- Email: [email protected]
Measurements of intersystem crossing kinetics using 3545 Å picosecond pulses: nitronaphthalenes and benzophenone
?olume
:
28, nu!nber 2’
CH&CAL
PHYSICS LEl-f’ERi’
lS%ptember -.
.’ .-
..
1954
..
_’
~
MEAiUREMENTS OF l&RSYSTEM CROSSINCKINETICS USING 3545 A PICbSECOND’PULSES: NITRONAPBTHAl$NES AND BENZOPtiENONEf -,
.
R.W. AmERS6N
H. LUTZ and”G.W. SCOTT+
Jr. %, R_M. HOCkSTFhjSSE’R,
Department of Chemistry atuflabomtory for Research on the Str?lcture of hiatfer, University of/Pennsylvar~ia, Plziladelphia, Fennsylvania 19174, USA
Rec&cd
11 J&e
1974
The buad-up ‘of triplet-tiplet absorption followin,o singlet excitation was rmamrkd fix l-and 2-niErotiphtha!ene and benzoohenone on a picosecond time scale in the condensed phase. A single, picasecond pull f’roma modelocked NdS+/glass laser was used to produce excitation at the laser third harmonic, 3545 rS, Fvhile the second tiarmanic at 5300 A was used to probe the known triplet--triplet absorption in the nitronaphthalenes and benzophenone.
Solvent assisted
effects on the build-up rates in these. molecules are exposed. vibrational relaxation 2nd intersystem crossing contributions
In the case of benzophenone,
we dir;cuss solvent
to these rates. The anomaly of_intersysten is dizcussed. s-l with a triplet yield of less than 1 for the nitronaphthalenes
crossing at 2 rate of ca. 10”
1. Introduction in benzophenone
[4] pointed to some hitherto unon electronic relaxation emphnsizing the need for more expctirnental results in order to generate a framework for a comparative study of ultrafast processes k the condensed phase. In this paper we piesent new measuremMts using near ultraviolet pulses of picosecond electronicrelaxation in l- and 2:nitronaphthalene’in.various solvents and of benzophenone in n-heptane. ?&e luminescence .,characteristicc, of nitronaphthalenes hatie recently been reviewed by, Seliskar et al. [51. Neither of the nitronaphthalenes have been found to fluoresce except in acidic glasses, [6]. Both exhibit phosphorescence in the naphtl&lene region with lifetimes of .0.047 s (I-nitro) and 0,206 S (2-nitro) [7] _ The trip!et yields have been measured by Hurley and Testa [8] and found to.be 0.63 (I-nitro) and 0183 (2.nitro). The triplet-triplet absorption has been previously smdied : [9, lo] for both l- and 2$tronaphthalene, and a : ” strong T-T absorption occuti in the region between 4275 and 6200. A. The existence of a tripIet yield less .. than I .O in a system that‘does not exhibit conventjonal‘..’ lv detectable-fluoresknce is an unusual phenomenon .’ .,. which iq itself merits ftirther study_. .
Molecules that do not exhibit fluorescence under non-nal conditions of excitatidq and detection are expected to be tindergoing very fast nonradiaiive processes. There is a paucity of experimental inform+on on the dynamics of very fast electronic relaxation in molecules. Until recentlythe internal conversion in azulene. [l, 21 and the intersystem crossing in benzophkoce [3] represented the only picosecond regime - electronic relaxation processes to have been studied’ apart from experiments with dye molecules. Clearly ,‘. the results from two isolate.d.situations might b” of limited value in answering questions in regard to the sub-nkosecond process& occurring in_widei classes of molecules. Recent studies of intersystem crossing. -. 7 ‘lIThisresearch
by the Nat&al Science Research on the. Structure of hidter at the Universjty.of P,ermsylvania. $ P&na..ent aftI!,iatiori: Pitman-Dunri Laboratories; Frankford Arsetil, Philadelphi3, PermsyJvuLia. fN.:.H. Postdoctdral Fellow; 197.3-74; Address aiter Sep .tenbsr 1,.1974: ,Dcpartment of.Chkmistry,Univirsity of ‘: Caiifornia, Riverside,California. w2s
supported
Foundation arid by the Laboretory.for
:
.. : .,,
.._‘.
.’
‘, :.
.. . -
. . .-. ‘,
. ‘. ‘8 . .
_’ .;
‘;
-;
., .,: .,._
known
solvent
:
I .. : .. .
_‘.
.,.’ _...
effects
,3 .,, .. :.
:;
_.‘..,.’ ., .:_.-.--
:
: .._,
153.
‘,
2J.'Mat&ls
,’ ,.
.
.’
‘.
-OiO- “.
”
:
-I-NITRONAPHiUALENE/ETHANOi
..
,. 1
I-‘f:ITRONAPHTHALENE
‘These concentrations result in an optical density .greater than 2.at 3545 A in a 1 mm pathlength.
