J . Phys. Chem. 1985, 89, 3512-3521
3512
4 * = ($M - $2)X + $2
(21) where X = [l - (x*/x2)/(z2/z*)], x2 and x * are the distance from the electrode surface to the oHp and to the reaction plane, respectively, and z2 and Z * are the average dielectric constants of the entire inner layer and a part of the inner layer between the metal surface and the reaction plane, respectively. It follows that $* could be independent of the inner layer thickness when X is independent of x2. Specific conditions at which A may be independent of x2 depend on the profile of the dielectric constant inside the inner layer. It is usually assumed that z is either uniform inside the entire region of the inner layer and discontinuously attains its high bulk value at the OHp or varies smoothly inside the inner layer from a small value near the electrode surface to the bulk value near oHp. The two profiles of the dielectric constant are shown in Figure 9. If e is homogeneous in the entire region of the inner layer z2 = Z * and hence X = 1 - x , / x z , so that X~ must vary with x2 for X to be constant. The electron is supposed to be transferred to the ion in the adsorbed state. If this step controls the rate of the overall reaction x* should be independent of x2. Hence, when x * is variable the reaction must be controlled by a slow ion transfer from oHp to the adsorbed site. The slope of the Tafel plot should then correspond to aaPp r. z(x2 - x ~ ) / x ~For . ~discharge ~ of Cd2+ = 0.25 the quotient ( x 2 - xt)/x2 would be with z = 2+ and aaPp (59) From eq 16 and 21 a, = ZX - zXat#J2/at#JM. However, at the surface of an electrode covered by a fifm of an organic surfactant and in the presence of a large concentration of a supporting electrolyte at#J2/at#JM is small and the second term of this expression could be neglected.
equal to 0.13. That means the reaction plane would be very close to the oHp. When e varies smoothly with the distance Z * decreases as x2 increases, Figure 9. Therefore the product (x2z*) may be either constant or weakly dependent on x2. For constant x * and z2, X should be constant or be a weak function of the inner layer thickness. In that case the electrode reaction could be either controlled by the ion or by the electron transfer. However, for = 0.25) X must the present experimental data ( z = 2+ and aaPp be close to unity. This means that $* must be close to 42 (see eq 24). This in turn implies nonlinear potential drop inside the inner layer with most of the potential drop between the metal and the oHp taking place in the immediate vicinity of the electrode surface. In summary, $* and hence aapp might be independent of the inner layer thickness when (a) the potential drop inside the inner layer is linear, x* is very close to x2, and the reaction is controlled by the ion transfer as the rate determining step or (b) the potential drop inside the inner layer is nonlinear, x 1 is located deep in the inner layer, but $* is not too different from 42;the reaction could be either controlled by electron transfer or by ion transfer.
Acknowledgment. The authors are indebted to Dr. W. R. Fawcett for helpful discussions and useful suggestions and to the Department of Chemistry and Biochemistry, University of Guelph for assisting with the publication costs. RegistrY NO. BD,110-64-5; P, 76-09-5; HD, 629-1 1-8; HT, 106-69-4; OD, 629-41-4; Cd, 7440-43-9; Hg, 7439-97-6; cadmium amalgam, 39451-77-9.
Vibrational Predissociation and Nonradiatlve Process of Electronically Excited 'van der Waals Complexes of Pyrimidine Haruo Abe, Yoshimi Ohyanagi, Minoru Ichijo, Naohiko Mikami, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: August 20, 1984; In Final Form: February 13, 1985)
The fluorescence excitation spectra of van der Waals complexes of pyrimidine with argon and nitrogen have been observed for the electronic transition lBl(nx*)-lAl of pyrimidine. Remarkable intensity changes were found for the vibronic bands of complexed pyrimidine. The dispersed fluorescence spectra obtained by excitation of the vibronic bands of the complex showed that vibrational predissociation is responsible for the large intensity changes in the excitation spectrum. For the Oo band of the complex, from which vibrational predissociation does not occur, a considerable shortening of fluorescence lifetime by complexation was found, with an estimated tenfold decrease of fluorescence quantum yield. The dissociation mechanism, the van der Waals bond energy, and the change of nonradiative electronic retaxation process of pyrimidine by the complexation are discussed.
I. Introduction Since the studies on van der Waals complexes of iodine with rare gases by Levy's group,' it has been recognized that supersonic jet laser spectroscopy is a powerful tool for the study of van der Waals complexes. In the supersonic jet, not only the van der Waals complex is effectively prepared but also the collision-freeconditions are ideal for the study of the intramolecular processes of the isolated van der Waals complexes. Particularly, vibrational predissociation of the complex is a mmt interesting intramolecular process which has recently attracted much attention as one of the typical unimolecular chemical reactions. However, the experimental investigations are rather few for van der Waals complexes involving a large polyatomic molecule; to our knowledge only van der Waals complexes of glyoxalZ and tetrazine3 have been sub(1) Blazy, J. A.; Dekoven, B. M.; Russell, T. D.; Levy, D. H. J . Chem. Phys. 1980, 72, 2439 and references therein.
jected to observations of v-v processes after vibrational predissociations. In this paper, we report the electronic spectra (fluorescence excitation and dispersed fluorescence spectra) of pyrimidineargon and -N2 complexes in supersonic jets, and discuss the v-v processes after vibrational predissociation. As a typical intermediate case molecule, it is well-known that pyrimidine shows dual fluorescence decay components? which has been a main subject of many theoretical and experimental studies on nonradiative electronic r e l a x a t i ~ n . ~Therefore, .~ it is also of particular interest to see (2) Halberstadt, H.; Soep, B. J . J . Chem. Phys. 1984,80, 2340 and references therein. (3) Brumbaugh, D. V.;Kenny, J. E.; Levy, D. H. J . Chem..Phys. 1983, 78, 3415. (4) Spears, K. G.; El-Manguch, M. Chem. Phys. 1977, 24, 6 5 . ( 5 ) Uchida, K.; Yamazaki, I.; Baba, H. Chem. Phys. Left. 1976, 38, 133. (6) Lahmani, F.; Tramer, A,; Tric, C. J. Chem. Phys. 1974, 60, 4431.
