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Inverse deuterium isotope effect on radiationless electronic transitions
CHEMICAL
Volume 100, number 6
INVERSE DEUTERIUM
ISOTOPE EFFECT
Aviv AMIRAV, Mark SONNENSCHEIN Deprtn~e~~t o_fChemists,
PHYSICS LElTERS
ON RADIATIONLESS
30 September
ELECTRONIC
1983
TRANSITIONS
and Joshua JORTNER
Tel A rir U?riversit_> , 699 76 Tel Aviv, Israel
Received 27 June 19S3
fiuorescencc quantum yields, Y, from photoselected vibrational states in the S1 manifold of anthracene and of perdeurcratcd ~nthncene in planar supersonic jets reveal a large imerse dcuterium isotope effect on the non-radiative relaxation from the SI origin ()-H/I’D = 5). while for high (21500 cm -t) excess energies above the S, ori$n the inverse isotope effect is eroded ( ~H/YD = 1). Nobel information emerges on intermediating states involved in intersystem crossing.
The deuteriuni isotope effect on tfle rates of intersystem crossing and internal conversion in large aromatic molecules, which m-anifests the reduction of the interstate Franck-Condon overlap upon deuteration [I] _has been well established since the early exploration of electronic radiationless transitions [2,3] . The lack of a significant deuterium isotope effect on the fluorescence of aromatic hydrocarbons [4] was rationalized by Siebrsnd [5] in terms of S, -+ T, intersystem crossing intermediated by a higher triplet (T,) state, while Sharf and Silbey [6] proposed that the S, -T, -T, coupling mechanism may exhibit an inverse deuterium isotope effect. Subsequently, Lim et al. [7,8] have demonstrated experimentally the existence of a modest inverse deuterium isotope effect on the rates of the S, --FT, intersystem crossing in naphthalene [S] and anthracene [7] in condensed phases, the reduction of the decay lifetimes and the fluorescence
quanfurn
yields
upon
deuteration
being =lO%_
Tins expcrlmental evidence [7,S] pertains to the thermally congested and inhomogeneously broadened level structure of large molecules, where photoselection of electronic-vibrational excitations is precluded_ Energy-resolved and time-resolved spectroscopy of large ultracold molecules in supersonic jets unveiled novel features of int ramolecular dynamics in “isolated-’ molecules originating from well-characterized photoselective states [9] _ We have applied the techniques of absorption and fluorescence excitation spectroscopy of large molecules in supersonic jets [lo] to deter-
mine the fluorescence quantum yields from photoselected vibrational states in the S, electronically excited manifold of anthracene (C,,Hlo) and of perdeuterated anthracene (C,4Dlo)_ We have studied the dependence of the emission quantum yield Y on the excess vibrational energy Ev above the electronic origin of S, , spanning the range Ev = O-3000 cm-l _We report the observation of a large inverse deuterium isotope effect on the non-radiative decay from low-lying vibrational excitations in the S, electronic state of anthracene. The ratio of the emission quantum yields from the S, electronic origin of anthracene, YH, and of perdeuterated anthracene, Y,, is YH/YD = 5.0, manifesting the implications of intersystem crossing intemlediated by low-energy vibronic components of a high triplet state. In the ener,y range Ev = 1 SOO3000 cm-l above the electronic origin of S, a cancellation of the deuterium isotope effect with YH/YD z 1 is exhibited,
providing
new information
on the na-
ture of intermediating states in intersystem crossing. Absorption spectra and fluorescence excitation spectra of anthracene and of perdeuterated anthracene (anthraceneillO) cooled in planar supersonic jets [l l] were determined by employing a pulsed xenon lamp and a monochromator. This “laser-free” technique [l&13] made it possible to record the absorption spectrum simultaneously with the lamp-i;iduced fluorescence (LMIF) spectrum- The ratio between the peak intensity in the LMIF and the peak absorption of each spectral feature gives the relative quantum yields [120 009~2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 100, number 6
CHEMICAL
PHYSICS LETTERS
30 September
1983
14]_ Pulsed planar jets were generated by expansion of seeded Ar through a nozzle slit (dimensions 0.27
mm X 90 mm, repetition rate 9 Hz, and width of gas pulse 300 ~.rs). Anthracene and anthracenedl10 were heated in the nozzle chamber to 120-140°C and mixed with Ar at the stagnation pressure of p = 60120 Torr. Light from a pulsed Xe flashbulb (pulse duration 2 W) was passed through a 03 m monochromator (spectral resolution 0.4 A) and focused onto the jet parallel to the slit at a distance of x = 10 mm from it. The light beam was split by a mirror and monitored by two vacuum photodiodes. The attenuation AI of the light beam due to absorption was determined from the difference in the light intensity before and after crossing the planar jet. The fluorescence intensity IF was monitored by a photomultiplier. The absorption spectrum m/lo and the LMIF spectrum IF/lo were normalized to the incident light intensity IO. The relative quantum yield, Q, is given by Q =I,lAI. The Q values were detemuned by fling the monochromator wavelength and recording simultaneously the values of IF and AI. The signals were recorded when temporal matching of the gas pulse from the nozzle and the light pulse occurs, while the base line was taken when the gas pulse is out of temporal coincidence with the light pulse. The Q values for each molecule were normalized relative to the S+(O) electronic origin of the S, electronic state. The relative fluorescence quantum yields for the S, (0) electronic origins of anthracene and anthraceneJ1O were determined in an expansion seeded with the two molecules. Absolute quantum yields [14] were determined by measuring the Q values from the Sl(0) states of anthracene and of 9,10dichloro-anthracene in an expansion containing these two molecules_ The quantum yield from Sr(0) of the reference 9 ,lO-dichloro-anthracene molecule was taken to be unity [ 141. In this manner the relative quantum yields of anthracene and of anthracenecllo were calibrated to give the absolute quantum yields, Y, from photoselected vibrational states of these two molecules. Fig. 1 shows the LMIF spectra in the region 36253375 A of anthracene and of anthracened10 in planar supersonic expansions of Ar. The most prominent SpcCtral features located at AH = 3610.5 A (VH = 27697 cm-l) * for anthracene and at h, = 3601.6 A * Accuracy of wavelength scale is k-O-2A_ Energies are accnrate within 21 cm-‘.
