P""oclo
J
.. PlIOrobioiogy,
1978,
Vol.27,pp.~S29. ~
Pre..
CONVENIENT AND SIMPLE METHODS FOR THE
OBSERVATION OF PHOSPHORESCENCE IN FLUID
SOLUTIONS. INTERNAL AND EXTERNAL HEAVY ATOM AND
MICELLAR EFFECTS
NICHOLAS J. TURRO,* KOU-CHANG
Department
of Chemistry,
Columbi~
Lro,
MING-FEA
University,
CHOW and PLATO LEE
New York, NY 10027, U.S.A.
(Received
30 August1977;~~pi~;~7~~tm
Abstract--Phosphorescence of organic molecules in ftuid solutions may be conveniently and readily
observed under certain conditions. If kp (radiative phosphorescencerate constant) is ~IOs-1, then
(in the absence of photoreaction) phosphorescenceis observable upon N2 purging. For example,
nitrogen purged; acetonitrile solutions of bromo and dibromonaphthalene display readily observable
phosphorescenceas a result of internal heavy atom enhancementof .p., and kp. External heavy atom
enhancementof kp (CH2BrCH2Br solvent) of aromatic hydrocarbons even allows observation of phosphorescencefrom these compounds in N2 purged fluid solutions. Although bromonaphthalenes are
not significantly phosphorescentin N2 purged aqueous solution, phosphorescenceis readily observed
in N2 purged detergent (HDTBr, HDTCI, and SDS) solutions above the critical micelle concentration.
The general factors which determine whether phosphorescenceis "readily" obervable in fluid solution
are briefty discussedand the results are interpreted in light of these factors.
INTRODUCflON
At 77 K, phosphorescenceis a commonly observed
phenomenon following photoexcitation of organic
compounds(Kasha, 1947;Lower and EI-Sayed,1966).
The acceptanceof the identity of the phosphorescent
state as the triplet state of organic moleculeswas not
general for nearly two decades after the classical
papers of Lewis and Kasha and of Terenin (Lewis
and Kasha, 1944;Terenin, 1943).In spite of the observation of phosphorescence
of biacetyl in fluid solution
(Lewis and Kasha, 1945; Backstrom and Sandros,
1962, 1960),this molecule was commonly viewed as
a pathological exceptionto the striking and persistent
failure to observephosphorescence
from organic moleculesin fluid solution. The elegantflash spectroscopy
of Porter (porter and Windsor, 1958; Porter and Wilkinson, 1961) provided convincing evidence for the
existenceof the triplet state in fluid solution, but the
problem of why phosphorescencewas "quenched"
(implication: tPp< 10-4) in fluid solutions remained
unsolved. Although simple specific bimolecular deactivation by adventitious diffusional quencherssuch as
O2 and solvent impurities qualitatively rationalizes
the absenceof phosphorescence,even in the middle
1960's more complicated interpretations such as
phonon involvement (Kellogg and Schwenker, 1964)
and coupling of molecular rotation with electronic
motion (Porter and Stief, 1962)continued to be proposed as alternative explanations.
Mter several scattered reports of the observation
of very weak phosphorescenceof molecules other
than biacetyl in fluid solutions near room temperature (Backstrom and Sandros, 1960;Parker and Hatchard, 1962),a breakthrough was published by Parker
and Joyce (Parker and Joyce, 1969) who demon-
'~
strated that the combination of an "unreactive solvent" (e.g. perfluorocarbons), low concentration of
oxygen, and careful solvent and solute purification
allows the ready observation of phosphorescences
whose quantum efficiency approachedthat observed
at 77 K! Shortly afterwards, numerous reports followed and de.monstrated that phosphorescenceof
organic molecules in fluid solution was a viable
means of studying photochemical reaction mechanisms (Clark et al., 1969;Saltiel et al., 1970).However,
there may still be a feeling among those photochemists who employ emission spectroscopymainly
as an analytical tool, that the general observation of
phosphorescencein fluid media requires very special
or exotic solvents (e.g. fluorocarbons), elaborate or
unconventional purflcation procedures,and specially
designed ultrasensitive equipment. The purpose of
this paper is to help to eradicate this misconception by demonstrating that phosphorescencein fluid
solution is, in fact, readily observablein common and
useful solvents(e.g.acetonitrile) and requires only (a)
simple Nz gaspurging, (b) attention to concentrations
and (c) conventional purification schemes.Of further
interest and importance to biochemists interested in
luminescencetechniques,we report that phosphorescence of simple molecules such as halonaphthalenes
may be observed in micellar solutions (models for
membranesolutions).Only a few examplesa:e known
of the useof triplets as probes of biologically relevant
systems(Saviotti and Galley, 1974;lmakubo and Kai,
1977; Cherry and Schneider, 1976). Hopefully, the
results reported here will serveto stimulate the application of phosphorescence
spectroscopyfor fluid solutions, in particular to areas involving the study of
the structure and dynamics of macromolecules.
