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Determination of triplet quantum yields by laser flash absorption spectroscopy
Volume 34. number
CHEhiKZAL PHYSICS LETTERS
1
1 July 1975
DETERMITVATION OF TRIPLET QUANTUM YIELDS BY L;ASER FEASH ABSi3EWTiON SPECTROSCOPY B. AMAND and R. BENSASSON ER 98: I.aboratoire
Received
de Chimie
PhyJique,
91 4Oi-Orw2y,
France
17 March 1975
using the laser flash spectroscopy technique has been developed for formation $T. &l-values for 17 aromatic compounds in cyclohexane or benzene at room temperature have been obtained in good agreement with previous meaarements. Triplet-triplet extinction coefficients necessary for the 0~ calculations and determined via enerm transfer are also reported. Conditions of application and limitations or this method are outlined. A general method
the determination
based an a relative actinometry
of the quantum
yield of trip!et state
1 .-Introdbction Quantitative information on aromatic quantum yields of triplet state formation h has become available during the past ten years as a result of at least a dozen different methods. Methods monitoring triplet formation by triplet energy transfer have been used by Lamola and Hammond [I] through an oiefin photosensitized isomerisation, by Parker and Joyce [2] through the sensitized P-type delayed fluorescence of perylerle, or by Sandros [3] through the sensitized biacety! phosphob,escence. M~‘re direct methods measured triplet yields by a flash calorimeter technique [4]: by the intensity of an ESR signrl [5] or by the conventional flash absorption spectroscopy which Bowers and Porter [6 ] first applied in a ca:e where the ground state is totally depleted. Horroi:ks and Wilkinson [7], VanderDonckt and Lietaer [8] combined flash spectroscopy and fluorescence quenching by a heavy atom to determine triplet yields. Recently, Soep et al. [9] recorded triplet populations created by a tunable dye laser and monitored with a photodiode the absolute number of
photons absorbed. In the method we shall describe, excited
Gi:h a monochromatic
the sample is also laser flash but the
energy input is measured by excitation of standard solutions. The principle of the method is to compare the concentration of triplets formed on excitation of a solution of a compound X, with the concentration of triplets formed by the same number of quanta from a solution of a standard A with a known triplet and cross-over efficiency. Actually, this relative actinometry was used for the first time by Richards and Thomas [lo] for the determination of TMPD triplet yield. Several studies on fluorescence efficiencies [ 1 l] have outlined the greater accuracy of a relative actinometry and detailed procedures which are also valid for our method.
3. Experimental The laser flash spectroscopy set-up used has already been described elsewhere [ 12,131. A Huet M 25 monochromator is used with band widths of 3 or 1.2 MI. Solutions deaerated by argon bubbling are irradiated with 35 ns flashes of 353 nm or 265 run light delivered by a Cilas neodymium laser after frequency tripling cr quadrupling. Exciting (laser) and analysing (xenon) flashes are in a crossed beam-arrangement. Transient absorption is observed in the sample along 1 cm in the first excited mm of solution.
Volume
34, number
3. Results
1
1 July 197s
CHEMICAL PHYSICS LE-ITERS
solutions OF standard A and compound X hatins the same ground state optical density EC [GA], =
and discussion
Both
3.1, Principle ofthe methA
62 [G’], and being irradiated with the same number of photons J f(t)dr, we can write:
The main step of our method for e determination is to use optically thin solutions (optical density OD at 1 mm depth
~i$,~JGAlrJ =&JGxlo
(1)
(EG is the ground state molar extinction coefficient, [Gjo the concentration at time L = 0 before excitation.) Under such conditions, both solutions absorb the same number of photons provided the following condi-
tions are fulfilled. The ground state depletion must be small enough to form rather low excited singlet, triplet or photoproduct concentrations [S], [T] or [P] and avoid any disturbing effect (excimer formation, T-T etc.). Moreover, laser absorption by excited states S and T or by a photoproduct P must be
annihi!aiion,
&[TXlmaK= @+/[TAimzu
.