/ EE&ZENE
of
,?ne Ndj’lglass laser, amplifier, and the 3545 8, pulse production technique was essentjally tha.same as that described previously [4]. A single pulse is extract&from a mode-locked Nd3i/glass laser, amplified, frequenc$ doubled, and then tripled by mixing the 1 .M’+ and 0.53 ‘p pulses in a KDP crystal. With this ~systkm we have available 4 ml at 53QO A and .I mJ at 3545 A. The.~30d_&pulsewidth was8 2 2 ps when. pulses near the beginning of the tram were used: Pulses in the middle of the train hvere typically I2 ? 2 ps. It-ithP experiments described below the pulses were always taken from the first third of the-pulse train. -.
TIME
Cps)
Fig. 1. Optical densities at.5300 A due to triplet l-nitronaph-
‘. thalene formation as a function of delay following singlet state excihtiqn as measured in benzene xnd ethanol solvents. ‘Iiie + are the measured optical densities, normalized to the ,aver~ge LTVpulse intensity. The smooth curves are the calculated R(t) described in the text.
The two probe pulses (before and after) and a sample of the pump pulse were arranged, by means of optical delays;to arrive at the photodiode ca. 2 r-r; after one anolher to ensure that lhey were clearly separated on the oscilloscope. trace. -2.3. ~2kethodof memuiemetit .. The point in time at which the assumed infmite.,_ sirnaL&thin active sample (< 0.5 mm actually) and .:’ The’ frequency tripled pu!ie at 3545 A was used to ._ the peaks of the two pulses coincide in space, is termed elicite the molecules to low’ energy .tibrational levels t = 0. The t= 0 position was-determined experimentalof, the fust singlet state. For both benzophenone [ 111 ly b!/.measurmg the apparent build-up time for an.Sl and the.uitronaphtbaienes [9, 1Qj th.ere is known to + Sr. r:bsorpt,ion spectrum following excitation of St be’a’triplet-tripjct.abso+tion at 5300-a that could: by 3545 &Th’ e t’rme‘necessary foi.the development be used to detect the occurrence of iritersystem.crosY ” &the S;A..S, absorption is~a’dtually very &se to, sing. Thus the experiment consisted’ & spa tia$y ,zero so the t = 0 position‘could be I,ocated accurately: -separating the single pulses of 3545 and 5300 A and; ‘. ‘. ” There calibration experiments involved pherianthrenes, arrqn&tg to have the green pulse’arril? a! :fhe.sample ,and:they’are described else&here [12]; .: / .’ at.knotin.times,before;during;,and after.the arrival of :- .. ; .., .::: ., .’ ,. ‘, ‘. ‘I ‘. .’ ,:. ,~W.,Eqm=iCjerithl remits ,’ ; $e.@5, Ej pulse..T+ probing pulse a? $300 4 was -* .. ,_ ” :,diffused_nfter beiiig optically delayed,such.that it qias’ . 1. -. .,‘. .,. ... : .. .:,T-,The, results are shown.in figs:l.,‘2, and 3 ‘and.‘are $np&iized at, the.~sample;T&354$ 8,liglit was :. ‘. : .-pqiarized:The probe’ pplse intensity .wasmcasttied i .,;‘. s;‘r&_narged jn tab& 1. The s.mboth~~cu.Gs in the.;-‘,,. r)kf&e hn$ after passing-through the sample. by m&s : ‘: .. .. figures,,ar,e drawn on-the assumpiion of a dual lineai., 1, : $f..?r[$t phoiodiode and‘s’ 5J.9 Teictronix“dsciUosco~e: ..’ r~++s~,~f the.‘systeti’to t+o,ga&ianpulses of .. ‘:, ,_ . ._,I, -.._I ,. ...
fit) = k j
F@. 2. &t&l densities at 5300 A due to tripIet 2-niironaphthalene formation as a function of delay follotig singlet state excitation as’measured in benzene and.ethanol solvents. The 8 are the measured optical densities, normalized to the average W puke intensity. ‘Ilk smooth curves are the calcuIated D(t) described in the text.
knownwidth
(8 ps), such that the triplet
state popula-
tioxi at time t is &en by:
,.;
‘.
..;,.
.
_._I .,, .: ‘.
: ‘.
.,. ._,
I$)&‘,
where I,(?) iS the ultraviolet pulse function (W use g for green below). The measured optical density for time r is then obtained from:
The constant cr &as chosen such that D(f; was equal to the optical density actually observed, say at the asymptote t 2~ k7 l. In these experiments it is not ex-
.,.
‘,’
,260.
,: ‘_
axp [-k&-t’)]
..
.?a. .
.’
