulse was 323 ns at fwhm which is short compared to (KS _T )-l [13] .) The stimulated emission cross sectioi wi calcillated ustig eqs. (8) and (9) and the spectroscopic data reported. The v&es of ue, over the lasing region studied, for kiton red S are given in fii. 4 (for comparison to rhodamine B see ref. [14] ). The lasing performance of kiton red S and rhodamine B were compared on the bases of measurements of
(1 -R,R.,)
dyes
in fig. 3. The value ofp
was z 7070
and constant for all lasing measurements.
(The shoz pulse data was taken with a coaxial flashlamp znd driver described in ref. [ 121. The flashlamp
emission cross section for kiton red S in
the slope efficiency (SE.), peak pcwer (P,) and energy per pulse. The resdts are tabulated in table 3. A
crude correlation between the ratio of the S.E., PO, Eo, and G+ was observed. This is not surprising based on spectroscopic difference reported irl table 2. The slope efficiency,
(11) where qINv is the inversion
(12)
1710ss=:pI(P+c3,
c =116.700
16.600
16.5C-O
16,‘mo
16,Mo
16.200
16.100
16.oK!
cm-’
Fig. 3. Self-absorption cross section crs~_~~ and rhodamine B in aixolute ethanol. (cso_s,
tiplied by 2.3.)
for k&r.
red S
must be mul-
exp (Nouso_s,
efficiency,
L)
03)
and vzFnrre is the ratio of the hri?g cross-sectional uea to the tota. laser aperture area. Eq. (11) simplifies to S.E. = TN (14)
Volume 35, number 2
1 September 1975
CHfMICAL PHYSICS LETERS
Table 3
Short pulse ia5.n: q%formance of kiton red S and rhodzmine B 2)
kiton red S
TP
Tf
@f
320
2.8
0.83
S.E.
QffW
!.53 1.2
rhodamine B
320
2.3 h)
S.E.(RZ
PO
1.35 1.2
0.72 cf
PO(R)
1.32
.&
E:! (RI
0.73 1.3
1.0
1.2 i3.60
a) The laser ourpu t of kiton red S peaked 2t 16150 cmel and rhodanine B at 16200 cm-l. TD = fkhm of the Da&J.amp pulse ia nr, Tf= fluorescence Lifetime in ix, Qf = fluo:escence quantum effidency, SE. = slope etticikxy - hser output to electrical enerw input - in %,Po s the peak power output in MW for 60 5 input at a dye conccntmtion OCt X Ii)- hi in ethanol, Po
ratio efE0 for KRS to RH-B. b)Ref. [i6]; c)ref. [I?]. if 0 9 C and ‘lapcfiure = 1 (which is the case for the coaxial laser and these dyes). The ratio of the slope efficiencies is S E _(ECRS)/S.E . @H-B) =
~f?‘f(KRS)&‘I’f@H-B),
when
where &,
is fhe pump& rate and is heId constant. The Iasing efficiency difference calculated and observed in the short pulse regime are in good agreement for such a crude model. The wr?velength dependence of the PO and E, ratio’s is shown by the tuning curves in fig. 5, indicating that the greater king efficiency of kiton red S is maintained at all wavelengths. In the long pulse regime to explain the differences in the lasing efs”iciencies of the two dyes c+, must include i&~~,_~ [A). ql,, must also be included in the defmition 0TS.E. as /3is not much greater than C Cpfor this laser was set at = 22%). The triplet state population (NT) can be described for two important cases assuming the steady approximation is valid un-
der these pumping conditions [14] NT = KS1_TI Nr --‘VT/T, - Kq EQI&4 1
(17)
NT =KSL-TL
(18)
‘T$
ifdye (TL) + quencher (SO) 5 dye (SO) +-quencher (T1) is not occurrir~g and NT =KS,_Ti assuming
Kq
occurring.