m
NICHOLAS J. Tuna
AND METHODS
aspossible« 1mm).The sampleswerebubbledas vigorously as possible (rate limited by capillary size) for
-IS min. The bubbling may be terminated by simply
snapping the capillary by turning the stopcock or by closing the stop cock immediately as the capillary is withdrawn. For degassing detergent solutions (foam!) a long
stem (-10 cm) is recommended as is a bubble breaker
bulb.
Although conventional N2 (Airco, Grade 4.5 TM) is
satisfactory, we found that Union Carbide-Linde, "oxygenfree" grade was superior (i.e. phosphorescenceintensities
were 2-3 times greater with the latter source). In the case.
of aromatic hydrocarbons in ethylene dibromide best
results were achievedby vacuum degassing(5 cycles)rather
than by N2 purgin(.
LuminescencespeCtrawere recorded on a Hitachi-Perkin Elmer fluorimeter (MPF-2A or 3L, equipped with
Hamamatsu 777 phototube). The spectra reported are uncorrected. Bands in the spectrum that were established as
being due to scattering or overtones are not shown in the
spectra reported.
~HOSPHORESCENCE
Solvent' Ac.tonitrile
Nltro,en purged
Aerated
3Z0
380
440
~
A(nm)spectrum
'60
-
~
,;.
O"
,
,.."
~T,---~~7"c~
C,"
.
..
,
'
...
~'
~FlUOR£SCENCE
/:'\~
Naphthalene and 1,4-dibromonaphthalene were crystalized six times from 95% ethanol, dried overnight and then
sublimed twice. Triphenylene was recrystalized six times
from 95% ethanol. Water was distilled twice. Acetonitrile
(Matheson, Coleman and Bell or Eastman; Spectro grade)
was used without further purification. Ethylene bromide
(Aldrich) was treated with conc. H2SO4 until the acid layer
was almost colorless. The bromide was then washed three
times with NaHC03, three times with water and then
dried over MgS04. The resulting material was passed
through a neutral alumina column and then twice distilled.
The detergents(HDTQ and HDTBr, Eastman; SDS, Eastman or Pierce) were purified by washing four times with
ether, followed by dissolution in boiling ethanol. Ether was
added to the cooled ethanol solutions until they became
slightly cloudy. Ethanol was then added in small amounts
until the solutions just becameclear. The resulting solution
was kept in a refrigerator until precipitation occurred and
the precipitate was collected by filtration. This procedure
was repeatedfour times. The final precipitate collected was
dried in vacuo overnight.
The N2 purging method employed in this study basically
involves vigorous bubbling of a sample through a small
(lessthan 1 mm) glasscapillary followed by immediate capping of the cell. The effectivenessof the degassingdepends
on (a) the purity of the N2 gas, (b) the rate of bubbling,
(c) the prevention of air leakage after thorough degassing
has been achieved. For example, for non-detergent solutions an ordinary quartz fluorescence curvette equipped
with a snug Teflon stopper is appropriate. The cell is filled
to two-thirds or one-half capacity with sample solution
and then vigorously (> 1 bubble/s) purged with N2. A disposable pipette (Beckman pasteur) may be employed and
should be set so that its tip is -1 mm from the bottom
of the cell. Bubbling for the order of 10min is usually
sufficient, but an unknown system should be tested empirically to determine whether shorter or longer times are
more appropriate. If the solvent is volatile, cooling to O°C
usually retards evaporation to the point that concentration
changes are insignificant. Although simple to execute the
successof this procedure dependson the ability to stopper
the sample quickly. It thus servesbest when only qualitative information is desired (i.e. is there phosphorescenceor
not?).