Therefore, C$ = Q+(OD$+)
(&OD+)
Q)
(where OD, and ET are the optical density
and the extinction coefficient measured at the maximum of the triplet-triplet absorption spectrum). It should be noticed that in certain cases, which were not met for the 17 rather similar aromatic compounds studied here, the photophysical parameters of A and X cotild be very different. For instance, ~4 2nd 4 might be much higher than of and 0:. Thus it might be necessary for the A solution to reduce [G*10 and use a fraction Q of the exciting photons by inserting an adequate filter, in order to have a low optical density OD$ and a low ground stale depletion G” -+ T*. Consequently in more general conditions, relation
such that relation (1) remains valid during the flash; in other words, that the sum of the optical densities of S, T and P of A or X must remain negligible during the flash, as compared to Q [G] _Speiser et al. [ 141 for a laser pulse excitation, Kling et al. [15] f-or a
(2) should be expressed as:
continuous irradiation have described by a general kinetic analysis the dependence of a primary quantum yield on excited states or photoproduct absorption. fiowever, in our simpler case where S, T or P absorption is negligible, the expression of the triplet quantum yield e, easi!y derived from the kinetic equations is
Relation (3) or relation (2) which has been used in work show that knowledge of ETA,E+, and @ is necessary to determine 6.
rm”:rT
QT=J
d[TlI
0
4
K,(r)[Glodr
0
of T is a (where 1, is the time when concentration maximum which is in 211our cases much smzller than the triplet lifetime rT; 0 is the pulse duration). K,(t) is the absorption (S + G) rate for dill;te solutions: ii’,(r) = 2.3 lG I(r) (where I(r) is the excitation intensity in millieinstein per second per cm2). Thus, the yield 4jrb ecomes:
+r = PI ma/z.3
EG [GIO
JI(r)dr .
0
--
.
(3)
the present
The extinction coefficients ET used were detcrmined by the ener,,z transfer method which consists of comparing an unknown according to the reaction T (donor)
+ G (acceptor)
triplet
with a known
4 T (acceptor)
one
+ G (donor).
E= values, either are those determined previously by Bensasson and Land [16] with a pulse radiolysis excitation, or have been determined in the present work by the same energy transfer method with a laser excitation; procedure; have been detailed elsewhere [17]. Table 1 presents the latter eT values and the donor acceptor pairs chosen. We should notice that in seven cases reported, the ratio eG(acceptor)/eG(donor) = C in other words that the acceptor did not abtit hexc,
Volume 34, number 1
CHEMICAL PHYSICS LETTERS
1 July 1975
Table 1 Triplet &iinction coefficients ET determined by laser flash spectroscopy. Data of present work are in cyclohesane to be in benzene (Liz). Solvents: chx = cyclohexanc; 3mp = 3-methylpentane; EPA = ethylether-S-isopentne-5+thanol-2;
unless
specified hx = II-
hcxane ncc
kXC
Triplet donor
EG don
(run)
L-1% at
Triplet
q-(Lmax)
(lo3 P mole-’
cm-‘)
aCCCptOi”)
litemtuie data
present workb)
_I ~XC
353 353 353 353 353 353 265 265 353
0 0 0 0 0 0 213 l/70 0
TMPD c, TMPD TMPD TIMPD TMPP ThlPD phcnan!hrene chrysene beniJphcnone/bz
1-mcthylN 2-methylN 1-methoxyN 2-methoxyN I-chloroN l-phenylN 2-phenylN anthracene triphenylene
353
2
bcnzophenone/bz
fluorenone
22.4 25.1 7.0 15.0 20.7 17.6 48.3 21.6 6.0
5 r + r f f f c r
3 (420) 3 (420) OS(440) 0.8(4X) 3 (420) 1 (480) 5 (425) 2 (565) 0.5(435)
20.7(4 17.5)/chx d) 18.4(495)/3mp77K e, 34_6(432)/3mp77Ke) 36.2(580)/EPA77K f) 4.1 (StS)/hx g) 16.8(437)/EPX77K f) 6.7 * o.7(43o)/bzh)
5.9 r 0.5(430)
a) N = naphthalene. b) Measured ET refer to compounds used as triplet acceptors, except for chrysenc used as triplet donor. c) ET (570 m-n) = 11900/chx, from ref. [IO]. We checked this value with naphthalene triplet as acceptor at353 nm. d)Ref. [22];e)ref. [23];f)~f. [24];6)ref. [25];h)ref.[26].