‘~~l~q
[Q1:
(19)
[QJ3+I/T',, when trip’,et quenching is
The triplet
state single
will be larger for rhodamine
pass absorption
loss (7T)
B based on cqs. (3) and
= OT,_T(KRS). Therefore, (4)if+1_T (RH-B) the lnsing efkiency of KEZS should be greater than RH-B. The results are tabulated in table 4 and indi-
Vo!ume 35, number
2
J September
CHEMICAL PHYSICS LETTEXS
197.5
Tablc4
i,ong pulse iaslng performance
of kiton red S and rhodamine
ltiton red S
B a)
*F
Tt-
@f
12
2.8
0.83
@f(R)
Eo
f x m-3 aided
hf COT
I2
2.3
iI,72
kiton
I2
2.8
0.83
red 5
3.3
B
12
2.3
2.J
7.5
0.05
1.2
1.7
3.03
Q.72
PO (RI.
2.7
0.009
l.2 rhodamine
PO 6.1
0.03 I.2
~~~d~~inc B
=%3(R)
6.2
as table 3 with these exceplionsr PO = peak power in k’lY at an input energy of 100 J, Ka = lasing far 100 1 input, Tp = exciting pulse fwhm in microseconds.
3.1The samesymbol dcilnitions efficiency
cate tfte predi&~~bly greater king efkiency of KRS. To illustrale that the power output ratio is strongly influenced by the magnitude of TT a second set of experiments were made using COT (cyclooctatetraene> to reduce the triplet state population il.51 _ The ‘result.ts given in table 4 suggest that a major difference between KRS and RK-3 is the farger triplet state ~o~ula~~of~ofrllodarkie B.
References It]
D.W. Greg_%MR. Querry, J.B. Mariing, S.J. Thorna.% C.V. Dobler, NJ. Davies and J.F. Belew, IEEE J. Quan-
tum Eiectron. QE-6 (1970) 270. [2] J.B. hfarling, D.W. Greg and L.L. Wood, Appl. Phys. Letters 17 (19711) 527.
[31 J.B. hbrlin_e, L.L. Wood and D.W. Gregg, IEEE J. Quantum Electron. QE-7 (1971) 498. [4I Colour Index, 2nd Ed., Vol. 4 (Society af Dyers and Calourists, PitcaUIy, 195&j p. 331. (5 1 Coiour Index, 2nd Ed., Vol. 3 (Society Qf Dyes and Coiourists, Picczdiliy, 1957) p. 3066. [6] i.$i Drake and RI. &forsr. Opt. Commun. 12 i147d)
Using a crude mc@:l we havi: been able to explain
the difference in the hsinp ~erforrna~~e of KRS and RH-E
tation re@mc. The fasing resul!s are consistent with
the spectroscopic differences in the molecules. The striking differences appear to be in Jrf and NT s The ~~~r~,~~e of the 9 carbon atom ~~~stitue~t~ lw been idetitified and a conelation with the inductive properties of R’ and the electronic energy level’ separation suggested.
(71 K.H. Drexhage, in: Topics ~~FpIied~h~f~ics~ ;bl. 1, cd. F.P. Schsfcr (Springer, Bctrlin, 1974) p. 119.
[8f H. Kuhn, in: Progress in chemistry of organic natural products,cd. D.L. Zrchmeister (Springer, Berlin, i9S8/ 59). 191 S.P. Xlfiynn, T. Azumi and hf. Kinahit3, Molecular spectroscopy of the rripier 5fafe (Prenrice-H&J, Engkwood Cliffs, 1969) p. 75. fXO] R. En&n~ aad 1. Jortner, J. Lu~~escGnce I,2 (1970) 134. f Zf 1 S. Fischer, E.‘tv, Schlag and S. Sthneide~, Chem. %ys. Letters I1 (1971) 583. [iZl J.M. Drake and R-1. Morse, Opt. Commun. 13 (1975) 109. G.K. Chibisort, li,A. Klezte, L,V. Levshin and T.D: Slavnova, J. Chcm. Sot. Chem. Commun. (1972) 1292. [ 14‘1R.A. lieiier, IEEE 5. Qucintum Electron. QE-6 (1970) 411. [ 15] 3. Poppzlardo, Ii. Sx?%efson md zi. Lzmpicki, Appt. Phys. Letters 16 (1970) 267. [If;] H.R. Stadelmam,J. Luminescence 3 (i470) 143. [ 17’1R.A. V&p~tdi, 5. R~s. Nati. Bui. Std. 76A (1972) &2. ff3f
The authors would Iike to express their spprecistion to Drs. I.B. Marlti~:: and Erich ?. Ippen for help ful discussion
during tile preparation
df the manuscipt.