I 3625
1
3575
t
I
35’25
3475
WAVELENGTH
3425
3: 75
6,
Fig. 1. Fluorescence excitation spectra in the region 362% 3375 A of anthracene and of anthracene-dro cooled in pulsed planar supersonic expansion of Ar at p = 100 Torr and expanded through a 0.37 X 90 mm nozzle slit. The nozzle temperature was 135°C. The light beam crossed the planar jet at x = 10 mm downstream. The spectral resolution was 4 cm-’ _
(Ye = 27765 cm-l) for anthraceneillo correspond to the O-O electronic origins of the So + S, (lb + lBuo) transition. The deuterium isotope bl
transition
of ultracold
anthracene
concur with the gas-phase (60°C) bulb data of Byrne and Ross [15], who found that X, = 3610.61 A, X,, = 3601-59 A and 6y(D-H) = 693 cm-l _ The excellent agreement between the present supersonic jet spectra and the bulb spectra of the O-O origin of the large anthracene molecule is a tribute to the pioneering study of Byrne and Ross [ 15]_ Fig. 2 portrays the absorption spectra together with the LMIF spectra of the electronic origins of anthracene and of anthracened10 obtained in a binary mlx-
4s9
Volume
100. number 6
CHEMICAL
___ ____._ -____
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1
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PHYSICS
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l-g. 7. Abzorption spcctr.~(upper curve) and tluoresccncc e.\cltntion spectra (loivcr curve) of the O-O electronic origins of .mthmccnc .md of .mthr.wcmA,o. Expcrimcntal conditions ac m fig. 1. except that a mixture of =50% anthrsccnc .md ==5Oc .mthr~ccnc+Ilo \\‘.I.\ used. rend the absorption and fluorcsccncc spccrra \\crc taken simultaneously on the same double pen recorder. The insert\ on the ri#-hand side of the tigure rcprcsrnt rJ\\ d.lta for the simult.meous mL’dsurenwnt of fluorcsccncc’ tcnlid line) and of absorption (dashed line) taken at liwd ~~.wclength~. which correspond to the O-O tr.msitions of .mthraccne .md of anthr.wcneiita. Thr measurement time. ~khich corresponds lo rhc horizont;ll scale is 100 5 for e.tch melsurcmCnt_ while the time conkmt of the mwsurinp sys-
LETTERS
30 September
1983
tial inverse deuterium isotope effect. A cursory examination of the LMF spectra (fig. 1) clearly indicates that the relative intensities at excess vibrational energies Ev > 1100 cm-l are considerably lower (by about a numerical factor of =:5) for anthracene than for anthracene-dlO. We thus expect that the large inverse deuterium isotope effect on the origin is considerably reduced at high Ev_ These observations are borne out by quantitative determinations of the relative quantum yields. We have measured Q values for fluorescence from the origin (fig. 2) and from photoselected vibrational states in the S, manifold of the two molecules_ These Q values were normalized to the value Q = 1 for S, (0) of anthracene. Absolute quantum yields were obtained subsequently by measuring the relative quantum yields from S,(O) of anthracene and of 9,10dichloro-anthracene, and assigning the absolute value Y = 1 for S1(0) of 9,lOdichloro-anthracene [ 14]_ The E, dependence of the Y values of anthracene and of anthraceneil10 are portrayed in fig. 3. From these results, the following conclusions emerge: (1) A large inverse deuterium isotope effect is exhibited at the electronic origin. The ratio of the quantum yield from S, (0) of anthracene and of anthracened,o is Y&Y, = 5.0 (fig. 4)_ Invoking the reasonable assumption that the pure radiative lifetime is invariant with respect to isotope substitution, then the ratio of the non-radiative decay lifetimes from the S, origin -&, is T,H,/ of anthracene, r,,,H and of anthracenedilO, ~fl, = (1 - Y,) YH/( 1 - Y,) Y, , resulting in the value -r&/7?& = 13.
trm 11JZ 10 \. The b~sc line is taken when the gas pulse from the nozrlc is out of tcmporai coincidence with the light pulse, N hilt the signal is rrcordrd when the tumporrtl matchins of the 2.1, pulse and the li_cht pulw occurs.
ture of these two molecules in a jet of Ar. The width (fwhm) of these spectral features is 4 cm-l, being derrrmined by the instrumental spectral resolution, which exceeds the intrinsic linewidth (l-2 cm-l) [ 161 of rhe rotational envelopes of the origins. An examination of the ratios between the peak LMIF and the peak absorption of the O-O origins in fig. 2 clearly indicates that the relative quantum yield, Q, from the S 1(0)state of anthracene is considerably higher than Q from S,(O) of anthracene~lO, exhibiting a substan490
. 011
0
I
500
,
t
IO00
1500
2000
2500
I 3000
Evkm-ll Fig. 3. The dependence of the absolure quantum yields of fluorescence from photoselected vibrational excitations in the S, state of anthncene and of anthracenwIt0 on the excess vibrational energy Ev above the electronic origin of St.