sorb the exciting wavelen:;th. For the other cases trip-
of ThlPD triplet excited
let acceptor could be formed by direct excitation and a suitable correction was applied [ 171.
still being valid during the laser flash, we checked for certain compounds that the 6 value remains unchanged when decreasing the ground state optical
For excitations performed at 265 nm the actinometer chosen was naphthalcne and its singlet to triplet
density portant
quantum efficiency has been measured in cyclohexane * = 0.75 * 0.03, applyin, 0 the method of fluorescence
sensible h
quenching by xenon. For excitations performed at 353 nm the standard chosen was anthracene. Its &,- = 0.71 f 0.05 in cyclohexane has been measured by the preserlt method versus naphthalene (X,x, = 265 nm) and duroquinone = 353 m-n, +Q = 1 [181). For nnthracene DQ G,,, in benzene, h = 0.67 ? 0.05 was obtained versus benzophenone BP (A,, = 353 run, @FP = 1 [ 1 I). 3.3. Derermimtion
of q$
Table 2 reports the yields QT derived from (2) and the corresponding ET used. It should be eilphasized that, for sharp T-T absorption peaks (e.g., znthracenej, Ed- and + measurements must-be and were achieved with the same monochrornator band widths: ‘To enstire that relation (1 j can be considered as 46.:
,.:’ “.
‘,
and the number of incident photons. It is imto notice that in our conditions of work, a
is obtained for anthracene excited at
3.53 nm, though
the first excited singlet state has an extinction coefficient fS 25 53000 M-l cm-l, routily ten times higher than eG at 353 nm [1’S]. In other words, even in this particular case, relation (1) remains valid during the flash by the fact that the optical density of the sample and the intensity of the laser beam are low enough to ensure a negligible depletion of the anthracene ground state and a negligible absorption of its singlet excited state. The accuracy of the method depends on the accuracy with which the eT values are krlown which is still a disadvantage as compared to other methods.
These values mi&t be improved by a calculation from an average oscillator strength [20]. Provided the eT values are correct within.1075 and numerous measurements on compound X are averaged, we can estimate that the error mqnitude
on @$ remains
below
15%.
VoIume 34, number
1
CHEMICAL PHYSICS LElTERS
I July 1975
Table 2 Intersystem crossing yields m in solution nt room temperature. Data or present work are in cyclohexane benzzsne (IX). Solvents: et = ethanol; Ip = liquid partin: other abbreviations, cf. table 1 ----Compound
@T literature
data
10-X ET(Xmax) cm-‘) Benssson and
unless specified
ec be in
Present work
(e mole-’ Parker and Joyce [2]/et
Horrocks and Wilkinson (71
others
Land
10-3 ET(A)
@
[ 161 ?xcxc = 265 run
NAPHTHALENE
0.71
O.EL/bz
G.68/chx [3 1
0.80lct
0.81/3mp-[ i?]