.:
-;:
‘.’ ..’ ‘..’ ‘,. ‘, .’ .‘,( -.:, ‘...’ .-i& 3. optical densities at 53110 A due to triplet .benzophenbne f&r@on ai a functktof delay foRowing W ,., asmeasured in n-heptane &vent: T’_he’Q~ti.ethe nieasq&d opt&al densities, norntalized’to’the~avemge ,’ .._:. ,_ ._:_. ,, smooth curv,es,qethe calculated D(t) de&ibkd”in the text. .. 1, :, ., _. :: .’ ,..‘,(.-. ..,.. r _.. _: ; : ! ,, ,, . .: :.. :. ,) ,., .. .‘.,.-I __ .’ .T,’ ‘.,: :, -. : ‘., .: y, ! ‘,,I ,(,’ ,_.. ., .1: ,:. ‘,’ : ..,,,, : ,: .. ., ; ._ : ., I
. .. sir&et’state excitation puti intensity. The
., -1.. ..
‘- ‘....
:
15.5
Volume 28, number 2 -: :
-CHEhiiCAL PHYSICS LEY-IERS .’
petted thatthe observed optical density will be obtamed Fr&m,the product ~2; where E is the extinction coefficient.for the T-T trkrsition and I is the path~~Ic&th, since the pump pulse has transverse inhomogenelties. The probable krrors given in tab!e Krefer. to the :Iunits beyond which the fits,of the data to the theoreti: cd curyCs were clearly worse. : 3.
Displssion
The results appear tb fit the simple model for the triplet formation relatively accurately. The relative magnitude of-the asymptq?ic and t = 0 absorption .- combined with our experimental measure of the location of r = Q-indicates that these measurements are not being severely influenced by S, + S,, absorption at early times. A possible source of long-term variations in the observed optical density is rotational relaxation, the effect of which we now consider. ‘3.1. The ~ffecr ofrotftfi~mtl r&xation In our experiment the 3545 B, pulse is polarized (x) and the 5300 aprobe is unpolarized. T’hus prior to rotational reiaxation the optical densities measured by the probe will be dependent on the relative polarization, in the molecular frame, of the So + SL and T, ? T, transitions. It follows that the optical density measured will change at later times as the solution once again’becomes random with respect to the probe. -4bsorption of the pump pulse causes the solution to become birefringent with principal directions x and y (say); The transmitted unpolarized light is I = $(IX+IY> where 1; = I0 10-E~c’, with eX being the ex:tinction coefficient for probe light poladzed along X. If the isotropic, or usual, extinction coefficient is Ei _then eX and 4 are g&en by
.’
15:Gptembeer
..
19.74
.,
D=log @.fD/(ld-nDa+Io-bDa ii (3) -.. where D,J is the optiCal density.for the isotropic distribution such as when the system has completely rotationally relaxed, and a and b are either p and 2
(for parallel Lransition moments) or 5, ayd 4 (for perpendicular transi&n moments). In our case D, is known from the asymptote to be ca. 6.3, and we find the following results for the two cases: (1) For parallel transition moments relaxation should cause the optical density to be reduced by about S%. (2) For perpendicular transition moments relaxation should cause the @ical density to be increased by about 12%. It is gratifying for our studies of intramolecular processes lhat Ihese changes are very small, but disappointing on the other hand that our data are not quite accurate enough to expose~definitively the effect of rotational relaxation. Chuang and Eisenthal [13] have studied notational reIaxation in rhodamine 6C which is not much larger than the molecules studied here, and the characteristic times are in the range 20 to 300 ps for solvents with viscosities similar to those we use. 3.2. Inrmysrmt
&o&g
ipt benzophenone
The earlier results [4] on benzophenone in ethanol and benzene suggested the possibility that iri the hydroxylic solvent the relative spacings of the rrrr* and nrr+ states might favor singlet+iplet mixing: From a conventional standpoint this no longer s3ems SO likely since n-heptane would then be predicted to give the least singlet-triplet~mix@g, whereas in fact it provides the environment in which the triplet-triplet absorption of benzophenone builds up fastest. Such a conventional argument assumes that vibrational relaxation occurs prior to intersystem crossing, whereasin tiis case they must be occurring on comparable time scales. Accordingly the solvent e,nhancement of k may be caused by the different .effectiveness of the various solvents in vibrationally.relaxing the excited benzophenone molecules to or fro& singlet levels that Ex=!Ei, c;,=gc (parallel OsciUatoiS) ; : are st?ongly coupled tothe’triplet manifold:Since .l the 3m* state is surely nearby but above the Inn* oscillators) _ k, = $Ei ) iy = fei’ I: (perpendicukr state .:he 3,~* admixture in the Ln& state (spinFor a given orientation of S,j + S, and TL + Tn transiortiit-vibronic interference) may be quite non-uniform, tion moments,-ihe optical density for the rotationally’ and ‘the frequency distribution of the %WL amplitude unrelaxed system will be: ‘, ‘-” : wqa]tl- be solventsensitive. 1. _: ,. : .’ I., /. :., ,:. ., : ,I_ ,;.::, : : ;; ._ _,.. ‘, ,‘, ., l& .., .‘.. .’ : .,. -‘.-. ‘. ,., .:: ... .:. ..,__. ..:. : : , :.; ._ . ‘. ,‘. :.‘_ -” .;-’ .~ .:
,.
