4622
J. Phys. Chem. 1984,88, 4622-4626
Optical and ODMR Studies on the Color Phenomenon of Silver Nitrite. The Effect of Ag’ Perturbation on the Radiative and Nonradiative Transitions in NO2Akira Yamashita and Tohru Azumi* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: August 29, 1983; In Final Form: April 10, 1984)
The effect of Ag+ perturbation on the T,-So transition of NO2- is examined, focusing attention on the triplet sublevels. The radiative rate constant for the T, phosphorescence is increased by a factor of 800 by Ag+ perturbation, whereas that for the T, phosphorescence is increased only by a factor of 4. The out-of-plane T, sublevel is practically nonemissive, and no quantitative analysis is made. The T1 So and SI T1 nonradiative decay rate constants are essentially unsusceptible to Agt perturbation. The source of the selective enhancement of the T, radiative rate constant is discussed with a concept that a charge-transfer state arising from the promotion of an electron from the nonbonding orbital of NOz- to the 5p, orbital of Ag+ plays an essential role.
-
-
Introduction The colors of nontransition-metal salts vary significantly with the metal counterion. For example, most of the A metal salts such as NaNO, and KNO, are almost colorless, whereas the heavy B metal salts such as A g N 0 2 and T l N 0 2 are yellow. In 1968, Maria, Wahlborg, and McGlynnl proposed that the color of the heavy B metal salts is caused by an enhanced TI So absorption of NOz- ion. The color induction, therefore, was attributed to a heavy atom effect on the spin-forbidden transitions. There are two ways of looking at this heavy atom effect. In the first place, one may regard the color enhancement phenomenon as the external heavy atom effect on the TI So transition of NOz- ion.2 Alternatively, one may regard the phenomenon as the internal heavy atom effect3 on the transitions in the cationNOz- “complex”. Whichever view one adopts, the interpretation of the color induction phenomenon in terms of the heavy atom effect encounters several difficulties. The first difficulty concerns the apparent lack of correlation between absorptivities and phosphorescence lifetimes. When Harris, Maria, and McGlynn3 succeeded in correlating the TI So absorptivities with the phosphorescence lifetimes for various heavy metal nitrites, the phosphorescence lifetime of N a N 0 2 was believed to be 3.1 f 1 ms as was reported by Hochstrasser and M a r ~ h e t t i . ~The shorter lifetimes observed for heavy metal nitritesS appeared to be consistent with the larger absorptivities. However, recent measurements carried out in this laboratory reveal that the phosphorescence lifetime of NaNO, is nearly identical with that of AgN02. As was reported previously,6 the phosphorescence lifetime of N a N 0 2 crystal observed at 4.2 K is 100 ps, significantly shorter than the 3.1 ms observed by Hochstrasser and Marchetti. Considering the possibility that impurities might have shortened lifetimes, we have endeavored to purify the crystal. N o change of lifetimes, however, was observed. The phosphorescence lifetime’ of AgNOz crystal, on the other hand, is -500 bs at 77 K. At 4.2 K or below, the spin-lattice relaxation is partially suppressed, and the decay behavior is more complicated; however, the low-temperature data are consistent with the 77 K datum provided the spin-lattice relaxation processes are properly considered. Thus, the phosphorescence lifetime of AgNOz is not shorter than that of N a N 0 2 ; in fact, the former is even slightly longer. It appears therefore that the correlation