0022-3654/85/2089-3512$01.50/00 1985 American Chemical Society
van der Waals Complexes of Pyrimidine how the change of environment by the complex formation affects the nonradiative process of pyrimidine. Usually, vibrational predissociation following intramolecular vibrational relaxation (IVR) is detected by dispersed fluorescence spectra. Therefore, as noted by Brumbaugh et al.,3 observed branching ratios for these processes depends .on the fluorescence lifetime of the parent molecule in the van der Waals complex. In the small molecule limit for molecules like 121 or glyoxal2 with long lifetimes, the fluorescence spectrum is only from fluorescent fragments produced by complete vibrational predissociation. However, because of its short lifetime due to photodissociation of tetrazine itself, for the tetrazine-Ar complex, single vibronic level (SVL) fluorescence of the complex is the largest contributor to the fluorescence spectrum? From this point of view, pyrimidine as a parent molecule is expected to be intermediate between the two limiting cases. It is well-known that the slow decay component of dual fluorescence of pyrimidine is effectively quenched by molecular According to the theories of the nonradiative electronic relaxation in the intermediate case: the quenching is explained by a mechanism in which after intersystem crossing from the initially prepared singlet level into the mixed S-T levels, the collision-induced rotational vibrational relaxation within the triplet state occurs resulting in pure triplet levels and prevents the recurrence into the singlet state leading to the quenching of the slow This quenching mechanism is known as so-called collision-induced intersystem crossing. In the case of the isolated van der Waals complex, vibrational predissociation is a consequence of vibrational relaxation in the complex. If the vibrational relaxation in the singlet state is slower than the intersystem crossing, vibrational predissociation resulting from the vibrational relaxation in the triplet state occurs, leading to the quenching of fluorescence as observed with molecular collisions. If it is faster, vibrational predissociation occurs in the singlet state and there is no fluorescence quenching.'" Thus we have two measures for the rate of vibrational predissociation which are the two distinct lifetimes of dual fluorescence of pyrimidine as a parent molecule in van der Waals complex. In the fluorescence excitation spectra of the van der Waals complexes of pyrimidine with argon and nitrogen molecule due to the electronic transition lBl(nr*) 'Al of pyrimidine, we found remarkable intensity changes in the vibronic bands of pyrimidine by complexation. From the results of the dispersed fluorescence spectra obtained by excitation of the vibronic bands of the complex, it was found that the large intensity change is related to vibrational predissociation of the complex. The v-v processes after the vibrational p r e d i i a t i o n and the dissociation energy of the complex will be discussed from the observed results. Vibrational predissociation does not occur from the Oo level of the complex. We observed the fluorescence decay from the Oo level of pyrimidineAr complex and found a disappearance of the long fluorescence lifetime component of pyrimidine. It was also found that the fluorescence quantum yield of the Oo band of pyrimidine greatly decreases by complexation. The large decrease closely correlates with the disappearance of the slow fluorescence. From these observations, we discuss the v-v relaxation processes in vibrational predissociation, van der Waals bond energy, and the influence of the complex formation on the nonradiative electronic relaxation process of pyrimidine.
-
11. Experimental Section The pulsed supersonic jet apparatus used in the present study is described elsewhere." The second harmonic of a dye laser (7) Knight, A. E. W.; Parmenter, C. S.Chem. Phys. 1976, 15, 85. (8) Yamazaki, I.; Fujita, M.; Baba, H. Chem. Phys. 1981, 57, 431. (9) Knight, A. E. W.; Jones, J. T.; Parmenter, C. S.J. Phys. Chem. 1983, 87, 973. (10) (a) Gelbart, W. M.; Freed, K. F. Chem. Phys. Lett. 1973, 18, 470. (b) Freed, K. F. Chem. Phys. Len.1976,37,47. (c) Freed,K.F. Adu. Chem. Phys. 1981,47,291. (d) Tramer, A.; Nitzan, A. Adu. Chem. Phys. 1981,47, 337. (1 1) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M. Chem. Phys. Left. 1980, 74, 531.
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3513 (Molectron DL-14) pumped by a pulsed nitrogen laser (Molectron UV-22 or UV-24) was used as the exciting light. The laser wavelength covered was from 325 to 309 nm with fwhm of 1 cm-'. The fluorescence excitation spectra were obtained by scanning the wavelength of the second harmonic whose output was kept constant by controlling the SHG crystal angle with the incident laser beam. Total fluorescence was detected with a HTV R562 photomultiplier. The measured peak positions were determined with an accuracy of f 3 cm-', while for the relative positions the reproducibilities were within the range of f l cm-l. For the dispersed fluorescence spectra, the laser wavelength was tuned to a specific vibronic band of the excitation spectrum and the fluorescence was dispersed by a 0.75-m, f/6 Nalumi grating monochromator in the second order. In most cases, the spectral resolution was less than 16 cm-' for pyrimidine-do and 22 cm-l for pyrimidine-d4. The dispersed light was detected by a HTV R928 photomultiplier and the photocurrent signal was amplified by a preamplifier (PAR 113). During the long-time recording, the total fluorescence intensity was monitored simultaneously by the same system as that used for the excitation spectra and kept constant by controlling the exciting light intensity. The spectra were not corrected for the wavelength dependence of the detection system. For both fluorescence excitation and dispersed fluorescence spectra, the pulsed signals were averaged and converted to dc voltage by a boxcar integrator (Brookdeal 9415/9425) and recorded on a chart recorder. The fluorescence decays from several bands of pyrimidine-do and its Ar complex were observed by a programmable digitizer (SONY/Tektronix 390 AD). In one decay curve measurement, the signals from 100 laser shots were accumulated and averaged. Time resolution of the instrumental response was 100 ns. Pyrimidinedo (Tokyo Kasei, G C grade) was used without further purification. Pyrimidine-d4 was kindly presented by Prof. R. Shimada of Kyushu University. N o signal due to pyrimidine-do impurity could be observed in the fluorescence excitation spectrum. Pyrimidine vapor (21 "C) diluted with 3 atm helium or 1.5 atm argon (Nippon Sanso, >99.9995%) or -80 torr of N 2 plus 3 atm helium was expanded into a vacuum chamber through a puised nozzle having an orifice of diameter 400 pm. The background torr. The pressure in the expansion chamber was 1 X fluorescence excitation and the dispersed fluorescence spectra were observed by exciting the jet typically 25 nozzle diameters downstream. By changing the laser-jet crosspoint (18-60 nozzle diameters), we examined the collisional effect on the spectra and found no appreciable effect except that the total fluorescence intensity varied. Therefore, with our experimental conditions, collisional effects12 can be safely neglected.
