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irulh•~eJbN. ~~ V:_ i 1.2. u 1079.!12. D1292-83 pp i 18-128.
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<br />• - .. .. _ ...-~;-rya. S. Marry. Aral. Chem.
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<br />- ~ '- "r^•.izs DT9, r0, 179-193.
<br />-~~ ..... :. 4h~. '-. $ ..r• ~. Chem. te84. 37, 733-117.
<br />- p~ ~~~•,m~,•~. ~t.am ,tool. Cbem. 1887. 58. 2073-2078.
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<br />1tECE1~~ED tM ret'IPa' A;:gust iU, 1J7U. Act.t•p:.dq ,,,,aunty 4,
<br />1991.
<br />Kinetic Titration Method To Determine the Excited-State
<br />Concentration of a Photochemical Sensitizer
<br />P. E. Poston and J. M. Herrie•
<br />Department o/ Chemistry, University o/ Utah, Salt Lake City, Utah 8J112
<br />
<br />A noncomparatlrs method is datarmina the excited-lHplst-
<br />stale concsntrallon a1 a photoeansltl:er Is descrlbad. Ths
<br />method is based on the klnetlw of reaetlon or quenehlnq of
<br />the ezclted state and avolde many of the WnXatlons of currsM
<br />techniques. Slnca the Illetlmas of excltad trlplet elates of
<br />molecules In fluid solution era generally mlcroseCOnd6 or
<br />longer, a dllfuslon-controlled quencher In less then mlg4nolar
<br />concentrations can slgnlttcantly Inlluance the decay rate of
<br />the trlplet poputatbn. i/ thts small concsMrallan o1 quencher
<br />la comparable to the Inlflal concsrttratlon of excited stelae,
<br />Then the klnatlca of the Irlplsl-elate decay ors no longer
<br />pseudo Ilrst order. Thla klnetlc behavior Is s:plolted to pro-
<br />rlde a elmple, noncamparatlve tlhatlon method for dstermlNrg
<br />sxcltad•irtplal•state coneen1ra11ons. The refs constant for
<br />quenching need not be known In advance, and any method
<br />of monltoring the quenehing klnetlcs can be used. The
<br />technlqua is evaluated In the present study la delerrnlning the
<br />sxclfetl•trlplsl•etete cancenlratlon of bensophenone by
<br />measuring the decay klnetlw of phoaphorascence quenched
<br />by blecetyt.
<br />Itt order to accurately dotertnine the quantum yiolda of
<br />phntoinitiated ronctions nr molar prapeRios of excited states,
<br />it is necessary Lo know Llte cuncetttratton of photoexcitcd
<br />moleculos. Concentrations of excited states con be estimated
<br />[rum Lhe absorption cross section, excited•stete lifetime, end
<br />grottttd-slate concentration of a species together with the
<br />excitation optical power, beam spot size, and pulse tluratinn.
<br />These estimates can he inaccurate. however, due to apotial
<br />inhumogetleity of rho excitation beam or depletion of the
<br />ground-state populotitin, neither of which pre easily chptac-
<br />Lerized. Determining excited•triplet-state concentrotions is
<br />particularly difficult, because yields of intersystem crntasing
<br />ate generally not known with accuracy.
<br />'fectutiyues for obtaining concentratimta, molar ebsorp-
<br />tivities, or quantum yields of fgrmation of triplet excited states
<br />fall generally into two classes, cumparat.ive mid nuncampa-
<br />rntivo methods. Comparative methods to estimate triplepstate
<br />molar ebsorptivities (1) are typicblly based on triplet-triples
<br />energy Lransfor from a standard excited triplet donor; rho
<br />decrease itt the optical absorption by the donor (having a
<br />known Tl ~ 1'„ nhsnrptivity) is mrrelstad with the increase
<br />in Absorption by the acceptor to obtain the acceptor molar
<br />absorptivity (2). The method assumes that all of the accoptor
<br />population arises from energy tragsfer from the donor, which
<br />may be difficult to assure. Uncerteintics also eriae from the
<br />molar absorptivity of the donor ur ire photoproducl and from
<br />any yield of donor phutaproduct drat ie not quenched by the
<br />acceptor (1 ), By use of the values:of'I'••Tm0lar absorptivity
<br />thw obtained, comparative metht>dc can also be used to de•
<br />twmine intersystem crossing yiolda (3). Hero, the triplet
<br />concentration of the unknown is tstimated from its'f-T ab•
<br />sorption and compared to that of a standud hoving known
<br />triplet yield nod absorptivity. For the results to be valid, the
<br />triplet states must not absorb the exciting light (4) and dc•
<br />pletion of the ground state must !ho negligible (51.
<br />To ovoid some of the uncertaihtiea and tusumptions of
<br />comparative motltuds, noncompatative methods fur deter•
<br />mining triplet•»taie populations ens molar absotptivities have
<br />been davoloped (G), These methods do nue rely on intermo-
<br />secular interactions such as onargy transfer nor do they require
<br />knowledge of the molar absorptivity or triplet yield of e
<br />standard. Fot exemplo, triplet•stat0 concentrations and muter
<br />absorpl.ivities can be determined by direct photolysis of an
<br />unknown while eimultoneously mposuring the triplot•»tete
<br />absorption and the ]nos of ground•atale ebaorptinn. in order
<br />to obtain excited state cuncentraL3one, rho method requirec
<br />either a spectra! region whore Lhe krau[td stale absorbs free
<br />of any Ltiplet-state absorption or aq isotx+stic point whero ChB
<br />molar ebsorptivities of the ground apd excited states nro equal
<br />(7), Other noncomparative moths are hgscd on the pho•
<br />tolysis kinetics for generating oxc led triplet states (6, A).
<br />Methods that do nut require prior knowledge of the triplet
<br />yield or molar absorptivity are based either on the intensity
<br />dependence of saturation by exciGtllon pulco» that are longer
<br />than the singlet lifetime 191 or on the Lime dependence of
<br />Criplrt-state saturat.ivn using a chopped continuous excitation
<br />OOOY^,700x9 VO?63.09895025010 ~. 7981 American Chemical Sadery
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