discussed by Harris, Maria, and McGlynn3 needs to be reexamined. Since the lifetime of the triplet state is mostly determined by nonradiative decays (as is revealed from the low phosphorescence quantum yield), the above observations indicate that the nonradiative decay rate is not susceptible to the heavy atom effect. This finding is in sharp contrast to the cases of most organic molecules. For all the molecules we have examined previously,8-I1 the nonradiative decays are susceptible to the heavy atom effect in ways quite analogous to the radiative decays. Thus, in order to interpret the color enhancement phenomenon in terms of the heavy atom effect, we should, first of all, understand why only the radiative transitions are susceptible to the heavy atom effect. The second difficulty is concerned with the fact that the lowest triplet state of NaNO, has been identified as 3(nn*). As long as nonbonding electrons are localized on the nitrogen and oxygen atoms of NO2- ion, no heavy atom effect should be expected. It is hard to believe that the delocalization of nonbonding electrons due to the interaction with u electrons could contribute to the large heavy atom effect. In view of these considerations, we are forced to believe that the color enhancement phenomenon of nitrite salts is not yet fully understood and that careful examinations are in order. Theoretical approaches to the color enhancement phenomenon exist. Carsey and McGlynn2 and Harris, Maria, and McGlynn3 made a theoretical analysis, and their computational results do indicate that the TI 6 So absorptivities of AgNO, and AgNa(NO2), are significantly larger than that of N a N 0 2 . Further, the results of their calculations appear to involve information which might answer part of our queries discussed above. For example, in the calculations of Carsey and McGlynn? the dominant source of the enhanced TI So absorptivity for AgNa(N02)2is not due to the mechanism involving the spin-orbit coupling on the metal center: the enhancement is mainly attributed to the existence of a low-lying NO2- Ag+ charge-transfer state and to the large transition moment associated with this state (expression 4 in their terminology). Thus, the governing factor is not the large spin-orbit coupling of Ag’ but the low electron affinityI2 of Ag+. This mechanism is different from the heavy atom effect in the conventional sense. In this way, if the interpretation of these authors is correct, all the odd features discussed above turn out to be understandable. Irrespective of this success, however, we are not fully satisfied with the present status of understanding. First of all, experiments’
-
-
-
+-
-
(1) H. J. Maria, A. Wahlborg, and S. P. McGlynn, J. Chem. Phys., 49, 4925 (1968). (2) T. P.Caney and S. P.McGlynn, J . Am. Chern. SOC.,101,1728 (1979). (3) L. E. Harris, H. J. Maria, and S. P. McGlynn, Czech. J . Phys., B20, 1007 (1970). (4) R.M. Hochstrasser and A. P. Marchetti, J . Chem. Phys., 50, 1727 (1969). (5) H. J. Maria, A. T. Armstrong, and S . P. McGlynn, J. Chem. Phys., 48,4694 (1968). (6) F. Kokai and T. Azumi, J. Phys. Chem., 86, 177 (1982). (7) A. Yamashita and T. Azumi, unpublished work.
0022-3654/84/2088-4622$01.50 IO , , I
(8) S. P.McGlynn, T. Azumi, and M. Kinoshita, ”Molecular Spectroscopy of the Triplet State”, Prentice Hall, Englewood Cliffs, NJ, 1969. (9) H. Saigusa and T. Azumi, J . Chem. Phys., 71, 1408 (1979). (10) H. Saigusa, T. Azumi, M. Sumitani, and K. Yoshihara, J . Chem. Phys., 72, 1713 (1980). (11)S . Yamauchi, H. Saigusa, and T. Azumi, J . Chem. Phys., 74,5335 (1 981). (12)S. P. McGlynn, T. Azumi, and D. Kumar, Chem. Rev., 81, 475 (1981).