-
-
111. Results III. 1. Fluorescence Excitation Spectra. Many spectroscopic investigations have been done on the 'Bl(nr*)-'Al transition of pyrimidine-do and -d4 vapors. Reliable assignments for the main vibronic bands have been given from a study of the SVL fluorescence spectra of pyrimidine-do vapor by Knight et al.13 Figure 1 shows the fluorescence excitation spectrum of the Oo region of pyrimidinedo in a supersonic jet. With helium carrier gas (a), no band is seen in the energy region lower than the Oo band of free pyrimidine. On the other hand, the spectrum of pyrimidine seeded in 1.5 atm argon (b) contains a band shifted by -41 cm-I from the Oo band of free pyrimidine. This band is not a hot band because its intensity is comparable to that of the 16bl hot band whose frequency is the lowest in the electronic ground state.I4 We assign the band as the Oo band of pyrimidine-Ar van der Waals complex. This assignment is supported by the analysis of the rotational structure of the Oo band which will be published e1~ewhere.I~ (12) Bemstein, E. R.; Law, K.; Schauer, M. J. Chem. Phys. 1984,80,634. (13) Knight, A. E. W.; Lawburgh, C. M.; Parmenter, C. S. J . Chem. Phys. 1975, 63,4336. (14) Sbrana, G.; Adembri, G.; Califano, S.Spectrochim. Acta 1966, 22, 1831.
3514 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
Abe et al. TABLE I: Fluorescence Excitation Spectra of Pyrimidine, Pyrimidine-Ar, and Pyrimidine-N,' Yobsd!
cm-'
31600
31100
30918 31 075 31 097 31 182 31 299 31 313 31 360 31 402 31 439 31 523 31 537 31 551 31 595 31 689 31 701 31 744 31 761 31 785 31 801 31 890 31 936 31 965 31 971 32 006 32017 32 043 32066 32 088 32 128 32 143 32 172 32 185 32 196 32218 32237 32253 32 266 32 277 32 287 32 296
31200 cm-1
Figure 1. Fluorescence excitation spectra of the Oo region of pyrimidine seeded in (a) 3 atm of helium, (b) 1.5 atm of argon, and (c) 80 torr of N2 plus 3 atm of helium in supersonic jets.
Figure I C shows the fluorescence excitation spectrum of pyrimidine-do seeded in the mixture of -80 torr of N 2 plus 3 atm of helium. Addition of a small amount of nitrogen gas to the H e carrier gas produces a new band shifted by -83 cm-I from the Oo band of free pyrimidine. This band can be assigned to Oo of pyrimidine-N2 complex. The same band is also seen in Figure 1b, and it probably arises from nitrogen gas included in the carrier gas as impurity. At first we assigned the band to Oo of pyrimidine-Ar2 complex because the magnitude of red shift is approximately twice as large as that of Ar complex following the so-called "additivity However, intended addition of nitrogen gas to the camer gas clearly revealed that the band should be assigned to Oo of pyrimidine-N2 van der Waals complex. The fluorescence excitation spectra of pyrimidinedo seeded in argon and in nitrogen plus helium over more extended spectral regions are shown in Figures 2 and 3. By comparison with the spectrum of free pyrimidine in helium-seed jet, the vibronic bands due to the complex are readily identified as indicated in the figures for a few vibronic bands. The spectral data are listed in Table I. Many weak fundamental and combination bands of pyrimidinedowhich are hidden by the hot bands in the room temperature absorption spectrum'* are found in a supersonic jet, but their assignments are not known at present. Innes et al. have given the assignments for many hot bands in the vapor absorption spectrum.Is If we refer to their results, no hot bands except those including 16al and 16bl are observed in the supersonic jet. To our knowledge, only seven fundamentals in the 'B1 state of pyrimidine-do have been established. It is of particular interest to note that the 16bh band appears a t 31 439 cm-I (0 364 cm-') with an appreciable intensity. The SVL fluorescence spectrum produced by exciting this band shows that 364 cm-' is the frequency of a nontotally symmetric vibration which corresponds to the 342-cm-' fundamental in the electronic ground state, Le., 16b mode,13*14 which belongs to bl symmetry in the Mulliken's notation. Therefore the band (0 364 cm-I) is due to a vibronically induced transition to A, (=B, X b,) vibronic state from the ground state, whose transition moment is parallel to the z axis in the molecular plane. More detailed discussion on this subject from the results of rotational structure analysis will be given e1~ewhere.l~
+
+
(15) Mikami, N.; Sukahara, Y.; Abe, H.; Ito, M., to be published. (16) Kenny, J. E.; Johnson, K. E.; Sharfin, W.; Levy,D. H . J. Chem. Phys. 1980, 72, 1109. (17) Amirav, A.; Even, U.; Jortner, J. J. Chem. Phys. 1981, 75, 2489. (18) Innes, K. K.; McSwiney, H. D.; Simmons, J. D.; Tilford, S. G. J. Mol. Spectrosc. 1969, 31, 76.
a
-
re1
cm-'
intens
yobsd
vm,
-157 0 22 107 224 238 285 327 364 448 462 476 520 614 626 669 686 710 726 815 861 890 896 93 1 942 968 99 1 1013 1053 1068 1097 1110 1121 1143 1162 1178 1191 1202 1212 1221
1 100 3
assign.
red shifts, cm-' Ar N2
16af 41
83
16a1
21
59
6ah
41
83
6b1
42
80
00
16b; 16ah 16bh
1 6ah16ai 4 1 53 2 47
40 2 4
40
76
11
1;
33
89
30
12;
40
79
5 9 4
9ah
41 38 39
lh16ah 1 1
Pyrimidine.