Volume 28, number 2
CHEMICAL PHYSICS LETTERS
3.3. Intersystem cmssing it2 the nitronuphthalen~ The 3545 A excitation falls close to:the long wavelength edge of.the So + S, transition of the nitronaphthalenes so only a small fraction of the energy input represents vibrational energy. The very fast triplet build-up is difficult to reconcile with the relatively low triplet yields. These yields were measured [a] using the Lamola and Hammond energy transfer technique [ 141, and in particular the isomerization of cispiperylene was employed. An assumption in this application is that the fraction of piperylene triplets which produce the tram isomar is Cl.55 as measured by Lamola and Hammond [ 141 for the benzophenone triplet induced isomerism of cis-piperylene. If instead this fraction were 0.35 then the triplet yield would become unity. Since isomerism and vibrational relaxation are competitive one might expect the above fraction to depend on the initial vibrational energy content of the triplet piperylene. Assuming thai’the triplet yields of Hurley and.Testa [8J are appropriate to the systems we have studied one is !eft with proposing a picosecond time, scale nonradiative relaxaticn from the singlet to the ground state. Hurley and Testa [8] have previously suggested such a step on the basis of the non-fluorescence and the triplet yields. As far as can b’e ascertained there is negligible photochemistry Ieading to removal of l- or 2nitronaphthalenes. If there were an internal conversion from S, + So one would expect on the basis of experience with naphthalene that the T, -+ S0 intersystem crossing rate would be slower than St -+ Ss internal conversion by an additional factor of_ca. ICI’: this figure would be reduced if spin-orbit coupling were enhanced. The anoma!ous feature of the nitronaphthalenes is that they phosphoresce with significant quantum yields [7, 151 which imp!ies that the mechanism that causes the singlets to disappear is not applicable to the triplets of these molecules. A possible explanation is that unlike naphthalene the singlet and triplet, states are not similar but that one of them (the singlet) is much more strongly Coupled to intramolecular electron transfer and ether nitro, group states. Excitation of the’singlets would then involve a considerable readjustment of the nuclei appropriate :
15 September 1974
to an enhanced internal conversion and S, + T, intersystem crossing. That the triplet state wavefunction is not strongly perturbed by the nitro group is evidenced by the phosphorescence spectral appearance which is basically naphthalenic [7, lo] _The shortened lifetime compared with naphthalene requires only an enhanced spin-orbit interaction due to coupling with distant states. The unexpected solvent effects on the triplet bui!dup in the nitronaphthaIenes could be due to a variety of causes such as changes in the internal conversion effectiveness, or the intersystem crossing, cr they could reifect the ability of the solvents to vibrationahy relax the excited molecules. These new effects p.ose challenging questions about the dynamics of fast processes in the ccndensed phase.
References
111P.M. Rentzcpis, Chem. Phys. Letters 2 (196% L17. I21 E. Drent, G. Makkes ian der Deijl end PJ. Zzndstra, Chem. Phys. Letters 2 (1968) 526.
[31 P.M. Rentzepis, Science 169 (1970) 239. (41 R.M. Hochstrasser, H. Lutz uld G-W. Scott, Chem. Phys. Letters 24 (1974) i62.
151 CJ. Seliskar, OS. KhsliJ and S.P. hlcGl~n;l, in: Excited states, ed. EC. Lim (AcademicPress, Nevi York. 1974) p. 231 (see p. 262). [61 O.S. Khahl, H. Bach and S.P. McGLynn, 1. hfol. Spectry. 35 (1970) 455. [71 R. Ruukowicz and A.C. Testa, Spectmchim. Acta 27A (1971) 787. Fl R. Hurley and A-C. Testa, J. Am.Chem. Sot. 90 (1968) 1949. 191 J.J. Mikula, R.W. Anderson Jr. and L.E. Harris, Advan. Mol. Relax. Processes 5 (1973) 193. [lOI J J. hl&la, R.W. Anderson Jr., LE. Harris and E.\C’. Stcubing, I. Mol. Spectry. 42 (1972) 350. [Ill DS. McClure and P.L. Hmst, J. Chem. Phys. 23 (1955) 1772. [I21 R.W. Ar.derscn Jr., R.hI. Hochstrzsser, H. Lutz and G.W. Scott, submitted for publication. t131 T.J; Chuang and K.B. Eiser.‘Jlal, Chem. Phys. Letters 11 (1971) 368: [ 141 A.A. Lamola and G.S. Hammond, J. Chem. Phys. 43 (1965) 2129. [ 15) G.S. McClure, J. Chem. Phys. 17 (i949) 905.