0 1984 American Chemical Societv -
Color Phenomenon of Silver Nitrite
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4623
indicate that the sixth-row metal ions such as T1+ and Pb2+ enhance the absorptivity significantly more than the fifth-row metal ions such as Ag+ and Cd2+. It is hard to believe that this trend could be interpreted without considering the spin-orbit coupling. Furthermore, theories developed so far are concerned only with the radiative transitions; the effect of metal ions on nonradiative transitions has not been elucidated either experimentally or theoretically. The above discussion leads us to conclude that the effect of metal counter ions on both radiative and nonradiative transitions of nitrite ion should be carefully examined. For this purpose, analysis focusing attention on the triplet spin sublevels is considered most fruitful. Unfortunately, our efforts to observe the PMDR (phosphorescencemicrowave double resonance) signals for AgNO, crystal were unsuccessful, which is likely due to the incomplete spin polarization. It is, therefore, impossible to analyze the spin sublevel properties of neat A g N 0 2 crystal at this moment. Accordingly, we have chosen, in this paper, a crystal of N a N 0 2 doped with a small amount of AgN0,. As was demonstrated by Clark and Tinti,l3?l4the phosphorescence of this mixed crystal consists of two series of spectra; one series is due to the unperturbed N a N 0 2 , and the other is due to NO2- that is strongly perturbed by AS+. We shall refer to the former as the host phosphorescence and to the latter as the trap phosphorescence. These two series of spectra may consequently be regarded as representing the spectra of NaNO, and AgN0,. The effect of the heavy metal ion can be best examined with this kind of mixed crystal. Of the two types of phosphorescence, the trap phosphorescence and its sublevel properties have been studied by Clark and Tinhowever, the host phosphorescence has not been studied quite in detail so far. In order to compare the radiative and nonradiative rate constants between the host and the trap, effort is devoted, in this paper, to study the sublevel properties of the host. Since the host phosphorescence is extremely weak and further since only one microwave transition could be detected with the instruments available to us, the determination of various kinetic parameters in the conventional MIDP method is fraught with difficulty. However, by utilizing the host-to-trap energy-transfer processes, we have succeeded to determine the rate constants. As will be described in detail below, the radiative rate constant for the Typhosphorescence is enhanced by the Ag+ perturbation by a factor of 800, whereas that for the T, phosphorescence is enhanced only by a factor of 4. The S1 T1 and T1 So nonradiative transitions are essentially unaffected by heavy metal ions. Interpretation of this remarkable feature is attempted. As in a previous paper: the z axis is chosen to be the principal axis of the C2, NO2- species and the x axis to be perpendicular to the plane.
-
-
Experimental Section NaNO, was recrystallized from aqueous solution. AgN0, was prepared from KNO, and AgNO, and was then recrystallized from aqueous solution. Mixed crystals of NaNO, doped with A g N 0 2 were grown from solutions. The fluorescence and the phosphorescence was observed by a 3/4-m Spex 1702 monochromator equipped with an EM1 62568 photomultiplier tube. As the excitation light source, either a 500-W high-pressure mercury lamp, a Molectron UV400 nitrogen laser, or a nitrogen laser pumped DL14P dye laser was used. In the phosphorescence-microwave double-resonance experiments, the microwave from an HP8690B sweeper together with an HP491C amplifier was fed to a helix through a coaxial cable. The MIDP and decay signals were digitized by a Kawasaki TMR- 10 transient digitizer, transferred to a microcomputer, and accumulated. Results ( I ) Phosphorescence Spectra. The phosphorescence spectra of Ag+-doped NaNO, crystals are shown in Figure 1 for three (13) S . E. Clark and D. S . Tinti, Chem. Phys. Leu., 60, 292 (1979). (14) S. E. Clark and D. S . Tinti, Chem. Phys., 51, 17 (1980).
1
a) AgNOz I %
/
I I
b) AgNOZ 0. I %
I
I I
WAVELENGTH/nm
Figure 1. The phosphorescence spectrum of AgN02/NaN02 mixed crystals observed at 4.2 K: (a) I%, (b) 0.176, (c) 0.01%. The ordinate scales are normalized with respect to the trap phosphorescence. F denotes
the fluorescence from the host. 0 Yo
w -20% W
z a
* 9-
0%
-20%
W
t-
z - +20% 0%
2.88 3. 30 FREQUENCY/GHz
Figure 2. The PMDR spectra of NaNOz doped with AgN02: (a) host phosphorescence for the 0.02% mixed crystal at 18 134 cm-', (b) trap phosphorescence for the 0.02% mixed crystal at 17986 cm-I, (c) trap phosphorescence for the 1% mixed crystal at 17986 cm-I.