As seen in Table I, the band shifts induced by complexation for the most vibronic bands are nearly equal to those for the electronic origin. This indicates that the fundamental frequencies in the 'B, state do not change substantial1 by complexation. However, one exception is seen for the 16ao band which shows the decrease in the band shift by half for the Ar complex and by a factor of 0.7 for the N2 complex. Brumbau h et al. have observed similar decrease in the band shifts for 16a0, 6ahl sa:, and 16bt of the s-tetrazine-argon complex and noticed that these vibrations are the out-of-plane modese3 It is expected that the argon atom (or nitrogen molecule) on the molecular ptane more strongly perturbs the out-of-plane modes than the in-plane modes. This would be also true for pyrimidine-Ar and -N2 complexes. More noticeable features in Figures 2 and 3 are the relative intensities among the vibronic bands due to the complexes. For example, the intensity ratio of the 6b; band to the Oo band of free pyrimidine, 162/12,is estimated to be 0.47, while those of complexes, 16c/IOc, are 9.4 and 43.4 for Ar and N,, respectively. Clearly, these large intensity changes cannot be attributed to the change of the absorption intensities by complexation. As shown later, this phenomenon can be explained by the fact that the quantum yield of pyrimidine Oo band greatly decreases by complexation, while vibrational predissociation from the higher vibronic levels of the complex increases the apparent fluorescence quantum yields as a result of the fragmentation. The corresponding but more tentative data for the excitation spectrum of pyrimidine-d, seeded in argon are listed in Table 11. As expected, the band shifts induced by complexation are nearly
Y
f
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3515
van der Waals Complexes of Pyrimidine
f Iuor escc nce excitation spectrum 41
I
41
-I
O0
Figure 2. Fluorescence excitation spectrum of argon-seeded pyrimidine in a supersonic jet. Lines connecting the bands represent the red shifts of the vibronic bands induced by complexation with argon. (a)
310
311
312
313
ab 315 316 wavenumber I ldcm-'
317
3ie
I
319
Figure 3. Fluorescence excitation spectrum of nitrogen-helium-seeded pyrimidine in a supersonic jet. Lines connecting the bands represent the red shifts of the vibronic bands induced by complexation with nitrogen.
30000
31000 wavenumber /cm-l
equal to those of pyrimidine-do complex. This is additional evidence that the newly observed bands in the argon jet are not hot bands or impurities. 111.2. Dispersed Fluorescence Spectra. Complete vibrational assignments for the fundamentals of pyrimidine-do and -d4in the electronic ground state are a~ailab1e.I~ The dispersed fluorescence spectra from the intense vibronic bands in the absorption spectrum of pyrimidinedo vapor have been analyzed in detail by Knight et The dispersed fluorescence spectra in the supersonic jets obtained by exciting free pyrimidine bands are essentially identical with those reported by Knight et al. Therefore, the assignments for the vibrational bands of free pyrimidine are straightforward. We have measured the fluorescence spectra by excitation of a few vibronic bands of pyrimidine-Ar and pyrimidine-N,. To obtain sufficient S / N ratio, only relatively strong bands in the excitation spectra were excited: Oo, sa;, 6b& and 12; for pyrimidine-do-Ar, and 6a; and 6bi for pyrimidine-do-N2. The fluorescence spectra after excitations of Oo, 6ah, 6bi, and 12; of pyrimidine-d,-Ar have also been observed. 111.2.1, 00 Excitation. The dispersed fluorescence spectra after Oo excitation of the pyrimidine-Ar complex and uncomplexed pyrimidine are shown in Figure 4, parts a and b, respectively. The spectral data are listed in Table I11 with the assignment^.'^ Except for the red shifts of the bands due to complexation, both spectra are almost identical. Since the total fluorescence intensity from Oo level of the complex is very weak under the experimental condition where its ratio of the complex to the free pyrimidine was 0.0025, the spectrum b is very noisy, resulting in the missing correspondences for some weak bands in Table 111. Nevertheless, it is sufficient to recognize that the fluorescence originates only from Oo level of the undissociated complex.
Figure 4. Dispersed fluorescence spectra after excitation of the Oo bands of (a) pyrimidine-Ar complex and (b) free pyrimidine. TABLE II: Fluorescence Excitation Spectrum of Pyrimidine-d, and Pyrimidine-d,-Ar VOM, vobsd re1 Ar red cm-l voo, cm-l intens assign shifts, cm-l ~~
31 053 31 145 31 176 31 183 31 194 31 205 31 732 31 787 31 810 31 836 31 851 31 902 32 005 32 028 32 045 32077 32 097 32110 32 121 32 196 32 229
-141 -49 -18 -1 1 0 11 538 593 616 642 657 708 81 1 834 851 883 903 916 927 1002 1035
16al 2 100 1
43
00
16bt
27 12 17 4
6ah
6b:
33 37 41
1
9aA
40 41 31
5 17
37 29 42 39
11
12;
+
111.2.2. 6aA Excitation. The band 6ah (0 614 cm-l) has been known to be a partner of the Fermi doublet with 6bi band.I3 The dispersed fluorescence spectrum after 6ah excitation of pyrimidineAr is shown in Figure 5b. We found that after 6ah excitation
3516 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
Abe et al.
TABLE III: Fluorescence Spectra of Pyrimidine-dnand Pyrimidine-dn-Ar after Oo Excitation ~
pyrimidine' wow,'
'
cm-l
we, - vow.
31 076
0
30 398 30391 30 094 30 086 30 026 30 004
678 685 982
29 937
29 929 29 837 29 723
29713 29 701 29 672
cm-'
assign.