Volume 100, number 6
CHEMICAL
PHYSICS
LETTERS
30 September
1983
simple effective hamiltonian matrix ANTHRACENE 51
E(S1) CV SO
i
1
v,, E(T,)
--$iA
0)
,
where E(SI) and E(T,) are the energies of the zeroorder states IS,> and IT,), respectively_ The eigenvalues of the effective hamiltonian are I
I
lokl
0
2000
El.2 =$ [E(T,)
I
- E(S,) - $iAj
3000
Evlcm-‘1 Fig. 4. The dependence of the ratio of the emission quantum yields YH/YD from pbotoselected electronic-vibntion~l excitations of anthracene (YH) and of anthnccne-dlo (YJJ) on the excess vibrational energy above the electronic origin of S1 The size of the circles exceeds the variance in the Ev values of the two isotopic molecules.
(2) The inverse deuterium isotope effect decreases with the increase of the excess vibrational ener,gy EV above S1(0). In the range Ev = O-1500 cm-l a gradual decrease of Y,/Y, is exhibited (fig. 4). (3) The inverse deuterium isotope effect disappears at high excess vibrational energies. In the energy range Ev = 1500-3000 cm-l, the ratio of the quantum = 1 (fig. 4), i.e. 7H yields is Y&Y, nr /Pnr = 1. The large inverse deuterium isotope effect at the electronic origin of anthracene is surprising_ The S, T, intersystem crossing in this molecule is intermediated by the vibronic level(s) of a TX state [5,6,17]. It can readily
be demonstrated
that indirect
coupling
in-
termediated by a single TX vibronic level cannot exhibit an inverse deuteriunr isotope effect_ Consider the model system where direct IS,>{ITI>} coupliogis negligible [ 171, so that the relevant coupling scheme is
!s,)
k
IT,) ‘.
{ITI)}
,
(1)
where vs,, and V,,ib represent spin-orbit and vibronic coupling, respectively. When the I TX) - { 1T, )} coupling corresponds to the statistical limit, the decay width of IT,) is [6,18] A =
;n(k$jbP) ,
(2)
where p is the density of states in the ITI >manifold. The dynamics of the system is then detemrined by the
- 4[E(Tx)E(S1)
-
V;o]}“’
_
(4)
The complex parts 6 = -lm E, and y = -Im E2 of these eigenvalues correspond to the decay lifetimes of the mixed state. Two limiting cases can now be distinguished: (A) Off-resonance limit: IE(T,) - E(S,) - $A1 % IV& This corresponds to the well-known perturbative result [ 171 S = A - V,‘,A/ { [E(T,) Y = V,‘,AlC ]E(T,)
- E(S,)] 2 + ($A)‘},
- E(S,)l’
+ (;A)‘],
(5) (6)
where y, eq. (6), corresponds to the decay rate of the state of IS1 >parentage. (B) Near-resonance limit: IE(T_X) - E(S1) -fin] < 1Vml_ This situation corresponds to a complete dilution of the width A among the two interacting states, whose decay widths are y=s=;A.
(7)
From this analysis it is apparent that in both cases (A) and (B) the lifetime -y of the state of IS, >parentage is y=A_
(8)
The width, eq. (2), appearing in eq. (S), is characterized by a normal deuterium isotope effect, i-e_ A(H)/A(D) > l_ Accordingly, we expect that for the coupling scheme (l), y(H)/y(D) L 1, and a normal deuterium isotope effect is exhibited. This conclusion is expected to hold provided that the ener-v denominator E(T,) - E(SI) between the vibronic levels, which appear in eq. (6), is invariant to isotopic substitution. We note in passing that the situation considered by Sharf and Silbey [6] corresponds to case (A) with 491
Volume 100, number 6
CHEMICAL PHYSICS LETTERS
Vso < W(T,) - E(Sl)I Q a. Such a rare accidental degeneracy can be excluded in the present case, in view of the smooth dependence of the inverse deuterium isotope effect on El, (fig. 4). What is the origin of the large inverse deuterium isotope effect on the intersystem crossing from S1 (0) and low vibronic levels of anthracene? Four general observations are relevant in this context_ Firstly, the electronic level structure, i.e. the electronic ener,v gap between S,(O) and the origin TX(O) of intermediating states is invariant to isotopic substitution. Secondly, on the basis of the foregoing analysis, the decay of a vibronic IS,) level is not intermediated by a single IT,) state. Instead, we propose that the decay of IS,) is intermediated by a manifold of low-lying ITi) (k = 1,2, 3, ___)at vibronic levels. The coupling scheme is now (9) Thirdly, the vibronic level structure, i.e. the energy separations lE(S,) - E(T$)I for some vibrationally excited IT-$> states. can be sensitive to isotopic substitution, Fourthly, the smooth decrease of Yr,/YD with increasing Ev in the range E, = O-1500 cm-r (fig. 4) implies that the IS, I T(;:) couplings for all k states correspond to the off-resonance situation, whereupon near-resonance coupling effects are not e_xhibited. In the off-resonance situation the coupling scheme (9) results in the following expression for the decay rate of IS,> [IS]: y =c