I-methylN 2-methylN l-methoxyN 2-methosyN I-chIcroN 1phenylN 2-phenylN biphcnyl phenanthcene
0.46
OSOjet
0.48/bz O.jl/bz 0.26/bz
0.80
0.85/et
O.El/chx 0.82/chx
ANTHRACENE
0.70
0.75Jlp 0.72/et
O.SS/lp [6]
0.77ihx 0.82/et
0.79/hx [ 271 0.78/lp [ 81 0.60/bz [8]
0.8S]et 0.38jet
0.81/hx [27] 0.38/hx [27]
0.75
24.5(414) 22.4(420) 25.1<420) 7.0(440) 15.0(435) 20.7(420) 17.6(480) 48.3(425)
[l] [l] [ 1J
OSR 0.56 0.45 0.50 0.79 0.52 0.43 0.84 0.73
42.8(361) 25.2(482.5)
[3] [3 ]
0.70/3mp [6 ]
1,2-benzanthracene tetnccnc chrysene pyrene duroquinone fluorenone
0.82 0.21
2) In ref. [16] table 2, T-T
(A in nm) of anthracene
Our h
values show good agreement with those obby other authors by different methods (see
table 2). This agreement supports the ET determination by the energy transfer method, at least for the compounds studied. It also confirms the value of the ketyl radical (dz COH) extinction coefficient E (550 nm) = 3220 M-l cm-l in .water, reference to which the ET of all our compounds are inrerconnected in a series of donor acceptor pairs with the approximation that oscillator strengths are independent of sol[16]. .Agreement between various 6 determined with different excitation energies, ranging from 3.1 to 6.2 eV, might 5e indicative that intersystem crossing vent
0.7 1
4S.S(43O)/bza! 205(490)/bz
= 353 nm
0.67/bz 0.82/bz
31.2(465)/bz
0.62/bz 0.85 0.50 0.94/bz 0.94/bz
21.6(565)
I/chx [ 181 0.93/bz [ 1,281 maximum
a)
A exe
4. Conclusion
tained
64.7c422.5)
30.4(412.5) 7.0(490)/bz 5.9(430)jbz
must he read as given in table
occurs from the same lowest excited singlet state in thermal equilibrium. Although the occurrence of intersystem crossing from 2 higher excited singiet should not be ruled
out,
it does
not appear
to be likely
for
the molecules studied herein. Fluorescence quenching methods [7,5] may provide more accurate e Mlues when the fluorescence lifetime is long enough and the quencher does not absorb X,,,. IUevereheless, the method described above, .based on a monochromatic excitation and a relative actinometry, stands out by its simplicity and its generality. It has $ready been applied to several quinones
and polyenes
of biological
interest
in recent
studies [17,21], and might prove a valuable the investigation of non-radiative aromatic compour?ds.
tool for parameters of
47
Volume 34, number
1
CHEMICAL
PHYSICS LETTERS
1 July 1975
; Refe.rences
[I] A.A. Lamola and G.S. Hammond,
[i4] J. Chem. Phys. 43
(1965) 2129. [2] CA. Parker and T.A. Joyce, Trans. Faraday Sot. 62 (1966) 2785. [ 3 ] K. Sandros, Acta Chem_ Snnd. 23 (1969) 2815. [4] J.B. CaUis, hl. Gouteman and J.D.S. Danielson, Rev. Sci. Inscr. 40 (1969) 1599. [S] M. GuCron; J. Eisirzpcr and KG. Schulman, Mol. Phys. 14 (19.58) 111. (6 ] P.G. Bowars and G. Porter, Proc. Roy. Sot. A299 (1967) 348. [7] A.R. tiorrocks and F. Wilkirtson, Proc. Roy. Sot. A306 (1968) 257. [8] E. VandcrDonckt and D. Lictacr, J. Chem. Sot. Faraday Trans. I.65 (1972) 112. 191 B. Soep, A. Kelbnarm, hll Martin and L. Lindqvist, Chem. Phys. Letters 13 (1972) 241. [lOI J.T. Richards and J.K. Thor&, Trans. Faraday Sot. 66 (1970) 621. (111 C.A. Parker, in: Advances in photochemistry, eds. W.A. Noyes, G.S. Hammond and J.N. Pitts (Interscience, New York, 1964) p_ 305. [ 121 R. Bensasson, C. Chnchnty, E.J. Iznd and C. Met, Photochem. Photobioi. 16 (1972) 27. [ 13] B. Amand, These IlIZme Cycic, UniversitC de Paris VI (1974).
IIS] [ 151 (171 [IS!