crystals of different Ag+ concentrations. The spectra are essentially identical with those reported by Calrk and Tinti,', except that slight differences in phonon structures are noted. As Clark and Tinti', demonstrated, the phosphorescence of these mixed crystals consists of two series of bands, each of which has a progression corresponding to the 830-cm-' v2 bending vibration of NO2- species. The series of bands starting at 18 959 cm-' is exactly identical with the series of bands observed for neat NaN0, crystal. Furthermore, the energy of the origin band coincides exactly with the energy of the origin band in the phosphorescence excitation spectrum. The bands are consequently regarded as the phosphorescence emission from the neat N a N 0 2 crystal. This series of bands will be referred to as the host phosphorescence. The other series of bands starting at 18 816 cm-' is observed only for Ag+-doped crystals. The intensity of these bands increases as the concentration of Ag+ increases. The bands are therefore regarded as emission from a trap that is strongly perturbed by Ag+; phosphorescence of this type will be referred to as trap phosphorescence. ( 2 ) PMDR Spectra. The effect of microwave transitions was examined for both the host phosphorescence and the trap phosphorescence. The PMDR spectra observed at 1.2 K under the steady-state excitation condition are shown in Figure 2. The host phosphorescence and the trap phosphorescence are in resonance with the microwaves of 2.88 and 3.30 GHz, respectively. The former transition nearly coincides with the T,-T, transition of neat NaNOz crystal observed by von Schutz and Dietrich,Is and the latter agrees with the results of Clark and Tinti.', It is remarkable that the trap phosphorescence is also sensitive to microwave of 2.88 GHz. This evidence clearly indicates that the energy transfer takes place from the host triplet state to the trap. (15) J. U. von Schiitz and W. Dietrich, Chem. Phys. L e x , 51,418 (1977).
4624
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
TABLE I: Total Decay Rate Constants, k,,Relative Radiative Rate Constants, k;, and Relative Populating Rates, Pi, for the Triplet Sublevels of the Host and the Trap host trap lO-’k,/ s-I 0.081‘ 10-3ky/s-1 33 22.3’ lO-’k,/s-l 1.6 0.23‘ k,’lk,’ 1/22’ k,’lk,’ I/3 70‘ 0.8
Pz/Px
’From ref
6Z
13.
remaining rate constant k,‘ was so small that no quantitative analysis was possible. The rate constants for the host thus determined are compared in Table I with those for the trap determined by Clark and Tinti.
Discussion
10-21
1
I
I
I
I
I
I
I
I
0.5
I
I
I
I. 0
TlME/ms Figure 3. Decays of the trap phosphorescence obtained by direct dye laser excitation (a) at 478.6 nm to the host triplet state or (b) at 482.3 nm to the trap triplet state.
( 3 ) Kinetic Datafor Individual Sublevels. As for the trap, the decay rate constants have been determined by Clark and Tinti, and the results are summarized in Table I. As for the host, the decay rate constants were determined anew in this paper. Since the host phosphorescence is extremely weak, it was impossible to detect the MIDP signals. Therefore, we tried to determine the rate constants by analyzing the phosphorescence decay curve. The decay obtained at 1.2 K was biexponential. Since the T, phosphorescence is forbidden, the two components must be due to the Tyand T,. We assigned the shorter and longer lifetime components to Ty and T,, respectively, in analogy to the host phosphorescence of an NaN0, crystal doped with KNO,. (The MfDP experiments carried out in this laboratory reveal that ky = 3.3 X IO4 s-l and k, = 7 X lo3 s-l for the NaNOz host.) Further, this assignment is supported by the theoretical analysis of the nonradiative decay rate (vide infra). The relative radiative rate constants were estimated from the phosphorescence decays in the following manner. In Figure 3 are shown the decays of the trap phosphorescence obtained by direct excitation by a nitrogen laser pumped dye laser to the host triplet state or to the trap triplet state. Both decays have two components corresponding to the Ty and T, emissions. The initial intensity ratio of the two components is markedly different. When the excitation is carried out directly to the trap triplet state, the initial intensity ratio should be expressed as the square of the radiative rate constant ratio, that is [k,’(trap)/k,’(trap)]*. When the host triplet is excited, on the other hand, the corresponding intensity ratio should be expressed as k,T(trap)k;(host)/(k,’(trap)k;(host)). We here assume that the host-to-trap energy-transfer rates do not depend on sublevels. The analysis of the decays shown in Figure 3 yields the following results:
k;(trap)/k,‘(trap)
k,’(trap)
+ k,‘(trap)
k,’(host)
+ k,‘(host)
(3)
= 200
The contribution of k,’ may well be neglected. By incorporating this expression into eq 1 and 2, we conclude that k,’ increases by a factor of -800 by Ag’ perturbation, whereas k,’ increases by a factor of only 4. (6) T I So Nonradiative Raze Constants. No quantitative analysis of the nonradiative decay rate constants has been made experimentally in this paper. However, in view of the finding6 that the decay of the triplet state is mainly governed by nonradiative paths, we may, to a good approximation, regard the total decay rate constants as the nonradiative decay rate constants. In the light of the data shown in Table I, therefore, we conclude that the nonradiative decay rate constants are essentially not affected by Ag+ perturbation for any sublevels. ( c ) SI T I Intersystem Crossing Rate Constants. Because of complexity due to the host-to-trap energy-transfer processes, comparison of the intersystem crossing rate cannot be made straightforwardly. We have estimated the effect of Ag+ perturbation in the following way. Unfortunately, it was impossible to determine the rate constants in terms of the triplet sublevels, and we are confined to the bulk intersystem crossing. As will be discussed below, however, the bulk intersystem crossing essentially corresponds to the intersystem crossing to the T, sublevel. As is clear from Figure 1, excitation of the host singlet state brings about the fluorescence and phosphorescence of both host and trap. The intensity of the fluorescence Zp and that of the phosphorescence Ipobtained under the steady-state excitation condition are related to the intersystem crossing rate constants kIsc as follows:
-
I0-4
0
-
( I ) Effect of Ag’ Perturbation on the Radiative and Nonradiative Rate Constants. ( a ) T I SoRadiative Rate Constants. How the radiative rate constants for individual sublevels are affected by Ag’ perturbation is estimated in the following way. Clark and Tinti13 reported that the radiative rate constant as a whole increases by a factor of 200 by Ag’ perturbation; that is
c
Y -4
w
Yamashita and Azumi
= 70
k,’(host)/k~(hosi) =
73
(1)
-
where k~ and kp are the radiative rate constants for fluorescence and phosphorescence, respectively, and k is the reciprocal of the phosphorescence lifetime.” The rate constant for the host triplet to trap triplet energy transfer is denoted by kET. We need further to estimate the magnitude of kET. This is estimated from the intensities of the host phosphorescence Ip’(host) and the trap phosphorescence Ip’(trap) brought about by the laser pulse excitation to the host triplet state. (The prime indicates the laser excitation to the host triplet.) The ratio of the two intensities should be expressed as zP’(trap) --kP(trap) --
(2)
The k,’/k,‘ ratio of 70 determined for the trap phosphorescence agrees with Clark and Tinti’s ODMR result;13 this agreement indicates the validity of the method. The k,’/k,’ ratio of determined for the host phosphorescence is qualitatively in agreement with Allen and Dixon’s polarization ratio16of 1/5. The (16) W. C. Allen and R. N. Dixon, Trans.Faraday Soc., 65, 1168 (1969).
Ip’(host)
kBT
(6)
kp(host) k( trap)
The experiments for the 1% AgNO,/NaNO, mixed crystal yielded the following results: (17) In terms of the rate constants associated with sublevels (see Table I),
kp and k are expressed as follows: kp = (k;
+ kyr + k,’)/3
k = (k, + ky
+ k,)/3
Color Phenomenon of Silver Nitrite
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4625
IF(trap)/zF(hOSt) = 1/(5.1 f 0.3)
little, because the perturbation introduced by this high-energy CT state would add little to the originally significant perturbation of the low-energy 'B,(nn*) state. ( b ) T I 4 SoNonradiatiue Transitions. The mechanisms of the T1 So nonradiative transitions are examined in terms of a theory described in detail in a previous paper.ls In essence, the theory, which is originally proposed by Metz,lg considers the spin-orbit and vibronic spin-orbit coupling between the initial triplet state and the final singlet state. The mechanisms are schematically expressed as follows:
Zp(trap)/Zp(host) = 110 f 30
-
Zp'(trap)/Zp'(host) = 13 f 2 k(trap)/k(host) = 0.65 (see Table I) Further, we assume that kF(trap)/kF(hOSt) = 1 and we adopt Clark and Tinti's result13 (eq 3) kp(trap)/kp(host) = 200 By inserting these ratios into eq 4-6, we obtain kIsc(trap)/kIsc(host) = 1.5 f 0.5
Tx: forbidden so
er
T,:
3B1(nn*)-1B,(na*)---1A,(G)
er