k
I’,‘,,(k)A;/{
[I:(S,)-E(T$)]’
+(;L\#}
,
(10) which constitutes a generalization of eq. (6). Here, Fso(k) is the spin-orbit IS, > lT$coupling, while Ax- is the decay width of lT$) due to its coupling to the statistical (lT1)) manifold. The intersystem crossing rate, eq. (IO). contains a cumulative contribution from several intermediating states. The normal deuterium isotope effect on the widths, 4k, is expected to be s~nall, in view of the moderate (10000-l 2000 cm-l) enerb?’ gaps between the electronic origins of the TX and T, electronic manifolds [7]. In addition, the V,,(k) coupling temts are expected to exhibit a small. normal. deuterium isotope effect_ The inverse deuterium effect can originate only from the sensitivity of the energy defects lE(Sl) - E(Tz)l which appear in eq. (10) on isotopic substitution_ The large inverse deuterium isotope effect on the 492
30 September 1983
decay of the S, electronic origin is ascribed to the coupling of S,(O) with a sparse {I T_$)) manifold, which involves a set of lowenerg vibrationally excited TX states. The level structure of these vibrational excitations in the TX manifold is sensitive to deureration. This sparse manifold of intermediating TX states is more congested in the vicinity of S1(0) of anthracened10 than near St (0) of anthracene. Accordingly, some ener=v defects IE(SI) -E(T$)l contributing ro the non-radiative rate expression, eq. (lo), are substantially larger for the decay of S,(O) of anthracened,c than for Sr{O) of anthracene, resulting in a considerable enhancement of y in the former case. It is thus apparent that the large inverse deuterium isotope effect on S,(O) of anthracene originates from coupling to low-frequency intermediating states, whose energies are sensitive to deuteration. Such low-frequency intermediating states may involve out-of-plane hydrogen vibrations. This physical picture provides a proper description for the disappearance of the deuterium isotope effect at high E, e 1500 cm-l), which is attributed to intersystem crossing intermediated by a congested manifold of T, states. At high values of Ev, the level structure of the intermediating TX states becomes dense. Under these circumstances, the coupling scheme (9) corresponds to sequential decay via the {lT$} quasicontinuum_ This state of affairs is analogous to internal conversion between high states in the statistical limit [ 193. The decay rate, yH, of a high S1 vlbronic level is then roughly T = 2nW~op_X), where pX is the density of states in the TX manifold.~ is expected to be characterized by a small, normal, deuterium isotope effect, whereupon the inverse deuterium isotope effect is eroded at high Eva Traditional theories of intramolecular radiationless transitions [l] focused attention on the details of the final states within the dissipative quasicontinuum, which were segregated into promoting modes and accepting modes. The present work provides novel information concerning the characteristics of intermediating states involved in intramolecular radiationless transitions_ This research was supported by the United States Amry through its European Research Office, and by the United States-Israel Binational Science Foundation, Jerusalem (Grant No. 2641).
Volume 100, number 6
CHEMICAL
PHYSICS LETTERS
References [l] [2] [3] [4] [5] 16) [ 71 [8] 191
J. Jortner, .%A_ Rice and R.hl. Hochstr&ser, Advan. Photochem. 7 (1969) 149_ C-A. Hutchison and B_\V_Mangum, J. Chem. Phys. 32 (1960) 1261. M.R_ Wright, RP_ Frosch and G-IV_ Robinson, J. Chem. Phys. 33 (1960) 934. E.C. Lii and JJ). Laposa, J. Chem. Phys. 41(1964) 3257. W. Siebrand, in: The triplet state, ed. A.B. Zahlan (Cambridge Univ. Press, London, 1967). B. Sharf and R. Silbey, Chem. Phys. Letters 5 (1970) 314. E-C. Lim and H.R. Bhattacharjee, Chem. Phys. Letters 9 (1971) 249. J-0. Vy and EC. Lim, Chem. Phys. Letters 7 (1970) 306. A. Amirrlv, U. Even and J. Jortner, J. Chem. Phys. 71 (1979) 2319.
[lo] [ ll] [12] 1131
.
[ 141 [15] 1161 1171 118 ] [19]
30 September
1983
D.H. Levy, Ann. Rev. Phys. Chem. 31(1980) 197. A. Amirav, I-L Even and J. Jortner. Chem. Phys. Letters 83 (1981) 1. A. Amirav and J. Jortner, Chem. Phys. Letters 94 (1983) 545. A. Amirav and J. Jortner,Chem. Phys. Letters 95 (1983) 295. M. Sonnenschein, A. Amirav and J. Jortner, to be published_ JP. Byrne and LG. Ross, Can. J. Phys. 43 (1965) 3253. A. Amirav, U. Even and J. Jortner, Anal. Chcm. 54 (1982) 1666. hf. Biion and J. Jortner, J. Chem. Phys. 48 (1968) 715. A. Nitzan and J_ Jortner, Theoret. Chim. Acia 29 (1973) :Nitzan, J. Jortner and PM (1971) 585.