[I91 [20] [21] (221 [23] 124) 1251 [26] [27] 128)
S. Speiser, R. van der Werf and J. Kommandeur, Chem. Phys. 1 (1973) 297. 0. K!ing, E. Nikolaiski and H.L. SchEfer, Z. Elektrothem. Ber. Bunsenses. Physik. Chem. 67 (1963) 683. R. Benslsson and E.J. Land, Trans. Faraday Sac. 67 (1971) 1904. E. Amouyal, R. Benssson and E.J. Lznd, Photochem. Photobiol. 20 (1974) 415. J. Nafisi-hIova&zr and F. Wilkinson, Trnnr Fanday Sot. 66 (1970) 2268; E. Amouyaf and R. Bensasson, unpublished. D. Beb+Iaar,Chem. Phys. 3 (1974) 205. D. Uvalette, R. Ben&m, B. Amand and E.J. Land. Chem. Phys. Letters 10 (1971) 331. R. Bensasson, E.J. Land and T.G. Truscott. Photochem. Photobiol. 17 (1973) 53. F. Dainton, MB. Ledger, R. May and G.A. Salmon, J. Phys. Chem. 77 (1973) 4.5. J.S. Brinenand M.K. Orloff, J. Chem. Phys. 51 (1969) 527. D. Lavalette, J. Chim. Phys. 66 (1969) 1853. G. Porter and hf.W. Windsor, Proc. Roy. Sac. t45A (1958) 238. E.J. Land and R. Bensasson, unpublished. W. Heinzelmnnn and H. Labhart, Chem. Phys. Letters 4 (1969) 20. R.A. Caldwell and R.P. Gajewski, J. Am. Chem. SOL. 93 (1971) 532.
CHEhiKZAL PHYSICS LETTERS
1
1 July 1975
DETERMITVATION OF TRIPLET QUANTUM YIELDS BY L;ASER FEASH ABSi3EWTiON SPECTROSCOPY B. AMAND and R. BENSASSON ER 98: I.aboratoire
Received
de Chimie
PhyJique,
91 4Oi-Orw2y,
France
17 March 1975
using the laser flash spectroscopy technique has been developed for formation $T. &l-values for 17 aromatic compounds in cyclohexane or benzene at room temperature have been obtained in good agreement with previous meaarements. Triplet-triplet extinction coefficients necessary for the 0~ calculations and determined via enerm transfer are also reported. Conditions of application and limitations or this method are outlined. A general method
the determination
based an a relative actinometry
of the quantum
yield of trip!et state
1 .-Introdbction Quantitative information on aromatic quantum yields of triplet state formation h has become available during the past ten years as a result of at least a dozen different methods. Methods monitoring triplet formation by triplet energy transfer have been used by Lamola and Hammond [I] through an oiefin photosensitized isomerisation, by Parker and Joyce [2] through the sensitized P-type delayed fluorescence of perylerle, or by Sandros [3] through the sensitized biacety! phosphob,escence. M~‘re direct methods measured triplet yields by a flash calorimeter technique [4]: by the intensity of an ESR signrl [5] or by the conventional flash absorption spectroscopy which Bowers and Porter [6 ] first applied in a ca:e where the ground state is totally depleted. Horroi:ks and Wilkinson [7], VanderDonckt and Lietaer [8] combined flash spectroscopy and fluorescence quenching by a heavy atom to determine triplet yields. Recently, Soep et al. [9] recorded triplet populations created by a tunable dye laser and monitored with a photodiode the absolute number of
photons absorbed. In the method we shall describe, excited
Gi:h a monochromatic
the sample is also laser flash but the
energy input is measured by excitation of standard solutions. The principle of the method is to compare the concentration of triplets formed on excitation of a solution of a compound X, with the concentration of triplets formed by the same number of quanta from a solution of a standard A with a known triplet and cross-over efficiency. Actually, this relative actinometry was used for the first time by Richards and Thomas [lo] for the determination of TMPD triplet yield. Several studies on fluorescence efficiencies [ 1 l] have outlined the greater accuracy of a relative actinometry and detailed procedures which are also valid for our method.