CHEMICALPHYSiCS LETfERS
1375
The tasing perforin~nc~ of kiton red S and r~od~rnin~ B (COOH) is shown to bc stron@y influenced b:~ the substlturnt bonded to the 9 carban atom of the main chromophore. The lasing efficiency of these dyes is compared under long and short puke flashlamp excitation. The results are shown to be predictable based on the relative differences in @f, Tf and K+T, for these dyes.
The use of&ton red S as a laser dye was reported first by Gregg et al. [I] _ Subsequent work by Marling and co-workers f&3] examined in greater detail the flashlamp Iasing properties of this dye. This work was done without Ihe benefit of knowing the molecular structure
of kiton
red S.
Kiton red S thou&t to b\t tbc monoazo dye j4,5] shown by structure I, but exhibited lasing properties similar, to the common santhene dye, rl~odan~ne B (structure II). We Ilave determined the structure of kiton red S based on nuclear magnetic resonance nnaiysis (see
iig. I) and cleme~t~l analysis*. The proposed structure is structure Hi.
* Preselrt address: Exxon Nuc!ear Company, Inc., R~SXW~I and Technofogy Center, Richhnd, Wshiagton 99357, USA.
* Elomcntol analysis indicated the coirect amount ofsodium znd su$hur present For the combositicm C27H29NZ07S2Nz.
C2%. C,H/N’
++%W5 N’C 2’Y5
181 ~..a
I
1 September 1975
CHEMICAL PHYSICS LETTERS
Volume 35, number :! I
1 cl80
I
I
420
CH:,CH2
I
I
I
360
300
I
I
I
I
I
160
240
I
I
I
I20
60
I
I
OHz
/ CH2 CHS
:N
‘CH2
CHs
_ NA-WH~
.
-
TMS
‘CHpCns
No
RING
9
PROTONS
1 ,s
CH3
‘CH2 -
CH3
-N
PHEN’IL PROTONS DMSO
I
I // I!
FiiJ. 1.60 MHz nuclear magnetic resonance spectrum of kiton red S in DMSO (d&. The striking similar-i-
bonded
to tile 9 carbon
atom is 11strong
structural determinate to the observed spectroscopy‘ and king of these xirnthene molecuIes. The chemistry of tile 9 carbon atom is found to irzfluence the rate of ~tersyst~m crossing (iCS1_T, ) and the energy separation between t:le electronic states of the molecule (S,,S, and TL). These spectroscopic differences can be used to explain the lasing properties of kiton red S and rhodamine B.
2. Experimental Xitorrred S was obtained Inc.., PlainviBe, New Yo&.
from K&X Laboratories, The material received
contained approximately seventy-five percent inorganic salts and twenty-five percent dyestuff by weight. The dye was purified by column chromatography on silica1 gel (Silica Gel Vioelm for absorption, Activity III) by gradient e&&ion using ~c~ton~/metb~nol. The major eluted fraction was homogeneous by thin chaomatographic analysis (TLC). The pyronin B (C-1752) and rhodamine B (P-4453) were obtained from Eastman Kodak and purified until the material was homogeneous by TLC analysis. The solvent used in all spectroscopic and lash-g measurements was absolute ethanol (with no 2-propa1-101,benzene or other denaturing additives present). AU solutions were deoxygenated either by argon saturation or a liquid nitrogen vacuum freeze/thaw cycle. The concentration of dye, for &I spectroscopic mea~remen~s involving the otservation of reradiated liJlt was G t X 10m6 M to avoid errors resulting from
c This [l-3]:
is the same dye sollice used by preview investigtors
CHEBflCAL
Volume 35, number 2
dye self-absorption,
concentration
que~c~~
PHYSKS
tween these electronic levels three spectroscopic meitsuremenfs were made: SI c- S, absorption, S, +- S1 fluorescence and So +- T, phosphorescence.