Rentzepis, hloL Phys. 22
493
Volume 100, number 6
INVERSE DEUTERIUM
ISOTOPE EFFECT
Aviv AMIRAV, Mark SONNENSCHEIN Deprtn~e~~t o_fChemists,
PHYSICS LElTERS
ON RADIATIONLESS
30 September
ELECTRONIC
1983
TRANSITIONS
and Joshua JORTNER
Tel A rir U?riversit_> , 699 76 Tel Aviv, Israel
Received 27 June 19S3
fiuorescencc quantum yields, Y, from photoselected vibrational states in the S1 manifold of anthracene and of perdeurcratcd ~nthncene in planar supersonic jets reveal a large imerse dcuterium isotope effect on the non-radiative relaxation from the SI origin ()-H/I’D = 5). while for high (21500 cm -t) excess energies above the S, ori$n the inverse isotope effect is eroded ( ~H/YD = 1). Nobel information emerges on intermediating states involved in intersystem crossing.
The deuteriuni isotope effect on tfle rates of intersystem crossing and internal conversion in large aromatic molecules, which m-anifests the reduction of the interstate Franck-Condon overlap upon deuteration [I] _has been well established since the early exploration of electronic radiationless transitions [2,3] . The lack of a significant deuterium isotope effect on the fluorescence of aromatic hydrocarbons [4] was rationalized by Siebrsnd [5] in terms of S, -+ T, intersystem crossing intermediated by a higher triplet (T,) state, while Sharf and Silbey [6] proposed that the S, -T, -T, coupling mechanism may exhibit an inverse deuterium isotope effect. Subsequently, Lim et al. [7,8] have demonstrated experimentally the existence of a modest inverse deuterium isotope effect on the rates of the S, --FT, intersystem crossing in naphthalene [S] and anthracene [7] in condensed phases, the reduction of the decay lifetimes and the fluorescence
quanfurn
yields
upon
deuteration
being =lO%_
Tins expcrlmental evidence [7,S] pertains to the thermally congested and inhomogeneously broadened level structure of large molecules, where photoselection of electronic-vibrational excitations is precluded_ Energy-resolved and time-resolved spectroscopy of large ultracold molecules in supersonic jets unveiled novel features of int ramolecular dynamics in “isolated-’ molecules originating from well-characterized photoselective states [9] _ We have applied the techniques of absorption and fluorescence excitation spectroscopy of large molecules in supersonic jets [lo] to deter-
mine the fluorescence quantum yields from photoselected vibrational states in the S, electronically excited manifold of anthracene (C,,Hlo) and of perdeuterated anthracene (C,4Dlo)_ We have studied the dependence of the emission quantum yield Y on the excess vibrational energy Ev above the electronic origin of S, , spanning the range Ev = O-3000 cm-l _We report the observation of a large inverse deuterium isotope effect on the non-radiative decay from low-lying vibrational excitations in the S, electronic state of anthracene. The ratio of the emission quantum yields from the S, electronic origin of anthracene, YH, and of perdeuterated anthracene, Y,, is YH/YD = 5.0, manifesting the implications of intersystem crossing intemlediated by low-energy vibronic components of a high triplet state. In the ener,y range Ev = 1 SOO3000 cm-l above the electronic origin of S, a cancellation of the deuterium isotope effect with YH/YD z 1 is exhibited,
providing
new information
on the na-
ture of intermediating states in intersystem crossing. Absorption spectra and fluorescence excitation spectra of anthracene and of perdeuterated anthracene (anthraceneillO) cooled in planar supersonic jets [l l] were determined by employing a pulsed xenon lamp and a monochromator. This “laser-free” technique [l&13] made it possible to record the absorption spectrum simultaneously with the lamp-i;iduced fluorescence (LMIF) spectrum- The ratio between the peak intensity in the LMIF and the peak absorption of each spectral feature gives the relative quantum yields [120 009~2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 100, number 6
CHEMICAL
PHYSICS LETTERS
30 September
1983
14]_ Pulsed planar jets were generated by expansion of seeded Ar through a nozzle slit (dimensions 0.27
mm X 90 mm, repetition rate 9 Hz, and width of gas pulse 300 ~.rs). Anthracene and anthracenedl10 were heated in the nozzle chamber to 120-140°C and mixed with Ar at the stagnation pressure of p = 60120 Torr. Light from a pulsed Xe flashbulb (pulse duration 2 W) was passed through a 03 m monochromator (spectral resolution 0.4 A) and focused onto the jet parallel to the slit at a distance of x = 10 mm from it. The light beam was split by a mirror and monitored by two vacuum photodiodes. The attenuation AI of the light beam due to absorption was determined from the difference in the light intensity before and after crossing the planar jet. The fluorescence intensity IF was monitored by a photomultiplier. The absorption spectrum m/lo and the LMIF spectrum IF/lo were normalized to the incident light intensity IO. The relative quantum yield, Q, is given by Q =I,lAI. The Q values were detemuned by fling the monochromator wavelength and recording simultaneously the values of IF and AI. The signals were recorded when temporal matching of the gas pulse from the nozzle and the light pulse occurs, while the base line was taken when the gas pulse is out of temporal coincidence with the light pulse. The Q values for each molecule were normalized relative to the S+(O) electronic origin of the S, electronic state. The relative fluorescence quantum yields for the S, (0) electronic origins of anthracene and anthraceneJ1O were determined in an expansion seeded with the two molecules. Absolute quantum yields [14] were determined by measuring the Q values from the Sl(0) states of anthracene and of 9,10dichloro-anthracene in an expansion containing these two molecules_ The quantum yield from Sr(0) of the reference 9 ,lO-dichloro-anthracene molecule was taken to be unity [ 141. In this manner the relative quantum yields of anthracene and of anthracenecllo were calibrated to give the absolute quantum yields, Y, from photoselected vibrational states of these two molecules. Fig. 1 shows the LMIF spectra in the region 36253375 A of anthracene and of anthracened10 in planar supersonic expansions of Ar. The most prominent SpcCtral features located at AH = 3610.5 A (VH = 27697 cm-l) * for anthracene and at h, = 3601.6 A * Accuracy of wavelength scale is k-O-2A_ Energies are accnrate within 21 cm-‘.