In this expression, so and er refer respectively to the spin-orbit and dipole couplings. The T, phosphorescence is forbidden; this is consistent with the finding that the T, sublevel is almost nonemissive. Both the Ty and T, sublevels couple with singlet (m*)states with one-center spin-orbit coupling. A difference exists only in the energy denominators. A 'Bz(m*) state is known to exist relatively close to the 3Bl(n7r*) state, whereas the lowest energy 'Al(?r?r*) state is believed to be located much higher. The finding that k,' is larger than ky' for the unperturbed species is consistent with this picture. We next examine the perturbation of Ag+. In addition to the above mechanisms, we need to further consider mixing with NO2Ag+ charge-transfer (CT) states, which are expected to exist at relatively low energy. We shall examine what sorts of C T states are responsible in what way. The lowest energy C T state would be the (n 5s) state, that is, the state arising from the excitation of the nonbonding orbital of NO2- to the 5s orbital of Ag+. This C T state, however, does not spin-orbit couple with 'Bl(n7*) and needs not be considered. The C T states to be considered are the (n 5p,) and (n 5p ) states. The former state couples with the Tu sublevel of 3B1(nnt() and the latter with the T, by two-center spm-orbit coupling. Of these two CT states, the (n 5pz) state is expected to be of lower energy since the vacant 5p, orbital of Ag+ should be stabilized to a great extent due to the interaction with the p, orbital of N. (If Ag+ is substitutionally incorporated into NaNOz crystal, the Ag-N bond lies along the z axis.) The energy of the (n 5p,) C T state may very well be lower than the higher lying 'Al(mr*) state. The large spin-orbit coupling and small energy denominator associated with this CT state should enhance the radiative decay of the Ty sublevel. The 800-fold enhancement found experimentally is interpreted in this way. The enhancement of k,' due to the (n 5py) C T state should not be nonexistent. However, the enhancement is expected to be
-
-
-
-
-
-
-
~
~
'B, (nn*)-'
SO
A, (G) vib
'B, (nn*)--'B,(nn*)-'A,
(G)
where vib stands for the vibronic coupling. As is elucidated above, the Ty sublevel directly couples with the ground state, 'A,(G), whereas the T, sublevel couples only in the second order. Thus, the nonradiative decay rate constants are expected to be in the order of kynr>> k,"'. This expectation is consistent with the experimental findings for the unperturbed species. (See Table 1.1
We next examine the effect of CT states for the Ag+ perturbed species. The effect of CT states is now greatly different as compared to the case of the radiative transitions. In the case of the nonradiative decay from the Tysublevel, 3Bl(n7r*)directly couples 5p,) C T state with the ground state. Perturbation of the (n is effective only in the second order and should be negligibly small. The effect of the (n 5py) CT state to the T, nonradiative decay is considered little in view of the energy denominators. Thus, none of the CT states are effective toward the nonradiative decays. The observed unsusceptibility of Ag+ perturbation to T, So nonradiative transitions is interpreted in this way. (c) Si T1Intersystem Crossing. In a similar manner as above, the mechanisms of intersystem crossing to individual sublevels are expressed as follows:
-
-+
-
-
(G)
so
T, : T,:
-
'B, (ns*)-'A,(Im*)-'A,
forbidden
SO
We thus see that the effect of Ag+ perturbation to intersystem crossing is only slight. This is in sharp contrast to the 800-fold enhancement found for the kyl radiative rate constant. (2) Mechanism of Ag' Perturbation on the Radiative and Nonradiative Transitions. As is discussed above, the Ag+ perturbation is significant only for the Ty radiative decay rate constant. As compared with this remarkable enhancement, the effect on all the other rate constants is regarded as negligibly slight. We now try to understand this remarkable selectivity. ( a ) T1 So Radiative Transitions. We first analyze the radiative transitions for the unperturbed nitrite ions in a framework of the perturbation treatment in which mixing of the singlet states via spin-orbit coupling plays an essential role. With the schematics we have used repeatedly,'* the mechanisms for the radiative decays from individual sublevels are expressed as follows:
T,:
T,:
Tx:
SO vib ' B, (n~r*)-'A~(nli*)-~B,
(nn*)
vib SO 'B, ( I I ~ * ) - ' A , ( ~ ~ * ) - - - ~ B ,
(na*)
T,: forbidden T, : forbidden