:
28, nu!nber 2’
CH&CAL
PHYSICS LEl-f’ERi’
lS%ptember -.
.’ .-
..
1954
..
_’
~
MEAiUREMENTS OF l&RSYSTEM CROSSINCKINETICS USING 3545 A PICbSECOND’PULSES: NITRONAPBTHAl$NES AND BENZOPtiENONEf -,
.
R.W. AmERS6N
H. LUTZ and”G.W. SCOTT+
Jr. %, R_M. HOCkSTFhjSSE’R,
Department of Chemistry atuflabomtory for Research on the Str?lcture of hiatfer, University of/Pennsylvar~ia, Plziladelphia, Fennsylvania 19174, USA
Rec&cd
11 J&e
1974
The buad-up ‘of triplet-tiplet absorption followin,o singlet excitation was rmamrkd fix l-and 2-niErotiphtha!ene and benzoohenone on a picosecond time scale in the condensed phase. A single, picasecond pull f’roma modelocked NdS+/glass laser was used to produce excitation at the laser third harmonic, 3545 rS, Fvhile the second tiarmanic at 5300 A was used to probe the known triplet--triplet absorption in the nitronaphthalenes and benzophenone.
Solvent assisted
effects on the build-up rates in these. molecules are exposed. vibrational relaxation 2nd intersystem crossing contributions
In the case of benzophenone,
we dir;cuss solvent
to these rates. The anomaly of_intersysten is dizcussed. s-l with a triplet yield of less than 1 for the nitronaphthalenes
crossing at 2 rate of ca. 10”
1. Introduction in benzophenone
[4] pointed to some hitherto unon electronic relaxation emphnsizing the need for more expctirnental results in order to generate a framework for a comparative study of ultrafast processes k the condensed phase. In this paper we piesent new measuremMts using near ultraviolet pulses of picosecond electronicrelaxation in l- and 2:nitronaphthalene’in.various solvents and of benzophenone in n-heptane. ?&e luminescence .,characteristicc, of nitronaphthalenes hatie recently been reviewed by, Seliskar et al. [51. Neither of the nitronaphthalenes have been found to fluoresce except in acidic glasses, [6]. Both exhibit phosphorescence in the naphtl&lene region with lifetimes of .0.047 s (I-nitro) and 0,206 S (2-nitro) [7] _ The trip!et yields have been measured by Hurley and Testa [8] and found to.be 0.63 (I-nitro) and 0183 (2.nitro). The triplet-triplet absorption has been previously smdied : [9, lo] for both l- and 2$tronaphthalene, and a : ” strong T-T absorption occuti in the region between 4275 and 6200. A. The existence of a tripIet yield less .. than I .O in a system that‘does not exhibit conventjonal‘..’ lv detectable-fluoresknce is an unusual phenomenon .’ .,. which iq itself merits ftirther study_. .
Molecules that do not exhibit fluorescence under non-nal conditions of excitatidq and detection are expected to be tindergoing very fast nonradiaiive processes. There is a paucity of experimental inform+on on the dynamics of very fast electronic relaxation in molecules. Until recentlythe internal conversion in azulene. [l, 21 and the intersystem crossing in benzophkoce [3] represented the only picosecond regime - electronic relaxation processes to have been studied’ apart from experiments with dye molecules. Clearly ,‘. the results from two isolate.d.situations might b” of limited value in answering questions in regard to the sub-nkosecond process& occurring in_widei classes of molecules. Recent studies of intersystem crossing. -. 7 ‘lIThisresearch
by the Nat&al Science Research on the. Structure of hidter at the Universjty.of P,ermsylvania. $ P&na..ent aftI!,iatiori: Pitman-Dunri Laboratories; Frankford Arsetil, Philadelphi3, PermsyJvuLia. fN.:.H. Postdoctdral Fellow; 197.3-74; Address aiter Sep .tenbsr 1,.1974: ,Dcpartment of.Chkmistry,Univirsity of ‘: Caiifornia, Riverside,California. w2s
supported
Foundation arid by the Laboretory.for
:
.. : .,,
.._‘.
.’
‘, :.
.. . -
. . .-. ‘,
. ‘. ‘8 . .
_’ .;
‘;
-;
., .,: .,._
known
solvent
:
I .. : .. .
_‘.
.,.’ _...
effects
,3 .,, .. :.
:;
_.‘..,.’ ., .:_.-.--
:
: .._,
153.
‘,
2J.'Mat&ls
,’ ,.
.
.’
‘.
-OiO- “.
”
:
-I-NITRONAPHiUALENE/ETHANOi
..
,. 1
I-‘f:ITRONAPHTHALENE
‘These concentrations result in an optical density .greater than 2.at 3545 A in a 1 mm pathlength.