and dye
aggregation. The spectral data wzs obttined using standard commercial instrumentation. The absorption data was
The molecules &ton red S and rhodamine B have identic& fused ring chromophores. They difier in the type and number of substituents on the pheny! group bonded to the 9 carbon atom. in ethanol the sulphonic acid group in the 2” position (see structure IV) is
taken with a Gary 14. The rluosescence spectra were
recorded using an Aminco43owmart spectrophotofluorimeter equipped with a selected S-20 or 3 c&ibrated S-l spectra! response photomultip~er_
LElTERS
The
&Qsphorescerke was observed using an iiminco phosphorimeter attachment and spectra were taken at 293K and 77K. Tke ~u~resceRce iifetime vfas measured with a TRW Efetime apparatus_ The hsing data ~2s obtained using a 100 joule coaxial lamp and driver (Phase-R, Inc.) 2nd 2 long pulse linear fkshknp pumped system described elsewhere [6’j _ fully dissociated. Kiton red S does not have an ass+ &ted rkon as does rh0damine.B 2nd the carboxylic acid jn the 2’ position of rhodamine B is not fully dissociated in aIco!~~lsolutions. We have made a distinction between the acid and basic form of rltodamiue B. To assure the predominaxe of one form a ~mll amount of HCI or NaOH was added. To understand ehe effect of the 9 carbon substituent
3. Visible spectroscopy The spectroscopic p2mmeters of interest are the energy separation between the ground state (So), the Erst excited singlet state (S1), and the lowest triplet state (T1 > (see fig_ 2). To establish the separation be-
40,000
lO,OOC
ABSORPTION
“i Fig. 2. The.rehtion between the obsened speci~oscopy and
the energy levels of 2 molecule.
183
CHEMICAL: PWSICS
,v01unie 35, nuTn~r 2
,
to be hx same for each molecule
of the 9 carbon substituents
(1) (2)
in
Dye
on the spectroscopy
9 Substitucnt
I
The influence of R’ on A,, might be viewed as a ~ert~rba~~on whose apparent effect is to change L (dist~bu~in~ the x electrons over a larger number of atoms). When R’ exhibits an eiectran withdrawing (--I) force, a net poia~ab~ty in the direction of R’, there is an increase in the effective region over which the n electroils of the main chromophore can be distributed. The result of this small perturbation to the patential of the li electrons is 3 rod-soft-~athoc’~omic skift) in-the absorption maximum. The reverse is observed when R’ is electron donating (4-r), a hypsochromic shift (~I~~-shift) resu!ts.
of the xanthene
Abs.max.
(cm-’
dyes
.%-lift
)
Phosphorescence x max km-“)
--PYRONlFi El
,cy+ Hw4ZCH3n
0
COOH
KITON
RED
-
18115
18060
1975
approximation the separation of So and S, win be dependent upon the conjugated length (L) of the molecu!e and the number of T?electrons (A’),
The property of the substituent (R’) bonded to the 9 carbon atom khich influences thi ener,g of the F electrons of the main ckromophorc, is the direction and magnitude of the inductive effect (I) of R’. The absorption transition S, Z-S, in these dye molecules is a resonance excitation of an electron from the highest filled bonding molecular orbitai to the lowest a~tibo~di~~ molecular orbital fn* C- n). A simple mode1 of the ?r electrons can be made based on the ona dimensional particle in a box problem Table 1 The inffuence
1 September
(ii., free ciectron gas model, Kuhn @3])_To a first
(R’).on the spectroscopy and basing of these dyes we chose the xanthcne series: pyronin B, rhodamine B (COOH); rhodamine f! (COO-) and kiton red S. The molecuIes have identical fused ring chrqtiophores. The substitution on the amino groups are’diethyl for each molecule (this WL\i verified for each dye sample by NMR), ami the i&heRCe Gf the free rOtatiOR Of thb alkylamine groups [‘if &n their relaxation properties is assumed the sertes.