A more satisfactory and quantitative method involves
the use of a fluorescence cell that has been modified by
attachment of a Teflon stop cock to replace the cap.
Nitrogen was bubbled through samplesby means of a very
thin capillary that was inserted through the stop cock
(open position!) with the tip as close to the cell bottom
"
MATERIAlS
et al.
6Z0
Figure 1. Total emission
of 0.3 mM
1,4-dibromonaphthalene (DBN) in acetonitrile at room temperature.
The spectra have been normalized
by arbitrarily
overlapping the fluorescence
portions,
since the latter are relatively
insensitive
to aeration.
RFSULTS
The photoluminescenceof 1,4-dibromonaphthalene
(as well as that of 1- and 2-bromonaphthalene) is
readily observablein N2 purged acetonitrile solution
(Fig. 1). From a comparison with data from low temperature emission,the observedspectral distributions
525
Phosphorescenci:in fluid solutions
~
-~BUB8LED (0.05 M HDTBrI
_-:~RATED(O.o5MHDTBr)
---AERATED
(Hz°I
A~SCENCE
v
\
\
s
n
~\
FLUOR£SCEr«:E
'
\.
'--F=--'-~
. -;---;-~~S~--;--=T
340 ~O 420 460 500 540 580 620 660 700
~ (nm)Figure 3. Total emission of I mM 1,4-dibromonaphthalene
in aqueous solutions of 0.05M hexadecyltrimethyl
ammonium chloride above the CMC. Below the CMC
(saturated solutions of DBN) phosphorescenceis barely
detectable under any conditions.
340 380 4W 460 500 540 580 620 660 700
~("m)Figure
in
4. Total
aqueous
ammonium
(saturated
emission
solutions
bromide
of
1 mM
of
above
aqueous solutions
not detectable
l,4-dibromonaphthalene
0.05 M
the
CMC.
hexadecyltrimethyl
Below
the
CMC
of DON), phosphorescence
(see also Fig. 5).
is
of the observed bands correspond to fluorescence where 11>..
is the quantum yield for intersystemcrossing from 81 to T 1 and kq[Q] is the "effective" pseudoThe phosphorescenceof N z purged, purified ace- unimoleculardecayof T1 inducedby a quencherQ.
tonitrile solutions of aromatic hydrocarbons such as Equation 1 assumesa wavelength independenceof
naphthaleneand triphenylene is too weak for detec- I1>p.It is important to note that kd and kq[Q] are
tion by a conventional fluorimeter. In vacuum
degassed,purified ethylene bromide, however, phosphorescenceis readily observablefrom these aromatic
e..)
hydrocarbons (Fig. 2).
The phosphorescenceof bromonaphthalenes is
-.
- Na BU~ED(O.05M SOS)
exceedingly weak in Nz-purged aqueous solutions.
--- - AERATED
(O.O5M
SDS)
Addition of detergent(hexadecyl-trimethylammonium '" ";;:""-AERA1"ED(HtO)r:J
li
bromide, HDTBr,
hexadecyltrimethylammonium
::>
>chloride, HDTCI or sodium dodecylsulfate, SDS)
~
results in the readily measurablephosphorescence,
the
a:
Im
latter being detectable even in the absence of Nz
.a:
purging (Figs. 3, 4 and 5).
>lThe intensity of both the fluorescenceand phosv;
Z
phorescence (Fig. 6) of l,4-dibromonaphthalene
...
Ishowedcritical micelle concentration (CMC) behavior
~
z
(Fendler and Fendler, 1975).The phosphorescenceof
Q
'"
OJ
l,4-dibromonaphthalene could not be observed in
~
aeratedaqueoussolution. The latter observation provides strong evidence th.at micelle formation and
FLUORESCENCE
\
micellar sequesteringof the naphthalene is respon\
sible for the observation of phosphorescence.
(- 340nm) and phosphorescence( - 530nm).
A~'
\
\
Kinetic model for interpreting phosphorescence
efficiency
The quantum yield of phosphores~nce 4Jpis given
by the deceptively
'tP =simple ~xpression:
.