4Cl
100 48
57
wow:
Oo excitation
1
6ay
pyrimidine-Arb ueX - wow: cm-I
cm-l
31 035
0
683
uoJ
~~~
- uOw: cm-' 41 46-39
-
16b:
30 352
-
30 047
989
39
10by16b:
29 983
29962 29 897 29 889
1052 1073 1138 1146
43 42
29 682
1353
29 674
1361
41 -39
29 632
1403
40
3
990
19
11
1050 1072 1139
41 24
39
1147
I
12:
9a0 1 1'16b:
18 7
1239 1353 1363
6bd
::> 12 14
1375
1404
6a;
- 6ay16b: - 16b:
19a:
40 40
"Spectral resolution R = 3 cm-I. bR = 16 cm-l. CFromref 12. TABLE IV: Fluorescence Spectrum after 6af,Excitation of Pyrimidine-Ar
uobsb>
cm31079 30926 30397 30246 30080 30028 30010 29 947 29936 29874 29 85 3 29787 29781 29722 29682 29566 29524
- yqbsd,
cm 571 724 1253 1404 1570
t 1 1776 1797 I
1622 1640 1703 1714
1863t 1869 1928 1968 2084 2126
emission from Oo O0
6ay 1: 12;
- 16b0,
assignment (uncomplexed pyrimidine) calcd displacement, cmemissi6n from 16a' 573 16a: 1255
- 10by16by -
- 16a: 16b:
16a:l; 16ai12:
- 16ailOby16by
730 1412
1712
gay, 11716by
6a; 6a!16b,0 19ay
16a:6ay 1563 1623 1645
calcd displacement, cm-'
1720
- 16b:
8ay
of pyrimidine-Ar the fluorescence originates from two levels: 16a' and of free pyrimidine resulting from vibrational predissociation. This can be readily confirmed by comparison of Figure 5b with the single vibronic level (SVL)fluorescence spectra following 16ai and Oo excitation of free pyrimidine shown in Figure 5 , parts a and d, respectively. The spectral data are listed in Table IV. Although small frequency deviations for Oo and 16af from the calculated ones can be found, they cannot be regarded as significant because the instrumental resolution was 16 cm-I. Such small deviations were very significant in the case of stetrazine-Ar studied by Brumbaugh et al.3 They observed the dispersed fluorescence spectrum after 16ai excitation of tetrazine-Ar and found that the spectrum consists of three types of fluorescence. Besides fluorescence from the initially excited level and from the Oo level of uncomplexed tetrazine arising from vibrational predissociation, they observed a 6a" progression built on the band a t 18041 cm-' whose deviation from 16at of uncomplexed tetrazine is only -6 cm-I. The shift of 16a' level induced by complexation with argon is also expected to be --I cm-I. Therefore, they considered this deviation to be an evidence that the progression is due to the emission from the 16a' level of tetrazine-Ar resulting from intramolecular vibrational relaxation. In the present case of pyrimidine-Ar, the situation is more evident because of the larger spectral shift induced by complexation. Perhaps the most apparent evidence which excludes the possibility of the emission from the levels resulting from the intramolecular vibrational relaxation will be obtained from the measurements of the fluorescence decay, which are shown later. We sought the sequence band as the candidate for the 30926-cm-' band other than 16ai of free pyrimidine but in vain. The estimate of the branching ratio after excitation and the absence of resonance
1720 1780 1802 1869 1877
16a:9ay, 16a:11:16by 1936 1977 2141
-
16a:6a,0 16ai6ay61b; 16a:19ay
- 16ai16b:
2093 2134 (a)
(b)
1 c
29000
30000
31000
wavenumber I cm-1
Figure 5. Dispersed fluorescence spectra (a) after 16a; excitation of free pyrimidine, (b) after 6ab excitation of pyrimidine-Ar complex, (c) after 6ba excitation of pyrimidine-Ar complex, and (d) after Oo excitation of free pyrimidine. Excitation positions are indicated by arrows. Broken lines indicate that the spectra b and c consist of the emissions from the Oo and 16aI levels of pyrimidine fragment produced after vibrational
predissociation. fluorescence and of intramolecular vibrational relaxation will be discussed later with the results of the other fluorescence spectra. 111.2.3. 6bg Excitation. The 6bi band (0 + 669 cm-l) is the other member of the Fermi doublet. The dispersed fluorescence spectrum after 6bg excitation of pyrimidine-Ar is, however, somewhat different from that of 6aA excitation. The spectrum is shown in Figure 5c. Again no resonance fluorescence could
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3517
van der Waals Complexes of Pyrimidine
29000
31000
30000
wavenumber /cm-l
Figure 7. Dispersed fluorescence spectra after Oo excitations of (a) pyI
'
30600
,
.
,
,
31600
,
,
.
32600
rimidine-d,-Ar complex and (b) free pyrimidine-do. Excitation positions are indicated by arrows.
wavenumber / cm-1
Figure 6. Dispersed fluorescence spectra (a) after 6aA excitation of free pyrimidine, (b) after 12; excitation of pyrimidine-Ar complex, and (c)
(a)
I
after 12; excitation of free pyrimidine. Excitation positions are indicated by arrows. Broken lines indicate that the spectrum b consists of two emissions; resonance fluorescence from 12' level of pyrimidine-Ar complex and fluorescence from 6a' level of pyrimidine fragment produced after vibrational predissociation. be detected and almost all emission can be attributed to fluorescence from 16a1level of pyrimidine fragment. Fluorescence from Oo level of pyrimidine fragment is only trace. It is clear that the small difference (55 cm-l) in the vibrational energy between 6a1 and 6bZis important in establishing the branching ratio of 16a' to Oo of pyrimidine fragment produced after vibrational predissociation. 111.2.4. 12; Excitation. The fluorescence spectrum after 12; (0 1013 cm-l) excitation of pyrimidine-Ar is shown in Figure 6b. The spectral data are listed in Table V. By comparison of part b of Figure 6 with parts a and c, it is easily seen that the main fluorescent levels are 12l of the complex and 6a' of pyrimidine fragment. The most noteworthy result is the appearance of resonance fluorescence, which was not present after 6a; or 6bi excitation. This indicates that resonance fluorescence from 12l of the complex competes with vibrational predissociation, while fluorescence from 6al and 6b2 does not. If the rate of vibrational predisscciation is independent of the exvibrational energy over the binding energy of van der Waals bond, the appearance of resonance fluorescence would closely correlate with the dependence of fluorescence lifetime of the parent molecule upon the excess vibrational energy of the initially prepared level. More detailed discussion on the subject will be given later. Only one prominent pathway is found for vibrational predissociation after 12; excitation; the 6a1 level of pyrimidine fragment is produced. It is of interest to consider why the 6b2 level of pyrimidine fragment is not produced since 6b2 is a counterpart of the Fermi doublet with 6a1 and is located at only 54 cm-' above 6a1. This fact indicates that some critical energy selection rather than the vibrational mode selection operates to determine the branching ratio of the levels resulting from the vibrational predissociation. 111.2.5. Pyrimidine-d4-Ar Complex. The fundamental frequencies of the ring vibrational modes 6a, 6b, and 12 of pyrimidined4 are not much different from those of pyrimidine-do. The binding energy of the van der Waals bond should be equal between -do and -d4 complexes. Therefore we can expect that the -d4 complex would give results parallel to the -docomplex. The fluorescence spectra after 0" excitation of pyrimidine-d,-Ar (a) and pyrimidine-d4 (b) are shown in Figure 7. Although no SVL fluorescence spectrum has been reported before for pyrimidine-d4, it is easy to assign the vibronic bands from the known fundamental frequencies in the ground electronic state.I4 The spectral data are listed in Table VI with the assignments. Similar to the case of -docomplex, the spectrum after 0" excitation of -d4 complex can be interpreted as resonance fluorescence from Oo of the undissociated complex. It is of particular interest to see the fluorescence spectrum after 6bi excitation of pyrimidine-d4-Ar complex since the 6b2 level (0 + 642 cm-l) of pyrimidine-d4 is located midway in the excess
+
/
1
,
l
1
1
1
,
30000
1
1
1
1
31000 WAVENl!MBER/cm-l
Figure 8. Dispersed fluorescence spectra (a) after 16al excitation of pyrimidine-d4and (b) after 6b$ excitation of pyrimidine-d4-Ar complex. Excitation positions are indicated by arrows.