Thus, intersystem crossing is allowed only to the T, sublevel. We next analyze the effect of C T states. The only C T state of Az symmetry that may couple with 3Bl(nn*)is the (n 5d,) state, which is considered to lie at significantly high energy. Thus, the effect of Ag+ perturbation on the S, T,intersystem crossing is expected to be little, in agreement with experimental finding. We have thus succeeded to qualitatively interpret the significant selectivity of Ag+ perturbation. The selectivity in the spin-orbit enhancement is largely due to the simplicity of the NO2- species. The electronic states are relatively far apart, and more importantly there exists only one nontotally symmetric vibration. States that may couple with the sublevels of 3Bl(nn*) by spin-orbit and vibronic spin-orbit coupling are therefore only coarsely spread. This leads to the high selectivity of coupling. It should be noted that we are not the first to have pointed out the role of charge-transfer states. Both Carsey and McGlynn2 and Clark and Tinti13 have already emphasized the importance of charge-transfer states. However, discussion given so far does not necessarily interpret all the known evidence. To be more specific, no discussion has been given so far why only k; is susceptible to Ag+ perturbation whereas the other two radiative rate constants are essentially unaffected. Nor has it been discussed why none of the S1 T1 and TI So nonradiative decay rate constants are susceptible to the perturbation. Furthermore, there has been some uncertainty with regard to the nature of the perturbing charge-transfer states. For example, Clark and Tinti13
-
-
-
~~~
(18) K. Asano, S.Aita, and T. Azurni, J . Phys. Chem., 87,3829 (1983).
(19) F . Metz, Chem. Phys. Left.,22, 186 (1973)
-
4626
J. Phys. Chem. 1984,88, 4626-4631
-
suggested that the (n 5s) charge-transfer state should be responsible. However, charge-transfer states involving the s orbital do not spin-orbit couple with the lowest triplet stateZoand should not act as a perturbing singlet state. These authors also proposed the (4d ?r*) reverse chargetransfer state as another candidate. Carsey and McGlynn also included these reverse charge-transfer states in their computation, and the results seem to indicate that the contribution from the reverse charge-transfer states is rather important. We are, however, of the opinion that the reverse
charge-transfer states leading to Ag2+NOz2-are at quite high energy and therefore do not contribute to the radiative transition from the lowest triplet state. Further, inclusion of the reverse charge-transfer states conflicts with the finding12that the colors of nontransition-metal salts is largely correlated with electron affinity of the metal cation. For these reasons, we believe that the (n 5p,) charge-transfer state proposed above is the mQst reasonable perturbing charge-transfer state.
-
-
Acknowledgment. We thank Professor Takeshi Nakajima and Professor S.P. McGlynn for stimulating discussions and criticism. Registry No. AgN02, 7783-99-5; Ag+, 14701-21-4.
-
(20) In a recent article, the role of the (n 5s) charge transfer-state is denied. K.E. Gotberg and D. S. Tinti, Mol. Phys., 47, 97 (1982).
The Time Dependence of the Low-Temperature Fluorescence of Tryptophan7 Eva F. Gudgin-Templeton and William R. Ware* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587 (Received: November 29, 1983; In Final Form: March 20, 1984)
The time dependence of the fluorescence of tryptophan (Trp) and indole in ethylene glycol/water solution has been investigated in the temperature region 77-300 K. It has previously been oberved that the large Stokes shift in the indole emission disappears at low temperature. Timeresolved spectra of indole as a function of temperature show that while at 77 K and room temperature, there is time dependenceof the fluorescence spectrum; in the temperature region where the fluorescence spectrum is changing, there is a red shift in the emission with time after excitation. Similar results are observed for Trp, except that at room temperature the early time spectrum is blue-shifted with respect to the late time spectrum. This is due to the differing spectral distribution of the two components in the Trp fluorescence decay. The fluorescence decay of indole is single exponential at room temperature and below 250 K, while it is nonexponential in the region of spectral change. The decay of Trp is double exponential at room temperature; in the region 150-250 K the decay is nonexponential, and below 150 K Trp decays with a single lifetime identical with that of indole. These results are discussed in terms of some of the models which have been proposed to explain the double-exponentialdecay of some Trp derivatives, and it is suggested that excited-state complexation with the solvent which lowers the 'Laelectronic state below the ILb state is necessary before double-exponentialdecay kinetics will be observed for Trp derivatives in general.