~P.c
<1>
PI
"'kp +kd+
kq[Q]
(1)
~:~~:~.~'__'.,~""
--;---;
340 380 420 480 500
,
,---=.->
-
540 580 620 660 700
X (nm)Figure 5. Total emission of 0.1 mM dibromonaphthalene
in aqueous solutions of 0.05 M sodium dodecylsulfate
above the CMC. Below the CMC (saturated aqueoussolutions of DBN), phosphorescenceis not detectable.
526
NICHOLAS J. TuRIOet
more generally a sum of quenching terms, i.e.
kd
= I k~ and kq[Q] ==Ik~ [Qj]
j
increases
the relativeprobabilityof S1 -+ T1 conver(2) sion /p.. will increase.The most common method is
i
where k~ is a ~icular
unimolecular decay constant
and k~[Qi] is the specific contribution of quencher
Qj to the total pseudo-unimolecularquenching. It is
also crucial to recognize that a term k~[Q12 may
become important for very long triplet lifetimes at
high intensitiesif Q' = Tl' In other words, under conditions such that triplet-triplet annihilation is significant, Eq. 1 must be rewritten in a more general form
as:
kp
!/Ip-- !/I" kp + kd + kq[Q]
+ k~[Q'Y'
al.
(3)
Discussionof the terms determining!/Ip
Consideration of Eq. 3 reveals the factors that are
generaIlyinvolved in determining whether phosphorescencemay be observedunder any experimentalconditions. We consider seriatim the impact of each of
these factors on the magnitude of !/Ip.
(1) The intersystem quantum efficiency, !/I". This
term representsthe probability that an absorbedphoton which excites 81 will produce a triplet state. 4>..
is not a molecular constant, but depends on experimental conditions (Wilkinson, 1975).In particular, if
81 is subject to reactions or quenching, !/I., will be
correspondingly decreased as will 4>P' We shall
assumethat chemicaldeactivation of 81 can be identified and eliminated and shall consider only physical
deactivation of 81, The formation of excimers and
exciplexes (Forster, 1969; Birks, 1970) constitutes a
general and commonly encounteredmeans of deactivating 81, Such a processis concentrationdependent,
so that !/I.. may be a function of solute (phosphorescer)concentration.
Importantly, !/I" may be increasedby variation of
structure or of reaction conditions. If a substituent
to enhanceks! relative to other deactivation pathways
available to SI' Quite often the same factors which
enhance/p.. (e.g.heavy atom effects)also enhancekp
(vide infra).
(2) The radiative rate constant, kp. The radiative
rate, kp, generally dependson the extent of spin orbit
coupling available to T1. The kp values for organic
molecules range from -102 S-I for molecules possessingheavy atoms affixed to the emitting lumophore or possessinglowest energy n, n* states, to
-10-1 S-I for aromatic hydrocarbons (McGly~
1969). The magnitide of kp may be increased by
external heavy atom perturbation (Kasha, 1952;
Karvarnos et al.. 1971).Under the assumption that
/Pp<: 10-3 is required for "ready" observation of
phosphorescence by conventional spectrophotometers, it is obvious that favorable conditions exist
only when kp is 10-3 X (kd + kq[Q] + k~[QY), even
if /p,. - I.
(3) The inherent unimoleculardecayof T I' k". The
magnitude of kd dependsstrongly on molecular structure and temperalure.For the moleculesemployed in
this study, photoreactions are inefficient and assumed
to be negligible. There is no general way to handle
the contribution of kd to /Ppexceptto usemodel compounds to calibrate expectations,or to make direct
measurementsof TT= ki I. Note that kd includes
"solvent quenching" in the broad sense that nonreaction interactions of triplets with solvent may
determine the magnitude of kd'
The factors which enhance /p,,!and maximize kp
may also lead to an enhancementof kd by (a) introducing new chemicalpathwaysfor triplet deactivation
or (b) by enhancing intersystem crossing back to So
(Cowan and Koziar, 1977).
(4) The pseudo-unimolecular
quenchingterm, k,[Q].
This term representsthe contribution of all specific
Phosphorescencein fluid solutions
diffusional bimolecular quenching by a ground state
species,Q. Q may be an adventitious quencher, residual oxygen,or the ground state of the phosphorescing
moleculeitselj: The latter point has not been generally
appreciatedin the earlier literature. Self-quenchingof
triplets is a well establishedphenomenon(Porter and
Ledger, 1972;Wolf et aI., 1977;Takemura et aI., 1976)
and may, in some cases,provide a limit to IPp.