I 3625
1
3575
t
I
35’25
3475
WAVELENGTH
3425
3: 75
6,
Fig. 1. Fluorescence excitation spectra in the region 362% 3375 A of anthracene and of anthracene-dro cooled in pulsed planar supersonic expansion of Ar at p = 100 Torr and expanded through a 0.37 X 90 mm nozzle slit. The nozzle temperature was 135°C. The light beam crossed the planar jet at x = 10 mm downstream. The spectral resolution was 4 cm-’ _
(Ye = 27765 cm-l) for anthraceneillo correspond to the O-O electronic origins of the So + S, (lb + lBuo) transition. The deuterium isotope bl
transition
of ultracold
anthracene
concur with the gas-phase (60°C) bulb data of Byrne and Ross [15], who found that X, = 3610.61 A, X,, = 3601-59 A and 6y(D-H) = 693 cm-l _ The excellent agreement between the present supersonic jet spectra and the bulb spectra of the O-O origin of the large anthracene molecule is a tribute to the pioneering study of Byrne and Ross [ 15]_ Fig. 2 portrays the absorption spectra together with the LMIF spectra of the electronic origins of anthracene and of anthracened10 obtained in a binary mlx-
4s9
Volume
100. number 6
CHEMICAL
___ ____._ -____
?1
1
ANTH~$EETJE-Y,~
li (f $1
4:
PHYSICS
ANTHQXEh’E
- D,,,
D
D
3
zi
ANTHRACENE
_I’
L
-__.-
.-.
i._
3626
_.__-L_
3610
____
A-_--J___ 3600 3590
____I
\VAVELENGTH
(i
.______
-----.i
I
1
l-g. 7. Abzorption spcctr.~(upper curve) and tluoresccncc e.\cltntion spectra (loivcr curve) of the O-O electronic origins of .mthmccnc .md of .mthr.wcmA,o. Expcrimcntal conditions ac m fig. 1. except that a mixture of =50% anthrsccnc .md ==5Oc .mthr~ccnc+Ilo \\‘.I.\ used. rend the absorption and fluorcsccncc spccrra \\crc taken simultaneously on the same double pen recorder. The insert\ on the ri#-hand side of the tigure rcprcsrnt rJ\\ d.lta for the simult.meous mL’dsurenwnt of fluorcsccncc’ tcnlid line) and of absorption (dashed line) taken at liwd ~~.wclength~. which correspond to the O-O tr.msitions of .mthraccne .md of anthr.wcneiita. Thr measurement time. ~khich corresponds lo rhc horizont;ll scale is 100 5 for e.tch melsurcmCnt_ while the time conkmt of the mwsurinp sys-
LETTERS
30 September
1983
tial inverse deuterium isotope effect. A cursory examination of the LMF spectra (fig. 1) clearly indicates that the relative intensities at excess vibrational energies Ev > 1100 cm-l are considerably lower (by about a numerical factor of =:5) for anthracene than for anthracene-dlO. We thus expect that the large inverse deuterium isotope effect on the origin is considerably reduced at high Ev_ These observations are borne out by quantitative determinations of the relative quantum yields. We have measured Q values for fluorescence from the origin (fig. 2) and from photoselected vibrational states in the S, manifold of the two molecules_ These Q values were normalized to the value Q = 1 for S, (0) of anthracene. Absolute quantum yields were obtained subsequently by measuring the relative quantum yields from S,(O) of anthracene and of 9,10dichloro-anthracene, and assigning the absolute value Y = 1 for S1(0) of 9,lOdichloro-anthracene [ 14]_ The E, dependence of the Y values of anthracene and of anthraceneil10 are portrayed in fig. 3. From these results, the following conclusions emerge: (1) A large inverse deuterium isotope effect is exhibited at the electronic origin. The ratio of the quantum yield from S, (0) of anthracene and of anthracened,o is Y&Y, = 5.0 (fig. 4)_ Invoking the reasonable assumption that the pure radiative lifetime is invariant with respect to isotope substitution, then the ratio of the non-radiative decay lifetimes from the S, origin -&, is T,H,/ of anthracene, r,,,H and of anthracenedilO, ~fl, = (1 - Y,) YH/( 1 - Y,) Y, , resulting in the value -r&/7?& = 13.
trm 11JZ 10 \. The b~sc line is taken when the gas pulse from the nozrlc is out of tcmporai coincidence with the light pulse, N hilt the signal is rrcordrd when the tumporrtl matchins of the 2.1, pulse and the li_cht pulw occurs.
ture of these two molecules in a jet of Ar. The width (fwhm) of these spectral features is 4 cm-l, being derrrmined by the instrumental spectral resolution, which exceeds the intrinsic linewidth (l-2 cm-l) [ 161 of rhe rotational envelopes of the origins. An examination of the ratios between the peak LMIF and the peak absorption of the O-O origins in fig. 2 clearly indicates that the relative quantum yield, Q, from the S 1(0)state of anthracene is considerably higher than Q from S,(O) of anthracene~lO, exhibiting a substan490
. 011
0
I
500
,
t
IO00
1500
2000
2500
I 3000
Evkm-ll Fig. 3. The dependence of the absolure quantum yields of fluorescence from photoselected vibrational excitations in the S, state of anthncene and of anthracenwIt0 on the excess vibrational energy Ev above the electronic origin of St.