3. Experimental The laser flash spectroscopy set-up used has already been described elsewhere [ 12,131. A Huet M 25 monochromator is used with band widths of 3 or 1.2 MI. Solutions deaerated by argon bubbling are irradiated with 35 ns flashes of 353 nm or 265 run light delivered by a Cilas neodymium laser after frequency tripling cr quadrupling. Exciting (laser) and analysing (xenon) flashes are in a crossed beam-arrangement. Transient absorption is observed in the sample along 1 cm in the first excited mm of solution.
Volume
34, number
3. Results
1
1 July 197s
CHEMICAL PHYSICS LE-ITERS
solutions OF standard A and compound X hatins the same ground state optical density EC [GA], =
and discussion
Both
3.1, Principle ofthe methA
62 [G’], and being irradiated with the same number of photons J f(t)dr, we can write:
The main step of our method for e determination is to use optically thin solutions (optical density OD at 1 mm depth
~i$,~JGAlrJ =&JGxlo
(1)
(EG is the ground state molar extinction coefficient, [Gjo the concentration at time L = 0 before excitation.) Under such conditions, both solutions absorb the same number of photons provided the following condi-
tions are fulfilled. The ground state depletion must be small enough to form rather low excited singlet, triplet or photoproduct concentrations [S], [T] or [P] and avoid any disturbing effect (excimer formation, T-T etc.). Moreover, laser absorption by excited states S and T or by a photoproduct P must be
annihi!aiion,
&[TXlmaK= @+/[TAimzu
.
Therefore, C$ = Q+(OD$+)
(&OD+)
Q)
(where OD, and ET are the optical density
and the extinction coefficient measured at the maximum of the triplet-triplet absorption spectrum). It should be noticed that in certain cases, which were not met for the 17 rather similar aromatic compounds studied here, the photophysical parameters of A and X cotild be very different. For instance, ~4 2nd 4 might be much higher than of and 0:. Thus it might be necessary for the A solution to reduce [G*10 and use a fraction Q of the exciting photons by inserting an adequate filter, in order to have a low optical density OD$ and a low ground stale depletion G” -+ T*. Consequently in more general conditions, relation
such that relation (1) remains valid during the flash; in other words, that the sum of the optical densities of S, T and P of A or X must remain negligible during the flash, as compared to Q [G] _Speiser et al. [ 141 for a laser pulse excitation, Kling et al. [15] f-or a
(2) should be expressed as:
continuous irradiation have described by a general kinetic analysis the dependence of a primary quantum yield on excited states or photoproduct absorption. fiowever, in our simpler case where S, T or P absorption is negligible, the expression of the triplet quantum yield e, easi!y derived from the kinetic equations is
Relation (3) or relation (2) which has been used in work show that knowledge of ETA,E+, and @ is necessary to determine 6.
rm”:rT
QT=J
d[TlI
0
4
K,(r)[Glodr
0
of T is a (where 1, is the time when concentration maximum which is in 211our cases much smzller than the triplet lifetime rT; 0 is the pulse duration). K,(t) is the absorption (S + G) rate for dill;te solutions: ii’,(r) = 2.3 lG I(r) (where I(r) is the excitation intensity in millieinstein per second per cm2). Thus, the yield 4jrb ecomes:
+r = PI ma/z.3
EG [GIO
JI(r)dr .
0
--
.
(3)
the present
The extinction coefficients ET used were detcrmined by the ener,,z transfer method which consists of comparing an unknown according to the reaction T (donor)
+ G (acceptor)
triplet
with a known
4 T (acceptor)
one
+ G (donor).