(5) The pseudobimolecularquenchingterm, k,[Q'Y.
At relatively high intensities,the interaction and annihilation of two triplets may determine IPp,i.e. when
k~[QY = k~[T1]2 and this term is > 103kp"
Numerous examples of triplet-triplet annihilation
appearin the literature (Yekta and Tuno, 1972).Even
for conventional spectrophotometerswhich are not
considered to possess"intense" light sources, this
term may become significant for exceedingly long
lived triplets (e.g. aromatic hydrocarbons).
527
carbons is not observedin acetonitrile as the result
of simple N2 purging, i.e. the residual [02] ~ probably> 10-8 M. More elaborate purification procedures demonstrate that it is possible to observe
phosphorescence
from aromatic hydrocarbonsin fluid
solution (Tsai and Robinson, 1968).It may also be
that triplet-triplet annihilation or decay from exciplexes determinesthe magnitude of 4>pfor aromatic
hydrocarbons.
Halonaphthalene.5and aromatic hydrocarbons in
ethylenebromide.Vacuum degassingallows observation of the phosphorescence
of both halonaphthalenes
and aromatic hydrocarbons in ethylenebromide (Fig.
2) but not in acetonitrile. The question arises as to
whether 4>..,kp or kq[Q] is affectedrelative to acetonitrile. Although 4>..certainly increasesupon proceeding from acetonitrile to ethylene dibrornide, this cannot be a major factor since 4>..is ~0.7 for naphthalene and ~0.9 for triphenylene (Wilkinson, 1975)so
that an increaseof less than a factor of two is possDISCUSSION
ible. We do not believe that ethylene bromide as solFrom the above analysis,solute concentration and vent suppressesthe kq[Q] term in Eq. 1. More likely,
light intensity, in addition to kp and kq[Q], can be kp is increasedin magnitude.This supposition is supseen to playa dominant role in determining IPp. ported by other experimentsconcerned with «heavy
Many studies in the past have tended to focus on atom induced enhancement of phosphorescence
the kq[Q] factor only in an attempt to optimize IPp. (Vander Donckt et al:, 1973; McGlynn et al., 1969;
Let us consider the results depicted in Figs. 1-5 in Karvarnos et al., 1971).
terms of the model for IPpdescribedabove.
Halonaphthalenesand aromatic hydrocarbons in
micellar solutions.The "ready" observation of phosHalonaphthalenesand aromati(~hydrocarbonsin acephorescence of halonaphthalenes is a remarkable
tonitrile
observation (its detection, although weak, is possible
The ready observation of phosphorescencefrom even in aeratedmicellar solution).
fluid solutions of the halonaphthalenesis the result
In Fig. 6, it is seenthat both fluorescenceand phosof an optimization of IPstand kp and a minimization phorescenceof 1,4-dibromonaphthalene(DBN) show
of kq[Q]. For aromatic hydrocarbons, IP., is variable critical micellar behavior in HDTBr solutions. The
and often much less than unity (Wilkinson, 1975). intensity of DBN fluorescenceis essentially indepenHowever, replacement of H with Dr usually causes dent of aeration or nitrogen purging, but the phosIP.,to approachunity by enhancingk., selectivity.The phorescenceof DBN is very sensitive to aeration or
heavy atom also enhanceskp. From low temperature nitrogen purging, even though a weak phosphoresstudies,kp- 10s-1 for the bromonaphthalenes
cence~ observedin aerated solutions near and above
(~arvarnos et al., 1971).Thus, for IPpto be ;<;10-3, the CMC. The CMC behavior of DBN luminescence
kq[Q] must be :51Q4s- 1 (we assumethat kd is neglimay depend both on enhanced solubility of DON
gible). If kq 1010M-1 S-I, i.e. the fastest allowable
at above the CMC and (for phosphorescence)on
rate for diffusional bimolecular quenching, then a reduced sensitivity to quenching. The CMC value
value of [Q] < 10-6 M is required for the "ready" observed(- 3 roM) is comparableto literature values
observation of phosphorescence.Evidently, simple (Mukerjee and Mysels, 1971).These results indicate
purging with N2 gas is sufficient to reduce the con- that the phosphorescerdoesnot leave its micellar encentration of O2 below this limit in acetonitrile, and vironment on the time scale of 1 ms (our lifetime
phosphorescence
of bromonaphthalenesis observable. measured for 1,4-dibromonaphthalene phosphoresIf kq < 1010M-1s-1, a higher concentration of cence), since only very weak phosphorescenceis
quenchercan be tolerated.