LETTERS
-
3190
14130
3133
bathochromic
14040
3230
bathochromic
13940
3385
-
S
Volume 35, number 2
CHEhlICAL PKYSICS LETTERS
A suitable reference, to establish the magnitude of the inductive effect of R’ on spectroscopy of tile xanthene dyes, is pyronin B. The inductive effect of 2 hydrogen atom is zero. The results shown in table 1 verify the predic’tible shifts in the absorption maximum of this series of dyes. The smallest energy gap (AEsQ_s, ) is associated with the strong electron withdrawmg properties of the sulphonic acid group of kiton red S and the largest energy gap results from
the electron donating property of the dissociated carboxylic acid group of rhodamine B. The inductive
propetiies
of R’ also influence
the
energy of the first excited triplet state (T1 ) of :he xanthene dyes. An argument based on the electrostatic interaction of electrons (the Fermi correlation effects are expected
to increase
as the box gets
smaller (L) 191) can be used to interpret the changes in the ener,T separation between the ground state (So) and T, (AE,,_T~). The phosphorescence m=imum for these dyes (see table 1) show the same characteristic dependence on the inductive properties of R’ as did the absorption maximum. The energy separation between S, and T, (AESI_TI) is also given in table 1. These results may be used to predict the relative magnitude of the rate of intersystem crossing (KS, _TI) and the relative triplet state lifetime (TV) for rhodamine B(COOH) and kiton red S. The relative and rhodamine
triplet
state !ifetime
of kiton
red S
B may be estimated using the equa-
tion [lo]
Table
1 September 1975
2
Spectroscopy of kiton red S md rho&mine B (C30H) 3j Kiton red S Rho&mine 6 (COOH) ES 0X0) km:‘) ET (o,o) (cm-‘)
17770 14385
gs,-T, (cm-‘) d Vs (cm-t)
3385 670 i8020 1 L9000 17330 2.8 + 0.3
17300 2.3 2 OS b)
0.83 c_0.02
0.72 k 0.07, c)
Am,lxS1 (cm-‘) lm3:: (Q Xi-' cm-'j FI.max. (cm-‘) Tf (ns) Of
17680 !4430 3250 750 18060 100000
“)Es(O.O)= e 1c c tronic energy of S, ET (0,O) = elecrronic energy of T, @S, -TV = energy separation of St and T,,
I sndt, .. Ama& = absorption maximum for S1 + SQ transitions, fmex = extinction coefficient at FL max. = fluorescence maximum, Tf = fILOieSAmxSt, cencc lifetime in ethanol at a dye conceniration < 1 x IO-’ M, @f= fluorescence quantum yield for a dye conccntiation c 1 x 10-6M. b)Kef. [ 161: c) ref. [17]. A Vs = Stokes
and indlrectly verify the importance of R’ on the relative rate of intersystem crossing [eq. (a)] .
4.
Iaiag
properties
The spectroscopic properties of a dye are related to their potential performance in 2 laser through an expression of the net gain (G). The net gain for, a round trip in the laser cavity is G(X) = 2(go -cY&,
where tlw, =;:3000 cm-l, y = 1. The relative rate of intersystem cross’flg may be determined from the equaticn [ 111 _
(5)
where the gain (go) and losses ((Yo), for a single pass through an active !ength of gain media (I,): are given by (6)
a0
The spectroscopy of these two molecules are summarized in table 2. It is of interest to noie that the fluorescence quantum yield of kiton red S is larger than rhodamine B. This is consistent with the result of eq. (4). The spectroscopic data may be used to explain the difference ir. the lasing behavior of these two dyes
=~o~&)_~,(~) +~pTI_TpPLJ’
(7)
respectively. The electronic state populations for So, S1 and T, are represented by ~\b, _Nl 2nd iv,, respectively (see fig. 1). The stimulated emission cross section (De) is oe = X4E(X>/8r; In2 rr and normalized
to the fluorescence
(8)
quantum yield (cP~) 185
Volume 35,pumber
1
CHEMICAL PHYSICS LETTERS
?.
Qr = j-E@) dX,
September
i97.5
(9)
where rf is the fluorescence lifetime md q is the refractive index of the media. The self-absorption cross section (u~,__~,) and triplet--triplet absorption cross section (oT,_T,).multiplied by the appropriate state population constitute the losses associated with the dye and 0 represerAs those cavity losses controlled by mirror transmhsion and scattering by the media G-q.
To fully characterize the flashlamp pumped !ssing properties of kiton red S and rhodamine B two temporal pumping regimes were studied. The short pulse regime where the exciting pulse (‘T’,) is short cu_mpared to the rate of intersystem crossing (Ks,_T,)-l. The long pulse regime where ‘TP > (KS1 _-., )- 1. In the short pulse regme eq. (7) reduces to cyO=N 0 u&-S,(X)
(10)
+fl.