/ EE&ZENE
of
,?ne Ndj’lglass laser, amplifier, and the 3545 8, pulse production technique was essentjally tha.same as that described previously [4]. A single pulse is extract&from a mode-locked Nd3i/glass laser, amplified, frequenc$ doubled, and then tripled by mixing the 1 .M’+ and 0.53 ‘p pulses in a KDP crystal. With this ~systkm we have available 4 ml at 53QO A and .I mJ at 3545 A. The.~30d_&pulsewidth was8 2 2 ps when. pulses near the beginning of the tram were used: Pulses in the middle of the train hvere typically I2 ? 2 ps. It-ithP experiments described below the pulses were always taken from the first third of the-pulse train. -.
TIME
Cps)
Fig. 1. Optical densities at.5300 A due to triplet l-nitronaph-
‘. thalene formation as a function of delay following singlet state excihtiqn as measured in benzene xnd ethanol solvents. ‘Iiie + are the measured optical densities, normalized to the ,aver~ge LTVpulse intensity. The smooth curves are the calculated R(t) described in the text.
The two probe pulses (before and after) and a sample of the pump pulse were arranged, by means of optical delays;to arrive at the photodiode ca. 2 r-r; after one anolher to ensure that lhey were clearly separated on the oscilloscope. trace. -2.3. ~2kethodof memuiemetit .. The point in time at which the assumed infmite.,_ sirnaL&thin active sample (< 0.5 mm actually) and .:’ The’ frequency tripled pu!ie at 3545 A was used to ._ the peaks of the two pulses coincide in space, is termed elicite the molecules to low’ energy .tibrational levels t = 0. The t= 0 position was-determined experimentalof, the fust singlet state. For both benzophenone [ 111 ly b!/.measurmg the apparent build-up time for an.Sl and the.uitronaphtbaienes [9, 1Qj th.ere is known to + Sr. r:bsorpt,ion spectrum following excitation of St be’a’triplet-tripjct.abso+tion at 5300-a that could: by 3545 &Th’ e t’rme‘necessary foi.the development be used to detect the occurrence of iritersystem.crosY ” &the S;A..S, absorption is~a’dtually very &se to, sing. Thus the experiment consisted’ & spa tia$y ,zero so the t = 0 position‘could be I,ocated accurately: -separating the single pulses of 3545 and 5300 A and; ‘. ‘. ” There calibration experiments involved pherianthrenes, arrqn&tg to have the green pulse’arril? a! :fhe.sample ,and:they’are described else&here [12]; .: / .’ at.knotin.times,before;during;,and after.the arrival of :- .. ; .., .::: ., .’ ,. ‘, ‘. ‘I ‘. .’ ,:. ,~W.,Eqm=iCjerithl remits ,’ ; $e.@5, Ej pulse..T+ probing pulse a? $300 4 was -* .. ,_ ” :,diffused_nfter beiiig optically delayed,such.that it qias’ . 1. -. .,‘. .,. ... : .. .:,T-,The, results are shown.in figs:l.,‘2, and 3 ‘and.‘are $np&iized at, the.~sample;T&354$ 8,liglit was :. ‘. : .-pqiarized:The probe’ pplse intensity .wasmcasttied i .,;‘. s;‘r&_narged jn tab& 1. The s.mboth~~cu.Gs in the.;-‘,,. r)kf&e hn$ after passing-through the sample. by m&s : ‘: .. .. figures,,ar,e drawn on-the assumpiion of a dual lineai., 1, : $f..?r[$t phoiodiode and‘s’ 5J.9 Teictronix“dsciUosco~e: ..’ r~++s~,~f the.‘systeti’to t+o,ga&ianpulses of .. ‘:, ,_ . ._,I, -.._I ,. ...
fit) = k j
F@. 2. &t&l densities at 5300 A due to tripIet 2-niironaphthalene formation as a function of delay follotig singlet state excitation as’measured in benzene and.ethanol solvents. The 8 are the measured optical densities, normalized to the average W puke intensity. ‘Ilk smooth curves are the calcuIated D(t) described in the text.
knownwidth
(8 ps), such that the triplet
state popula-
tioxi at time t is &en by:
,.;
‘.
..;,.
.
_._I .,, .: ‘.
: ‘.
.,. ._,
I$)&‘,
where I,(?) iS the ultraviolet pulse function (W use g for green below). The measured optical density for time r is then obtained from:
The constant cr &as chosen such that D(f; was equal to the optical density actually observed, say at the asymptote t 2~ k7 l. In these experiments it is not ex-
.,.
‘,’
,260.
,: ‘_
axp [-k&-t’)]
..
.?a. .
.’