E= values, either are those determined previously by Bensasson and Land [16] with a pulse radiolysis excitation, or have been determined in the present work by the same energy transfer method with a laser excitation; procedure; have been detailed elsewhere [17]. Table 1 presents the latter eT values and the donor acceptor pairs chosen. We should notice that in seven cases reported, the ratio eG(acceptor)/eG(donor) = C in other words that the acceptor did not abtit hexc,
Volume 34, number 1
CHEMICAL PHYSICS LETTERS
1 July 1975
Table 1 Triplet &iinction coefficients ET determined by laser flash spectroscopy. Data of present work are in cyclohesane to be in benzene (Liz). Solvents: chx = cyclohexanc; 3mp = 3-methylpentane; EPA = ethylether-S-isopentne-5+thanol-2;
unless
specified hx = II-
hcxane ncc
kXC
Triplet donor
EG don
(run)
L-1% at
Triplet
q-(Lmax)
(lo3 P mole-’
cm-‘)
aCCCptOi”)
litemtuie data
present workb)
_I ~XC
353 353 353 353 353 353 265 265 353
0 0 0 0 0 0 213 l/70 0
TMPD c, TMPD TMPD TIMPD TMPP ThlPD phcnan!hrene chrysene beniJphcnone/bz
1-mcthylN 2-methylN 1-methoxyN 2-methoxyN I-chloroN l-phenylN 2-phenylN anthracene triphenylene
353
2
bcnzophenone/bz
fluorenone
22.4 25.1 7.0 15.0 20.7 17.6 48.3 21.6 6.0
5 r + r f f f c r
3 (420) 3 (420) OS(440) 0.8(4X) 3 (420) 1 (480) 5 (425) 2 (565) 0.5(435)
20.7(4 17.5)/chx d) 18.4(495)/3mp77K e, 34_6(432)/3mp77Ke) 36.2(580)/EPA77K f) 4.1 (StS)/hx g) 16.8(437)/EPX77K f) 6.7 * o.7(43o)/bzh)
5.9 r 0.5(430)
a) N = naphthalene. b) Measured ET refer to compounds used as triplet acceptors, except for chrysenc used as triplet donor. c) ET (570 m-n) = 11900/chx, from ref. [IO]. We checked this value with naphthalene triplet as acceptor at353 nm. d)Ref. [22];e)ref. [23];f)~f. [24];6)ref. [25];h)ref.[26].
sorb the exciting wavelen:;th. For the other cases trip-
of ThlPD triplet excited
let acceptor could be formed by direct excitation and a suitable correction was applied [ 171.
still being valid during the laser flash, we checked for certain compounds that the 6 value remains unchanged when decreasing the ground state optical
For excitations performed at 265 nm the actinometer chosen was naphthalcne and its singlet to triplet
density portant
quantum efficiency has been measured in cyclohexane * = 0.75 * 0.03, applyin, 0 the method of fluorescence
sensible h
quenching by xenon. For excitations performed at 353 nm the standard chosen was anthracene. Its &,- = 0.71 f 0.05 in cyclohexane has been measured by the preserlt method versus naphthalene (X,x, = 265 nm) and duroquinone = 353 m-n, +Q = 1 [181). For nnthracene DQ G,,, in benzene, h = 0.67 ? 0.05 was obtained versus benzophenone BP (A,, = 353 run, @FP = 1 [ 1 I). 3.3. Derermimtion
of q$
Table 2 reports the yields QT derived from (2) and the corresponding ET used. It should be eilphasized that, for sharp T-T absorption peaks (e.g., znthracenej, Ed- and + measurements must-be and were achieved with the same monochrornator band widths: ‘To enstire that relation (1 j can be considered as 46.:
,.:’ “.
‘,
and the number of incident photons. It is imto notice that in our conditions of work, a
is obtained for anthracene excited at
3.53 nm, though
the first excited singlet state has an extinction coefficient fS 25 53000 M-l cm-l, routily ten times higher than eG at 353 nm [1’S]. In other words, even in this particular case, relation (1) remains valid during the flash by the fact that the optical density of the sample and the intensity of the laser beam are low enough to ensure a negligible depletion of the anthracene ground state and a negligible absorption of its singlet excited state. The accuracy of the method depends on the accuracy with which the eT values are krlown which is still a disadvantage as compared to other methods.