These losses are cold cavity losses (present when the gain media is not pumped) and may be determined with little difficulty. The function u~~_~,(A) is shotin
Fig. 4. Stimulated absolute ethanol.
for both
pulse was 323 ns at fwhm which is short compared to (KS _T )-l [13] .) The stimulated emission cross sectioi wi calcillated ustig eqs. (8) and (9) and the spectroscopic data reported. The v&es of ue, over the lasing region studied, for kiton red S are given in fii. 4 (for comparison to rhodamine B see ref. [14] ). The lasing performance of kiton red S and rhodamine B were compared on the bases of measurements of
(1 -R,R.,)
dyes
in fig. 3. The value ofp
was z 7070
and constant for all lasing measurements.
(The shoz pulse data was taken with a coaxial flashlamp znd driver described in ref. [ 121. The flashlamp
emission cross section for kiton red S in
the slope efficiency (SE.), peak pcwer (P,) and energy per pulse. The resdts are tabulated in table 3. A
crude correlation between the ratio of the S.E., PO, Eo, and G+ was observed. This is not surprising based on spectroscopic difference reported irl table 2. The slope efficiency,
(11) where qINv is the inversion
(12)
1710ss=:pI(P+c3,
c =116.700
16.600
16.5C-O
16,‘mo
16,Mo
16.200
16.100
16.oK!
cm-’
Fig. 3. Self-absorption cross section crs~_~~ and rhodamine B in aixolute ethanol. (cso_s,
tiplied by 2.3.)
for k&r.
red S
must be mul-
exp (Nouso_s,
efficiency,
L)
03)
and vzFnrre is the ratio of the hri?g cross-sectional uea to the tota. laser aperture area. Eq. (11) simplifies to S.E. = TN (14)
Volume 35, number 2
1 September 1975
CHfMICAL PHYSICS LETERS
Table 3
Short pulse ia5.n: q%formance of kiton red S and rhodzmine B 2)
kiton red S
TP
Tf
@f
320
2.8
0.83
S.E.
QffW
!.53 1.2
rhodamine B
320
2.3 h)
S.E.(RZ
PO
1.35 1.2
0.72 cf
PO(R)
1.32
.&
E:! (RI
0.73 1.3
1.0
1.2 i3.60
a) The laser ourpu t of kiton red S peaked 2t 16150 cmel and rhodanine B at 16200 cm-l. TD = fkhm of the Da&J.amp pulse ia nr, Tf= fluorescence Lifetime in ix, Qf = fluo:escence quantum effidency, SE. = slope etticikxy - hser output to electrical enerw input - in %,Po s the peak power output in MW for 60 5 input at a dye conccntmtion OCt X Ii)- hi in ethanol, Po
ratio efE0 for KRS to RH-B. b)Ref. [i6]; c)ref. [I?]. if 0 9 C and ‘lapcfiure = 1 (which is the case for the coaxial laser and these dyes). The ratio of the slope efficiencies is S E _(ECRS)/S.E . @H-B) =
~f?‘f(KRS)&‘I’f@H-B),
when
where &,
is fhe pump& rate and is heId constant. The Iasing efficiency difference calculated and observed in the short pulse regime are in good agreement for such a crude model. The wr?velength dependence of the PO and E, ratio’s is shown by the tuning curves in fig. 5, indicating that the greater king efficiency of kiton red S is maintained at all wavelengths. In the long pulse regime to explain the differences in the lasing efs”iciencies of the two dyes c+, must include i&~~,_~ [A). ql,, must also be included in the defmition 0TS.E. as /3is not much greater than C Cpfor this laser was set at = 22%). The triplet state population (NT) can be described for two important cases assuming the steady approximation is valid un-
der these pumping conditions [14] NT = KS1_TI Nr --‘VT/T, - Kq EQI&4 1
(17)
NT =KSL-TL
(18)
‘T$
ifdye (TL) + quencher (SO) 5 dye (SO) +-quencher (T1) is not occurrir~g and NT =KS,_Ti assuming
Kq
occurring.