pendence of the low-temperature fluorescence of other indole derivatives such as Trp, which decay with a double exponential, has not been reported. The timeresolved spectra and fluorescence decays of tryptophan and indole in ethylene glycol/water glass as a function of temperature have been investigated here in an effort to elaborate on the mechanism responsible for the double-exponential decay kinetics of tryptophan,
Introduction The low-temperature emission properties of tryptophan (Trp) are of great interest in attempting to understand the reason for its double-exponential decay kinetics in solution at room temperature. The emission spectra, lifetimes, and fluorescence quantum yields of indole and tryptophan in polar solvents are extremely temperature sensitive. However, while spectra and quantum yields as a function of temperature over a wide range have been reported previo~sly,'-~ it is only recently that the time dependence of the low-temperature fluorescence of indole derivatives has been investigated. Three recent reports have appeared which describe investigations of this aspect. Lakowicz and Balter6 have measured the time-resolved fluorescence spectra of N-acetyltryptophanamide (NATA) in propylene glycol from 205 to 3 13 K. Lami' has measured the fluorescence lifetime of indole and 2,3-dimethylindole in glycerol down to 213 K. Meech et a1.* have observed the time-resolved fluorescence spectra and fluorescence decay of 1,3-dimethylindole in 1-butanol from 85 to 280 K. In all of these cases, the fluorescence spectrum was observed to blue shift as the temperature was lowered. In the temperature region where the maximum of the steady-state spectrum was shifting, a red shift of the emission maximum with time after excitation was observed with the use of time-resolved spectroscopy. They concluded that the fluorescence in fluid polar solvent at high temperature arises not from a pure 'La state but from a state with significant intramolecular charge-transfer character. All of the indole derivatives used in the above studies give single-exponential decay at room temperature. The time de+ Publication
Experimental Section Materials. L-Tryptophan (99%, Aldrich) was recrystallized three times from 1-propanol. Indole (99+%, Aldrich) was recrystallized once from ethanol. Ethylene glycol (EG, Fisher reagent) was purified by adding activated charcoal and distilling under vacuum at 5 mmHg. Ethylene glycol/water (EGW) solutions were 1:l (v/v) ethylene glycol to triply distilled water. Solutions were generally lC3M in the fluorophore for front-face measurement of fluorescence and la" M otherwise. All solutions were air saturated and were prepared immediately before use. (1) J. Eisinger and G. Navon, J. Chem. Phys., 50, 2069 (1969). (2) L.C. Pereira, I. C. Ferreira, M. P. F. Thomaz, and M. I. B. M. Jorgc, J. Photochem., 9,425 (1978). (3) C. Conti and L.S . Fmter, Biochem. Eiophys. Res. Commun., 57,1287 (1974). (4) W.C. Galley and R. M.Purkey, Proc. Natl, Acad. Sci. U.S.A., 67, 1116 (1970). (5) S. Suzuki, T. Fujii, A. Imai, and H. Akahori, J. Phys. Chem., 81,1592 (1977). (6) J. R. Lakowicz and A. Balter, Photochem. Photobiol., 36, 125 (1982). (7) H. Lami, 11, Nuovo Cimento SOC.Ita/. Fis. E, 63B, 241 (1981). (8) S. R. Meech, D.Philips, and A. G. Lee, Chem. Phys. Lett., 92,523 (1982).
No. 326.
0022-3654/84/2088-4626%01.50/0 , , I
0 1984 American Chemical Societv -