I
(
"
~
30000
l
~
'
31000 wavenumber / cm-1
Figure 9. Dispersed fluorescence spectra (a) after 6ah excitation of pyrimidine-N, complex and (b) after Oo excitation of free pyrimidine. Excitation positions are indicated by arrows.
energy between 6b2 (669 cm-l) and 6a1 (614 cm-') levels of pyrimidine-do while the 161a'(232 cm-') level has almost same excess energy as -do(238 cm-'). Figure 8 shows the fluorescence spectra after 6bi excitation of pyrimidine-d4-Ar (a) and 16a; excitation of pyrimidine-d4 (b). These two spectra can be considered to be identical except for the band at the exciting position which is due to the stray light of the laser. Fluorescence from Oo of pyrimidine-d4 fragment could not be detected. The pyrimidine-d4 molecule resulting from the vibrational predisscciation from 6b2 of Ar complex finds only one pathway to 16aI level of pyrimidine-d4, which is almost the same result as that for pyrimidinedo complex. The excitation of 12; also gives the same result as that of -do complex. In general, nearly identical excess energy gives similar branching ratios after vibrational predisscciation for both -d4and -docomplexes. 111.2.6. Pyrimidine-N2 Complex. Since the vibronic bands due to pyrimidine-N2 complex are weak, only the 6a; and 6bi bands could be excited. For bothcases the fluorescence spectra give identical results. The fluorescence spectrum after 6a; excitation of pyrimidineN2 is shown in Figure 9a. All fluorescence
1
3518
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985
Abe et al.
TABLE V: Fluorescence Spectrum after 12; Excitation of Pyrimidine-Ar calcd uob_sb - uqbsd* displacement, cm cmemission from 12'' cm0 356
32 048 31 692 31 372 31 360 31 021 31 014 30898 30765 30690 30672 30 642 30625 30551 30444 30 399 30316 30288 30233 30219 a Of
688 676 1027 1031 1150 1283 1358 1376 1423 1406 1497 1604 1649 1732 1760 1815 1829
pyrimidine-Ar.
0
12;
I
12i6ay
- 12;16b,0
0 659 832 845 955 975 1048 1106 1206 1316 1488 1538 1592 1613 1632 1706 1762 1878 1977 2000 2022 2094
100 106 1 1 1 8 149 10 4 73 2 15 5 2 11 182 11 4 33 4 9 111
359
6a' 6 18 685 6a:
12A9a:
1139
12;6a,0
1356
I
- 6aA16b:
I
12 :6a:9a:
-1037 1349
6a;l: 6aA12; 6ai9a: 6a;6b,0
-
6a;lOby16b?
6ai16a,0
1409 1431 1498 1598 -1722 1763
.
1817
- 6ay16b:16a,0
-1833
Of uncomplexed pyrimidines.
TABLE VI: Fluorescence Spectra of Pyrimidine-d, and Pyrimidm-d,-Ar after od Excitation
31 193 30.534 30361 30 348 30 238 30218 30 145 30 087 29 987 29 877 29 705 29 655 29 601 29 580 29 561 29 487 29 43 1 29315 29216 29 193 29 171 29 099
emission from 6 r 1
calcd displacement, cm- I
00
6ay
'I
31 151 30488
0 663
42 46
30091
1060
54
29 826
1325
51
29612
1539
43
29505 29433
1646 1718
56 58
1987
52
51 9a0 12.i lob: 6bi 6a8 6adly 8a, 6ai9ai
6a 12,
6a110b; d 1327 6a; 29164 9a012y 12* b
is found to originate from Oo of pyrimidine fragment (Figure 9b) resulting from vibrational predissociation. This indicates that the vibrational excess energy of 614 cm-' for 6a' or 672 cm-' for 6bZ of pyrimidine-Nz is large enough to break the pyrimidine-N, van der Waals bond. In contrast to the case of the Ar complex, no fluorescence from 16a' level of pyrimidine fragment could be detected after 6aA and 6ba excitations of the pyrimidine-Nz complex. This indicates that the dissociation energy of Nz complex is greater than that of Ar complex and that after vibrational predissociation the 16a' level of pyrimidine fragment is not produced because of the lack of the residual internal energy. 111.3. Fluorescence Decay. In order to investigate the correspondence between the observed small fluorescence yields from 0" levels of the complexes and the well-known collisional quenching of the long lifetime fluorescence, we have monitored the fluorescence decays after excitation of Oo levels of pyrimidine and pyrimidine-Ar in a supersonic jet. We found that the fluorescence decay from Oo of free pyrimidine shows only the slow component, which is consistent with the results from the more detailed observation around the band origin with the higher time resolution by Saigusa and Lim.Ig Making the assumption that the decay
curve is a single exponential, the decay rate constant is O.SO(f 0.03) X lo6 s-' which is somewhat smaller than the result (0.84 X lo6 s-I) obtained from the zero pressure extrapolation of the decay observed by Spears and E l - M a n g u ~ h .Since ~ the apparent decay rate of slow component is very sensitive to the vapor pressure, Le., collision efficiency, the above result confirms that collisional effects could be safely neglected under our experimental conditions. In striking contrast to the case of free pyrimidine, we found that the fluorescence from Oo of pyrimidine-Ar exhibits only a fast decay component. Although the response time resolution of our devices is not sufficient to trace the fast decay, it is clear that the slow component is absent in the fluorescence. The disappearance of the slow decay component is reflected in the decrease of fluorescence quantum yield of Oo band by complexation. As shown in the excitation spectrum (Figure 2), the relative intensity of the Oo band of Ar complex is abnormally small. This tendency is exaggerated for the N2complex as shown in Figure 3. Although we could not observe the decay of the fluorescence from the Oo level of the N 2 complex because of its low intensity, the above results should also hold