These values mi&t be improved by a calculation from an average oscillator strength [20]. Provided the eT values are correct within.1075 and numerous measurements on compound X are averaged, we can estimate that the error mqnitude
on @$ remains
below
15%.
VoIume 34, number
1
CHEMICAL PHYSICS LElTERS
I July 1975
Table 2 Intersystem crossing yields m in solution nt room temperature. Data or present work are in cyclohexane benzzsne (IX). Solvents: et = ethanol; Ip = liquid partin: other abbreviations, cf. table 1 ----Compound
@T literature
data
10-X ET(Xmax) cm-‘) Benssson and
unless specified
ec be in
Present work
(e mole-’ Parker and Joyce [2]/et
Horrocks and Wilkinson (71
others
Land
10-3 ET(A)
@
[ 161 ?xcxc = 265 run
NAPHTHALENE
0.71
O.EL/bz
G.68/chx [3 1
0.80lct
0.81/3mp-[ i?]
I-methylN 2-methylN l-methoxyN 2-methosyN I-chIcroN 1phenylN 2-phenylN biphcnyl phenanthcene
0.46
OSOjet
0.48/bz O.jl/bz 0.26/bz
0.80
0.85/et
O.El/chx 0.82/chx
ANTHRACENE
0.70
0.75Jlp 0.72/et
O.SS/lp [6]
0.77ihx 0.82/et
0.79/hx [ 271 0.78/lp [ 81 0.60/bz [8]
0.8S]et 0.38jet
0.81/hx [27] 0.38/hx [27]
0.75
24.5(414) 22.4(420) 25.1<420) 7.0(440) 15.0(435) 20.7(420) 17.6(480) 48.3(425)
[l] [l] [ 1J
OSR 0.56 0.45 0.50 0.79 0.52 0.43 0.84 0.73
42.8(361) 25.2(482.5)
[3] [3 ]
0.70/3mp [6 ]
1,2-benzanthracene tetnccnc chrysene pyrene duroquinone fluorenone
0.82 0.21
2) In ref. [16] table 2, T-T
(A in nm) of anthracene
Our h
values show good agreement with those obby other authors by different methods (see
table 2). This agreement supports the ET determination by the energy transfer method, at least for the compounds studied. It also confirms the value of the ketyl radical (dz COH) extinction coefficient E (550 nm) = 3220 M-l cm-l in .water, reference to which the ET of all our compounds are inrerconnected in a series of donor acceptor pairs with the approximation that oscillator strengths are independent of sol[16]. .Agreement between various 6 determined with different excitation energies, ranging from 3.1 to 6.2 eV, might 5e indicative that intersystem crossing vent
0.7 1
4S.S(43O)/bza! 205(490)/bz
= 353 nm
0.67/bz 0.82/bz
31.2(465)/bz
0.62/bz 0.85 0.50 0.94/bz 0.94/bz
21.6(565)
I/chx [ 181 0.93/bz [ 1,281 maximum
a)
A exe
4. Conclusion
tained
64.7c422.5)
30.4(412.5) 7.0(490)/bz 5.9(430)jbz
must he read as given in table
occurs from the same lowest excited singlet state in thermal equilibrium. Although the occurrence of intersystem crossing from 2 higher excited singiet should not be ruled
out,
it does
not appear
to be likely
for
the molecules studied herein. Fluorescence quenching methods [7,5] may provide more accurate e Mlues when the fluorescence lifetime is long enough and the quencher does not absorb X,,,. IUevereheless, the method described above, .based on a monochromatic excitation and a relative actinometry, stands out by its simplicity and its generality. It has $ready been applied to several quinones
and polyenes
of biological
interest
in recent
studies [17,21], and might prove a valuable the investigation of non-radiative aromatic compour?ds.
tool for parameters of
47
Volume 34, number
1
CHEMICAL
PHYSICS LETTERS
1 July 1975
; Refe.rences
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