.:
-;:
‘.’ ..’ ‘..’ ‘,. ‘, .’ .‘,( -.:, ‘...’ .-i& 3. optical densities at 53110 A due to triplet .benzophenbne f&r@on ai a functktof delay foRowing W ,., asmeasured in n-heptane &vent: T’_he’Q~ti.ethe nieasq&d opt&al densities, norntalized’to’the~avemge ,’ .._:. ,_ ._:_. ,, smooth curv,es,qethe calculated D(t) de&ibkd”in the text. .. 1, :, ., _. :: .’ ,..‘,(.-. ..,.. r _.. _: ; : ! ,, ,, . .: :.. :. ,) ,., .. .‘.,.-I __ .’ .T,’ ‘.,: :, -. : ‘., .: y, ! ‘,,I ,(,’ ,_.. ., .1: ,:. ‘,’ : ..,,,, : ,: .. ., ; ._ : ., I
. .. sir&et’state excitation puti intensity. The
., -1.. ..
‘- ‘....
:
15.5
Volume 28, number 2 -: :
-CHEhiiCAL PHYSICS LEY-IERS .’
petted thatthe observed optical density will be obtamed Fr&m,the product ~2; where E is the extinction coefficient.for the T-T trkrsition and I is the path~~Ic&th, since the pump pulse has transverse inhomogenelties. The probable krrors given in tab!e Krefer. to the :Iunits beyond which the fits,of the data to the theoreti: cd curyCs were clearly worse. : 3.
Displssion
The results appear tb fit the simple model for the triplet formation relatively accurately. The relative magnitude of-the asymptq?ic and t = 0 absorption .- combined with our experimental measure of the location of r = Q-indicates that these measurements are not being severely influenced by S, + S,, absorption at early times. A possible source of long-term variations in the observed optical density is rotational relaxation, the effect of which we now consider. ‘3.1. The ~ffecr ofrotftfi~mtl r&xation In our experiment the 3545 B, pulse is polarized (x) and the 5300 aprobe is unpolarized. T’hus prior to rotational reiaxation the optical densities measured by the probe will be dependent on the relative polarization, in the molecular frame, of the So + SL and T, ? T, transitions. It follows that the optical density measured will change at later times as the solution once again’becomes random with respect to the probe. -4bsorption of the pump pulse causes the solution to become birefringent with principal directions x and y (say); The transmitted unpolarized light is I = $(IX+IY> where 1; = I0 10-E~c’, with eX being the ex:tinction coefficient for probe light poladzed along X. If the isotropic, or usual, extinction coefficient is Ei _then eX and 4 are g&en by
.’
15:Gptembeer
..
19.74
.,
D=log @.fD/(ld-nDa+Io-bDa ii (3) -.. where D,J is the optiCal density.for the isotropic distribution such as when the system has completely rotationally relaxed, and a and b are either p and 2
(for parallel Lransition moments) or 5, ayd 4 (for perpendicular transi&n moments). In our case D, is known from the asymptote to be ca. 6.3, and we find the following results for the two cases: (1) For parallel transition moments relaxation should cause the optical density to be reduced by about S%. (2) For perpendicular transition moments relaxation should cause the @ical density to be increased by about 12%. It is gratifying for our studies of intramolecular processes lhat Ihese changes are very small, but disappointing on the other hand that our data are not quite accurate enough to expose~definitively the effect of rotational relaxation. Chuang and Eisenthal [13] have studied notational reIaxation in rhodamine 6C which is not much larger than the molecules studied here, and the characteristic times are in the range 20 to 300 ps for solvents with viscosities similar to those we use. 3.2. Inrmysrmt
&o&g
ipt benzophenone
The earlier results [4] on benzophenone in ethanol and benzene suggested the possibility that iri the hydroxylic solvent the relative spacings of the rrrr* and nrr+ states might favor singlet+iplet mixing: From a conventional standpoint this no longer s3ems SO likely since n-heptane would then be predicted to give the least singlet-triplet~mix@g, whereas in fact it provides the environment in which the triplet-triplet absorption of benzophenone builds up fastest. Such a conventional argument assumes that vibrational relaxation occurs prior to intersystem crossing, whereasin tiis case they must be occurring on comparable time scales. Accordingly the solvent e,nhancement of k may be caused by the different .effectiveness of the various solvents in vibrationally.relaxing the excited benzophenone molecules to or fro& singlet levels that Ex=!Ei, c;,=gc (parallel OsciUatoiS) ; : are st?ongly coupled tothe’triplet manifold:Since .l the 3m* state is surely nearby but above the Inn* oscillators) _ k, = $Ei ) iy = fei’ I: (perpendicukr state .:he 3,~* admixture in the Ln& state (spinFor a given orientation of S,j + S, and TL + Tn transiortiit-vibronic interference) may be quite non-uniform, tion moments,-ihe optical density for the rotationally’ and ‘the frequency distribution of the %WL amplitude unrelaxed system will be: ‘, ‘-” : wqa]tl- be solventsensitive. 1. _: ,. : .’ I., /. :., ,:. ., : ,I_ ,;.::, : : ;; ._ _,.. ‘, ,‘, ., l& .., .‘.. .’ : .,. -‘.-. ‘. ,., .:: ... .:. ..,__. ..:. : : , :.; ._ . ‘. ,‘. :.‘_ -” .;-’ .~ .:
,.