true for the N 2 complex. If we take into account the resemblance to the collisional quenching of the slow decay component, the disappearance of the slow component from Oo of the complexes should be understood as a result of a similar mechanism to the collision-induced intersystem crossing. Comparison of the quenching mechanism of the Oo fluorescence of the complexes with that due to molecular collisions will be discussed later. Unlike the case of the Oo band of pyrimidine-Ar, the dispersed fluorescence spectra show that vibrational predissociation within the singlet state takes place following excitation of all higher vibronic levels studied. We observed the fluorescence decay after 6ah excitation of pyrimidine-Ar. Single-exponential analysis gives the decay rate constant as 0.58 (f0.03)X IO6 s-I which is slightly larger than that for the Oo level of free pyrimidine. This is consistent with the fact that emission after 6ah excitation of pyrimidine-Ar is mainly due to fluorescence from 16al (0 238 cm-') of free pyrimidine. Note that vibrational predissociation from the higher SI levels does not quench the fluorescence. On the contrary, it enhances the apparent quantum yield of fluorescence because the resultant vibronic levels of the free pyrimidine fragment have larger fluorescence quantum yields than the initially pumped level. This phenomenon is expected to be more prominent for the Nz complex than the Ar complex because of larger energy loss accompanied by the vibrational predissociation. This is
+
(19) Saigusa, H.; Lim, E.C . J . Chem. Phys. 1983, 78, 91.
van der Waals Complexes of Pyrimidine
The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 3519
TABLE VII: Estimation of Upper Limit of Dissociation Energy for Pyrimidine-Ar and -N2Complexes in the First Excited Singlet States
upper limit, cm-' 376 430
excitation do-Arl 6a' 6bi
400
129
37 1 412 410
d4-ArI 6a 6bi
614 672
do-N, 6al
500
16a,
120
6b;
observation (pyrimidine fragment) 16al 16aI 6aI
16aI 16al
6aI 00
OO. 00
"
Indirect observation; see text. consistent with the larger relative intensity, 16bC/ZOCas described in previous section.
IV. Discussion ZV.1. van der Waals Bond Energy. Since the occurrence of vibrational predissociation indicates that the vibrational energy at the initially prepared level exceeds the van der Waals bond energy, it is possible to estimate the upper limit of dissociation energy. The estimated values are listed in Table VII. For the Ar complex the values for pyrimidine-d4 are also included because its binding energy is expected to be equal to that of -do complex. Brumbaugh et al. have observed the hot band 16ai for tetrazine-Ar which allows them to evaluate the lower limit of the binding energy.3 Unfortunately we could not observe a hot band in the complex under our experimental condition. Therefore the lower limit could not be obtained. Our best value for the upper limit is 371 cm-I, which is comparable with that of tetrazine-Ar (381 cm-l). Although we did not observe dispersed fluorescence spectrum after the excitation of 16ai of N2complex because of its weakness, it is instructive that in the excitation spectrum (Figure 3), the relative intensity ratio of 16ai band of N2 complex to that of free pyrimidine, I16:/I16af, is 0.049 which is nearly equal to the ratio for 6aA (0.0485) or for 6b; (0.0499). This suggests that the fluorescent level after vibrational predissociation from 16aZlevel of N 2 complex would be the Oo level of pyrimidine fragment as in the case after 6aA and 6bi excitations. This indirect observation gives our best value for the upper limit to be 500 cm-'. If we can expect the same predissociation mechanism for N 2 complex as Ar complex, the fact that the 16a' level of the pyrimidine fragment is not produced after vibrational predissociation from 16a2, the 6a1 and 6b2 levels of N 2 complex may give the lower limit value for the dissociation energy. Thus we can obtain the range for the dissociation energy of N 2 complex as 434 cm-' C Do'< 500 cm-I. IV.2. Intramolecular Vibrational Relaxation. For pyrimidine-Ar, the dispersed fluorescence spectra following excitation of all vibronic levels studied show no spectral feature due to intramolecular vibrational relaxation (IVR), whereas for tetrazine-Ar, IVR is prominent in dispersed fluorescence spectrae3 This does not imply that IVR is absent in pyrimidine-Ar. As noted by Freed and Nitzan,20 IVR must occur when the excess energy at initially prepared level flows into the van der Waals bond stretch leading to vibrational predissociation. A possible reason for the absence of IVR in the spectra is that fluorescence from the levels resulting from IVR is a small fraction of total fluorescence because JVR is a fast process in comparison to the fluorescence lifetime of pyrimidine. In the case of tetrazine-Ar, the lifetime of the excited electronic state is limited by fast photodissociation (C1 ns) whose rate is comparable with that of IVR. Therefore, not only IVR but also resonance fluorescence of the complex are seen in dispersed fluorescence spectra. As pointed out by Brumbaugh et a long excited-state lifetime of the parent molecule in the complex generally causes the observed quantum yield of vibrational predissociation to be unity as in the cases of iodine' and glyoxaL2 (20) Freed,K. F.;Nitzan, A. J. J . Chem. Phys. 1980, 73, 4765.