Volume 100, number 6
CHEMICAL
PHYSICS
LETTERS
30 September
1983
simple effective hamiltonian matrix ANTHRACENE 51
E(S1) CV SO
i
1
v,, E(T,)
--$iA
0)
,
where E(SI) and E(T,) are the energies of the zeroorder states IS,> and IT,), respectively_ The eigenvalues of the effective hamiltonian are I
I
lokl
0
2000
El.2 =$ [E(T,)
I
- E(S,) - $iAj
3000
Evlcm-‘1 Fig. 4. The dependence of the ratio of the emission quantum yields YH/YD from pbotoselected electronic-vibntion~l excitations of anthracene (YH) and of anthnccne-dlo (YJJ) on the excess vibrational energy above the electronic origin of S1 The size of the circles exceeds the variance in the Ev values of the two isotopic molecules.
(2) The inverse deuterium isotope effect decreases with the increase of the excess vibrational ener,gy EV above S1(0). In the range Ev = O-1500 cm-l a gradual decrease of Y,/Y, is exhibited (fig. 4). (3) The inverse deuterium isotope effect disappears at high excess vibrational energies. In the energy range Ev = 1500-3000 cm-l, the ratio of the quantum = 1 (fig. 4), i.e. 7H yields is Y&Y, nr /Pnr = 1. The large inverse deuterium isotope effect at the electronic origin of anthracene is surprising_ The S, T, intersystem crossing in this molecule is intermediated by the vibronic level(s) of a TX state [5,6,17]. It can readily
be demonstrated
that indirect
coupling
in-
termediated by a single TX vibronic level cannot exhibit an inverse deuteriunr isotope effect_ Consider the model system where direct IS,>{ITI>} coupliogis negligible [ 171, so that the relevant coupling scheme is
!s,)
k
IT,) ‘.
{ITI)}
,
(1)
where vs,, and V,,ib represent spin-orbit and vibronic coupling, respectively. When the I TX) - { 1T, )} coupling corresponds to the statistical limit, the decay width of IT,) is [6,18] A =
;n(k$jbP) ,
(2)
where p is the density of states in the ITI >manifold. The dynamics of the system is then detemrined by the
- 4[E(Tx)E(S1)
-
V;o]}“’
_
(4)
The complex parts 6 = -lm E, and y = -Im E2 of these eigenvalues correspond to the decay lifetimes of the mixed state. Two limiting cases can now be distinguished: (A) Off-resonance limit: IE(T,) - E(S,) - $A1 % IV& This corresponds to the well-known perturbative result [ 171 S = A - V,‘,A/ { [E(T,) Y = V,‘,AlC ]E(T,)
- E(S,)] 2 + ($A)‘},
- E(S,)l’
+ (;A)‘],
(5) (6)
where y, eq. (6), corresponds to the decay rate of the state of IS1 >parentage. (B) Near-resonance limit: IE(T_X) - E(S1) -fin] < 1Vml_ This situation corresponds to a complete dilution of the width A among the two interacting states, whose decay widths are y=s=;A.
(7)
From this analysis it is apparent that in both cases (A) and (B) the lifetime -y of the state of IS, >parentage is y=A_
(8)
The width, eq. (2), appearing in eq. (S), is characterized by a normal deuterium isotope effect, i-e_ A(H)/A(D) > l_ Accordingly, we expect that for the coupling scheme (l), y(H)/y(D) L 1, and a normal deuterium isotope effect is exhibited. This conclusion is expected to hold provided that the ener-v denominator E(T,) - E(SI) between the vibronic levels, which appear in eq. (6), is invariant to isotopic substitution. We note in passing that the situation considered by Sharf and Silbey [6] corresponds to case (A) with 491
Volume 100, number 6
CHEMICAL PHYSICS LETTERS
Vso < W(T,) - E(Sl)I Q a. Such a rare accidental degeneracy can be excluded in the present case, in view of the smooth dependence of the inverse deuterium isotope effect on El, (fig. 4). What is the origin of the large inverse deuterium isotope effect on the intersystem crossing from S1 (0) and low vibronic levels of anthracene? Four general observations are relevant in this context_ Firstly, the electronic level structure, i.e. the electronic ener,v gap between S,(O) and the origin TX(O) of intermediating states is invariant to isotopic substitution. Secondly, on the basis of the foregoing analysis, the decay of a vibronic IS,) level is not intermediated by a single IT,) state. Instead, we propose that the decay of IS,) is intermediated by a manifold of low-lying ITi) (k = 1,2, 3, ___)at vibronic levels. The coupling scheme is now (9) Thirdly, the vibronic level structure, i.e. the energy separations lE(S,) - E(T$)I for some vibrationally excited IT-$> states. can be sensitive to isotopic substitution, Fourthly, the smooth decrease of Yr,/YD with increasing Ev in the range E, = O-1500 cm-r (fig. 4) implies that the IS, I T(;:) couplings for all k states correspond to the off-resonance situation, whereupon near-resonance coupling effects are not e_xhibited. In the off-resonance situation the coupling scheme (9) results in the following expression for the decay rate of IS,> [IS]: y =c