Volume 28, number 2
CHEMICAL PHYSICS LETTERS
3.3. Intersystem cmssing it2 the nitronuphthalen~ The 3545 A excitation falls close to:the long wavelength edge of.the So + S, transition of the nitronaphthalenes so only a small fraction of the energy input represents vibrational energy. The very fast triplet build-up is difficult to reconcile with the relatively low triplet yields. These yields were measured [a] using the Lamola and Hammond energy transfer technique [ 141, and in particular the isomerization of cispiperylene was employed. An assumption in this application is that the fraction of piperylene triplets which produce the tram isomar is Cl.55 as measured by Lamola and Hammond [ 141 for the benzophenone triplet induced isomerism of cis-piperylene. If instead this fraction were 0.35 then the triplet yield would become unity. Since isomerism and vibrational relaxation are competitive one might expect the above fraction to depend on the initial vibrational energy content of the triplet piperylene. Assuming thai’the triplet yields of Hurley and.Testa [8J are appropriate to the systems we have studied one is !eft with proposing a picosecond time, scale nonradiative relaxaticn from the singlet to the ground state. Hurley and Testa [8] have previously suggested such a step on the basis of the non-fluorescence and the triplet yields. As far as can b’e ascertained there is negligible photochemistry Ieading to removal of l- or 2nitronaphthalenes. If there were an internal conversion from S, + So one would expect on the basis of experience with naphthalene that the T, -+ S0 intersystem crossing rate would be slower than St -+ Ss internal conversion by an additional factor of_ca. ICI’: this figure would be reduced if spin-orbit coupling were enhanced. The anoma!ous feature of the nitronaphthalenes is that they phosphoresce with significant quantum yields [7, 151 which imp!ies that the mechanism that causes the singlets to disappear is not applicable to the triplets of these molecules. A possible explanation is that unlike naphthalene the singlet and triplet, states are not similar but that one of them (the singlet) is much more strongly Coupled to intramolecular electron transfer and ether nitro, group states. Excitation of the’singlets would then involve a considerable readjustment of the nuclei appropriate :
15 September 1974
to an enhanced internal conversion and S, + T, intersystem crossing. That the triplet state wavefunction is not strongly perturbed by the nitro group is evidenced by the phosphorescence spectral appearance which is basically naphthalenic [7, lo] _The shortened lifetime compared with naphthalene requires only an enhanced spin-orbit interaction due to coupling with distant states. The unexpected solvent effects on the triplet bui!dup in the nitronaphthaIenes could be due to a variety of causes such as changes in the internal conversion effectiveness, or the intersystem crossing, cr they could reifect the ability of the solvents to vibrationahy relax the excited molecules. These new effects p.ose challenging questions about the dynamics of fast processes in the ccndensed phase.
References
111P.M. Rentzcpis, Chem. Phys. Letters 2 (196% L17. I21 E. Drent, G. Makkes ian der Deijl end PJ. Zzndstra, Chem. Phys. Letters 2 (1968) 526.
[31 P.M. Rentzepis, Science 169 (1970) 239. (41 R.M. Hochstrasser, H. Lutz uld G-W. Scott, Chem. Phys. Letters 24 (1974) i62.
151 CJ. Seliskar, OS. KhsliJ and S.P. hlcGl~n;l, in: Excited states, ed. EC. Lim (AcademicPress, Nevi York. 1974) p. 231 (see p. 262). [61 O.S. Khahl, H. Bach and S.P. McGLynn, 1. hfol. Spectry. 35 (1970) 455. [71 R. Ruukowicz and A.C. Testa, Spectmchim. Acta 27A (1971) 787. Fl R. Hurley and A-C. Testa, J. Am.Chem. Sot. 90 (1968) 1949. 191 J.J. Mikula, R.W. Anderson Jr. and L.E. Harris, Advan. Mol. Relax. Processes 5 (1973) 193. [lOI J J. hl&la, R.W. Anderson Jr., LE. Harris and E.\C’. Stcubing, I. Mol. Spectry. 42 (1972) 350. [Ill DS. McClure and P.L. Hmst, J. Chem. Phys. 23 (1955) 1772. [I21 R.W. Ar.derscn Jr., R.hI. Hochstrzsser, H. Lutz and G.W. Scott, submitted for publication. t131 T.J; Chuang and K.B. Eiser.‘Jlal, Chem. Phys. Letters 11 (1971) 368: [ 141 A.A. Lamola and G.S. Hammond, J. Chem. Phys. 43 (1965) 2129. [ 15) G.S. McClure, J. Chem. Phys. 17 (i949) 905.