observed in pure water. It also requires an unusual
Inclusion of a heavy atom can be detrimental to "protection" of T1 from O2 as a quencher.The basis
IPpif kd is greatly enhancedrelative to kp. This would of this "protection mechanism" is not yet clear since
be the caseif k.. were more sensitiveto the influence evidence exists that O2 can penetrate and exist in
of heavy atoms than kp or if the heavy atom intro- micelles(Hautala, et al~ 1973; Dorrence and Hunter,
duced fast chemical deactivation pathways.
1972; Wallance and Thomas, 1973). It should also
In the case of aromatic hydrocarbons, kp
10-1
be noted that compartmentalization of the phosphorS-1. This requires [Q] < 10-8 M to assure the escersuppressesbimolecular quenching mechanisms
"ready" observation of phosphorescence.It is thus such as triplet-triplet annihilation and self-quenching.
apparent why phosphorescenceof aromatic hydro- Unless the occurrenceof O2 and a phosphorescerin
M
-
-
528
NICHOLAS J. TtlRRo et aI.
the samemicelle causespeculiar behavior with respect
to quenching, we take these results to cast doubt on
our earlier interpretation (Hautala et al., 1973) that
O2 is preferentially sequesteredinto micelles from the
aqueousphase.
We have been unable to detect phosphorescence
from N 2 purged micellar solutions of naphthalene,
triphenylene and pyrene.The lack of phosphorescence
of aromatic hydrocarbons in micellar solutions
requires that a quenching mechanism occur which
dominates emission. Reaction of T 1 with halide ion
or water (that penetratesthe micelle interior) may be
important for HDTCI and HDTBr micellar solutions.
Thomas has been able to induce phosphorescence
from aromatic hydrocarbons in micellar solutions by
the clever application of thallous ion in anionic micellar solutions (Thomas,et al., 1977).Evidently the thallous ion increaseskp (i.e. servesas a heavy atom perturber) or decreaseskq[Q] (i.e.by suppressingquenching by O2 or some other species).
CONCLUSION
The "ready" observation of phosphorescencefrom
organic moleculesat room temperature in fluid solution is neither a phenomenondisplayed by "odd ball"
structures nor an experiment requiring extraordinary
or unconventional precautions,purification, or equipment. In the absenceof specific reaction with solvent
and/or unimolecular reactions from T 1; conditions
discussed in this paper should allow the "ready"
observation of phosphorescencefor many organic
compounds. The impact of "purity" of a sample or
solution has relevanceonly with respectto a specific
measurement.Thus, elaborate solvent purification for
phosphorescenceis irrelevant when O2 quenching,
triplet-triplet annihilation, or self-quenching determine the lifetime of T 1. From the work reported here,
it can be concluded that conventional purification,
a careful analysis and selection of the experimental
conditions is a proper strategy for observation of
phosphorescencein fluid solution. The use of phosphorescenceprobes for the examination of mi~lIar
and macromolecular systemsof biological interest is
apparent and offer a rich potential for future investigations.
Acknowledgements-The authors thank the Air Force
Office of Scientific Research and the National Science
Foundation for their generous support of this research.
We wish to thank Professor J. K. Thomas, University of
Notre Dame, for infonning us of his results on the observation of phosphorescencein micellar solutions. They also
thank Mr. Evan Watkins for his assistancein obtaining
some of the data reported herein.
REFERENCES
Backstrom,H. L. 1. and K.Sandr~ (1960) Acta Chern.scam. 14,48-62.
Backstrom,H. L. J. and K. Sandr~ (1962) Acta Chern.scam. 16, 958-968.
Birks, J. B. (1970) Photophysics
of AromaticMolecules,Wiley-Interscience,
New York.
Cherry,R.I. and G. Schneider (1976) Biochemistry
IS, 3657.
aark, W. D. K., A. D. Litt and C. Stiel (1969) Chern.Commlm.,
1087-1089.