‘~~l~q
[Q1:
(19)
[QJ3+I/T',, when trip’,et quenching is
The triplet
state single
will be larger for rhodamine
pass absorption
loss (7T)
B based on cqs. (3) and
= OT,_T(KRS). Therefore, (4)if+1_T (RH-B) the lnsing efkiency of KEZS should be greater than RH-B. The results are tabulated in table 4 and indi-
Vo!ume 35, number
2
J September
CHEMICAL PHYSICS LETTEXS
197.5
Tablc4
i,ong pulse iaslng performance
of kiton red S and rhodamine
ltiton red S
B a)
*F
Tt-
@f
12
2.8
0.83
@f(R)
Eo
f x m-3 aided
hf COT
I2
2.3
iI,72
kiton
I2
2.8
0.83
red 5
3.3
B
12
2.3
2.J
7.5
0.05
1.2
1.7
3.03
Q.72
PO (RI.
2.7
0.009
l.2 rhodamine
PO 6.1
0.03 I.2
~~~d~~inc B
=%3(R)
6.2
as table 3 with these exceplionsr PO = peak power in k’lY at an input energy of 100 J, Ka = lasing far 100 1 input, Tp = exciting pulse fwhm in microseconds.
3.1The samesymbol dcilnitions efficiency
cate tfte predi&~~bly greater king efkiency of KRS. To illustrale that the power output ratio is strongly influenced by the magnitude of TT a second set of experiments were made using COT (cyclooctatetraene> to reduce the triplet state population il.51 _ The ‘result.ts given in table 4 suggest that a major difference between KRS and RK-3 is the farger triplet state ~o~ula~~of~ofrllodarkie B.
References It]
D.W. Greg_%MR. Querry, J.B. Mariing, S.J. Thorna.% C.V. Dobler, NJ. Davies and J.F. Belew, IEEE J. Quan-
tum Eiectron. QE-6 (1970) 270. [2] J.B. hfarling, D.W. Greg and L.L. Wood, Appl. Phys. Letters 17 (19711) 527.
[31 J.B. hbrlin_e, L.L. Wood and D.W. Gregg, IEEE J. Quantum Electron. QE-7 (1971) 498. [4I Colour Index, 2nd Ed., Vol. 4 (Society af Dyers and Calourists, PitcaUIy, 195&j p. 331. (5 1 Coiour Index, 2nd Ed., Vol. 3 (Society Qf Dyes and Coiourists, Picczdiliy, 1957) p. 3066. [6] i.$i Drake and RI. &forsr. Opt. Commun. 12 i147d)
Using a crude mc@:l we havi: been able to explain
the difference in the hsinp ~erforrna~~e of KRS and RH-E
tation re@mc. The fasing resul!s are consistent with
the spectroscopic differences in the molecules. The striking differences appear to be in Jrf and NT s The ~~~r~,~~e of the 9 carbon atom ~~~stitue~t~ lw been idetitified and a conelation with the inductive properties of R’ and the electronic energy level’ separation suggested.
(71 K.H. Drexhage, in: Topics ~~FpIied~h~f~ics~ ;bl. 1, cd. F.P. Schsfcr (Springer, Bctrlin, 1974) p. 119.
[8f H. Kuhn, in: Progress in chemistry of organic natural products,cd. D.L. Zrchmeister (Springer, Berlin, i9S8/ 59). 191 S.P. Xlfiynn, T. Azumi and hf. Kinahit3, Molecular spectroscopy of the rripier 5fafe (Prenrice-H&J, Engkwood Cliffs, 1969) p. 75. fXO] R. En&n~ aad 1. Jortner, J. Lu~~escGnce I,2 (1970) 134. f Zf 1 S. Fischer, E.‘tv, Schlag and S. Sthneide~, Chem. %ys. Letters I1 (1971) 583. [iZl J.M. Drake and R-1. Morse, Opt. Commun. 13 (1975) 109. G.K. Chibisort, li,A. Klezte, L,V. Levshin and T.D: Slavnova, J. Chcm. Sot. Chem. Commun. (1972) 1292. [ 14‘1R.A. lieiier, IEEE 5. Qucintum Electron. QE-6 (1970) 411. [ 15] 3. Poppzlardo, Ii. Sx?%efson md zi. Lzmpicki, Appt. Phys. Letters 16 (1970) 267. [If;] H.R. Stadelmam,J. Luminescence 3 (i470) 143. [ 17’1R.A. V&p~tdi, 5. R~s. Nati. Bui. Std. 76A (1972) &2. ff3f
The authors would Iike to express their spprecistion to Drs. I.B. Marlti~:: and Erich ?. Ippen for help ful discussion
during tile preparation
df the manuscipt.