k
I’,‘,,(k)A;/{
[I:(S,)-E(T$)]’
+(;L\#}
,
(10) which constitutes a generalization of eq. (6). Here, Fso(k) is the spin-orbit IS, > lT$coupling, while Ax- is the decay width of lT$) due to its coupling to the statistical (lT1)) manifold. The intersystem crossing rate, eq. (IO). contains a cumulative contribution from several intermediating states. The normal deuterium isotope effect on the widths, 4k, is expected to be s~nall, in view of the moderate (10000-l 2000 cm-l) enerb?’ gaps between the electronic origins of the TX and T, electronic manifolds [7]. In addition, the V,,(k) coupling temts are expected to exhibit a small. normal. deuterium isotope effect_ The inverse deuterium effect can originate only from the sensitivity of the energy defects lE(Sl) - E(Tz)l which appear in eq. (10) on isotopic substitution_ The large inverse deuterium isotope effect on the 492
30 September 1983
decay of the S, electronic origin is ascribed to the coupling of S,(O) with a sparse {I T_$)) manifold, which involves a set of lowenerg vibrationally excited TX states. The level structure of these vibrational excitations in the TX manifold is sensitive to deureration. This sparse manifold of intermediating TX states is more congested in the vicinity of S1(0) of anthracened10 than near St (0) of anthracene. Accordingly, some ener=v defects IE(SI) -E(T$)l contributing ro the non-radiative rate expression, eq. (lo), are substantially larger for the decay of S,(O) of anthracened,c than for Sr{O) of anthracene, resulting in a considerable enhancement of y in the former case. It is thus apparent that the large inverse deuterium isotope effect on S,(O) of anthracene originates from coupling to low-frequency intermediating states, whose energies are sensitive to deuteration. Such low-frequency intermediating states may involve out-of-plane hydrogen vibrations. This physical picture provides a proper description for the disappearance of the deuterium isotope effect at high E, e 1500 cm-l), which is attributed to intersystem crossing intermediated by a congested manifold of T, states. At high values of Ev, the level structure of the intermediating TX states becomes dense. Under these circumstances, the coupling scheme (9) corresponds to sequential decay via the {lT$} quasicontinuum_ This state of affairs is analogous to internal conversion between high states in the statistical limit [ 193. The decay rate, yH, of a high S1 vlbronic level is then roughly T = 2nW~op_X), where pX is the density of states in the TX manifold.~ is expected to be characterized by a small, normal, deuterium isotope effect, whereupon the inverse deuterium isotope effect is eroded at high Eva Traditional theories of intramolecular radiationless transitions [l] focused attention on the details of the final states within the dissipative quasicontinuum, which were segregated into promoting modes and accepting modes. The present work provides novel information concerning the characteristics of intermediating states involved in intramolecular radiationless transitions_ This research was supported by the United States Amry through its European Research Office, and by the United States-Israel Binational Science Foundation, Jerusalem (Grant No. 2641).
Volume 100, number 6
CHEMICAL
PHYSICS LETTERS
References [l] [2] [3] [4] [5] 16) [ 71 [8] 191
J. Jortner, .%A_ Rice and R.hl. Hochstr&ser, Advan. Photochem. 7 (1969) 149_ C-A. Hutchison and B_\V_Mangum, J. Chem. Phys. 32 (1960) 1261. M.R_ Wright, RP_ Frosch and G-IV_ Robinson, J. Chem. Phys. 33 (1960) 934. E.C. Lii and JJ). Laposa, J. Chem. Phys. 41(1964) 3257. W. Siebrand, in: The triplet state, ed. A.B. Zahlan (Cambridge Univ. Press, London, 1967). B. Sharf and R. Silbey, Chem. Phys. Letters 5 (1970) 314. E-C. Lim and H.R. Bhattacharjee, Chem. Phys. Letters 9 (1971) 249. J-0. Vy and EC. Lim, Chem. Phys. Letters 7 (1970) 306. A. Amirrlv, U. Even and J. Jortner, J. Chem. Phys. 71 (1979) 2319.
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1983
D.H. Levy, Ann. Rev. Phys. Chem. 31(1980) 197. A. Amirav, I-L Even and J. Jortner. Chem. Phys. Letters 83 (1981) 1. A. Amirav and J. Jortner, Chem. Phys. Letters 94 (1983) 545. A. Amirav and J. Jortner,Chem. Phys. Letters 95 (1983) 295. M. Sonnenschein, A. Amirav and J. Jortner, to be published_ JP. Byrne and LG. Ross, Can. J. Phys. 43 (1965) 3253. A. Amirav, U. Even and J. Jortner, Anal. Chcm. 54 (1982) 1666. hf. Biion and J. Jortner, J. Chem. Phys. 48 (1968) 715. A. Nitzan and J_ Jortner, Theoret. Chim. Acia 29 (1973) :Nitzan, J. Jortner and PM (1971) 585.
Rentzepis, hloL Phys. 22
493