Cowan,D. O. and 1. Koziar (1977)J. Am. Chern.Soc.98, 1001.
Dorrence,R. C. and T. F. Hunter (1972) 1i'ans.FaradaySoc.68, 1312-1321.
Fendler,J. and E. Fendler (1975) Catalysisin Micellar Systems,
AcademicPress,New York.
Forster,T. (1969) Angew.Chern.,Internat.Ed. 8, 333-343.
Hautala,R. R., N. E. Schoreand N. J. Turro (1973) J. Am. Chern.Soc.95, 5508-5514.
Imakubo,K. and Y. Kai (1977) J. Phys.Soc.Jpn. 42, 1431.
Karvarnos, G., T. Cole, P. Scribe, 1. C. Dalton
1032-1034.
and N. J. Turro
(1971)
J. Am. Chern. Soc. 93,
Kasha,M. (1947) Chern.Rev.41, 401-419.
Kasha,M. (1952) J. Chem.Phys.20, 71-74.
Kellogg,R. E. and R. P. Schwenker (1964) J. Chern.Phys.41, 28ro-2863.
Langelaar,
1.,R. P. H. Rettschnick
andG. J. Hoijtink
(1971)
J. Chem.
Phys.S4,1-7.
Lewis,G. N. and M. Kasha (1944) J. Am. Chem.Soc.66. 2100-2116.
Lewis,G. N. and M. Kasha (1945) J. Am. Chell!.Soc.67, ~1003.
Lower,S. K., and M. El-Sayed (1966) ChelrLRev.66, 199-241.
McGlynn,S. P. (1964) Photochern.
Photobiol.3, 269-294.
McGl~
S. P., T. Azumi and M. Kinoshita
Prentice Hall, Engelwood aiffs, NJ.
(1969) Molecular Spectroscopyof the 1i'iplet States,
Mukerjee,P. and K. 1. Mysels (1971) Critical Micelle Concentrations
of AqueousSurfactantSystems,
National Bureauof StandardsNSRDS-NBS36, p. 107.Washington,D.C.
Parker,C. A. and C. G. Hatchard (1962) J. Phys.Chem.66, 2506-2511.
Parker.C. A. and T. A:1oyce (1968) J. Chem.Phys.49, 3184-319L
Parker,C. A. and T. A. loyce (1969) Trans.FaradaySoc.65, 2823-2829.
Porter,G. and M. B. Ledger (1972) Trans.FaradaySoc.I. 68, 539-335.
Porter,G. and L. 1. Stief (1962) Nature195,991-992.
Porter,G. and F. Wilkinson (.1961) Proc.R. Soc.A 264, 1-18.
Porter,G. and M.W. Windsor (1958) Proc.R. Soc.A 245,238-258.
Saltiei,J., H. C. Curtis, L. Metts,1. W. Miley, J. Winterle and M. Wrighton (1970) J. Am. Chern.
Soc. 92, 410-411.
Saviotti,M. L. and W. C. Galley (1974) Proc.Natl. Acad.Sci. U.S.71, 4154.
Takemura,T., M. Aikawa, H. Baba and Y. Shindo (1976) J. Am. Chem.Soc.98, 2205-2210and
references therein.
Phosphorescencein fluid solutions
Terenin,A. (1943)Acta Phvs.U.S.S.R.18,211-241.
Thomas,
1.and
K., K.
and F.
(1977)
Chem.phys.Left., 51, 501..
Tsai,S. C.
G. Kalyanasundaram
W. Robinson (1968)
J. Grieser
Cliem.Phys.
49, 3184-3191.
Turro, N. 1. (1967) MolecularPhotochemistry,
W. A. Benjamin,New York.
VanderDonckt, E., M. Matagne,and M. Sapir (1973) Chem.Phys.Lett. 10, 81-83.
Wallace,S. C., and 1. K. Thomas (1973) Radial.Res.54, 45-62.
Wilkinson,F. (1975) In OrganicMolecularPhotophysics,
Vol. 2; pp.95-158,(Editedby 1. B. Birks),
Wiley,New York.
Wolf, M. W., R. E. Brown and L. Singer (1977) J. Am. Chern.Soc.99, 526-531.
Yekta,A. and N. J. Turro (1972) Mol. Photochem.
3, 307-322.
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