TABLE VI11 Branching Ratios for Vibrational hedissociations after 69: and 6bi Excitations of Pyrimidine-Ar
fluorescence quantum yield 16al
0.057
I,.? 79
00
0.16656
21
branching ratioC
I..,"
branching ratiod
92
96
99
8
4
1
"Relative intensity (estimated from 16a16ay and 6ay bands) in the excitation. d6bi excifluorescence spectra. *Taken from ref 17. tation. It is of particular interest to note that in pyrimidine-Ar, resonance fluorescence could be detected from 12' but not from 6a' and 6b2. As mentioned earlier, just after excitation the fast decay component of fluorescence is the only competitor against IVR and vibrational predissociation. With increasing excess vibrational energy, the fluorescence lifetime of pyrimidine decreases to nearly the statistical limit case. However, the ratio of the quantum yield of the fast decay component to the total fluorescence quantum yield, @f/@f+s and decay rate of the fast component, K+,increase rapidly as increasing the excess energy. According to the data presented by Uchida et al.2' for 6a' (613 cm-'), @f/@f+s = 0.14 and K+ = 4.8 X lo8 s-I while for 12' (1012 cm-'), af/Qf+, = 0.24 and K+ = 6.3 X lo8 s-'. Considering the weakness of resonance fluorescence from 12' (see Figure 6b), the above difference between 6a' and 12' is critical for minimal detection of resonance fluorescence. From these considerations it is reasonable that fluorescence from a widespread population of the states resulting from IVR would be beyond our detection limit. IV.3. Branching Ratio after Vibrational Predissociation. Vibrational predissociations from 6a' and 6b2 of pyrimidine-Ar yield only the levels 16a' and Oo in free pyrimidine. To obtain their relative populations, the relative intensities of fluorescence from these levels corrected for the Franck-Condon factors and for fluorescence quantum yields must be known. The fluorescence spectrum after 16a' excitation of pyrimidine vapor has been observed by Knight et and is identical in spectral features with the fluorescence spectrum after Oo excitation (see also Figure 5 ) . This is reasonable because 16ai is a sequence transition of the Oo parent transition. Therefore correction for Franck-Condon factors is unnecessary and the relative fluorescence intensities from 16a' and Oo can be calculated from the relative intensities of the corresponding bands, for example, 16ai6a: and 6a7, in the fluorescence spectrum. The fluorescence quantum yield for 16al has not been reported yet. As a typical intermediate case molecule, the pyrimidine fluorescence quantum yield decreases monotonically as the excess vibrational energy of the pumped level increases. Since the fluorescence quantum yields for the bands which form main progressions in the absorption spectrum are known,21interpolation from the fluorescence quantum yields vs. excess energy allows us to estimate the quantum yield at any excess energy. Thus we obtained the fluorescence quantum yield of 16a' to be 0.057. The estimated branching ratios are listed in Table VIII. The schematic representation is shown in Figure 10. Although from these data alone it is not sufficient to discuss a propensity for the pathways after the vibrational predissociation, it appears that the energy gap law rather than some mode selectivity is dominant to determine the v-v process. The lower vibrational level closest in energy gains the largest population, following simply the prediction of the energy (momentum) gap law.22 Among many possible pathways, the route which gives the least relative translational momentum of the fragments has the largest dissociation rate. From this point of view, we do not recognize the special role of the 16a vibrational mode for the dissociation as found in the tetrazine-Ar ~ o m p l e x . ~ In Figure 10 we drew the levels of Ar complex by assuming its dissociation energy Do'to be -365 cm-'. If Do'C 348 cm-' the energy gap law predicts that 6b2 level of pyrimidine fragment (21) Uchida, K.; Yamazaki, I.; Baba, H. Chem. Phys. 1978, 35, 91. ( 2 2 ) Ewing, G. E. J . Chem. Phys. 1979, 71, 3143.
Abe et al.
3520 The Journal of Physical Chemistry, Vol. 89, No. 16, 1985 N7complex Ar complex
free Wrimidine
TABLE u[: &timates of the Upper Limits of the Fluorescence Quantum Yields of Oo Bands of Pyrimidine-Ar and -N2Complexes ioc/iJa 16bc/161a concn [c]/[fl @oc/@2c 5.0 X 2.5 X 0.10 Ar complex 0.25 X N2complex 0.54 X lo-' 48.5 X lo-' 8.2 X lo-' 0.066
rcc2 IVU
16a3
6b116b' 6b2 6a>lGa'16b'
12'
1
'Observed. CRelativequantum yield. 16a2
16b'
P - 400 - 300 - 200 - 100 -0
SVL
Flgure 10. Schematic representation of branching ratios after vibrational predissociation and upper limit of van der Waals bond energy. Possible mechanism for the disappearance of slow fluorescence from Oo level of pyrimidine-Ar complex is also shown, which is discussed in IV-4.
should have the largest population after the predissociation from 12l of the Ar complex which conflicts with the observation. Therefore, it may be reasonable that the lower limit value of DO' is 348 cm-I, Le., 348 cm-' < DO'< 371 cm-I. ZV.4. Fluorescence Quenching by Complex Formation. The most prominent effect of the complex formation on the fluorescence excitation spectra is the increase of apparent fluorescence quantum yields due to vibrational predissociation because emission occurs from levels with less vibrational energy. On the contrary, the band of the complex, where vibrational predissociation does not occur in the singlet state, has very small fluorescence quantum yield resulting from the quenching of slow fluorescence decay component. Now we can estimate the quantum yields from the results given so far. The intensities of the vibronic bands in the fluorescence excitation spectrum are given by where the superscript i (=c or f) denotes the complex or free pyrimidine. [i] is the concentration of the i species; m is the vibronic notation; A is the absorption cross section; @ is the fluorescence quantum yield; L is the laser intensity. With constant laser intensity and the assumption that the absorption cross section does not change by complex formation, i.e., AmC= Amf,then eq 1 gives
-z6bc =-162
iC1 @6bC [fl @ 6 2
(3)
for the relative intensities of the Oo and 6bi bands. The observed values for eq 2 and 3 are given in Table IX. 00'and @6bf are already knownz1 to be 0.1665 and 0.028, respectively. The ob= 0.057 served branching ratios give the results that 06,' = for the Ar complex and @6bc = @po' = 0.1665 for the N, complex.
Inserting these quantities into eq 2 and 3, we obtain the relative concentrations under our experimental condition and fluorescence quantum yields for Oo bands as given in Table IX. In the above calculation, we make a tacit assumption that the quantum yield of vibrational predissociation after 6bi excitation is unity. This is not realistic because the other nonradiative processes might be possible. Therefore the calculated values for the relative fluorescence quantum yields are the upper limit values. The large decrease of the fluorescence quantum yields due to complexation should be closely correlated with the disappearance of slow decay as shown in previous section. Phenomenological resemblance between the fluorescence quenching due to collisions and due to complexation is instructive. In the vapor phase it is well-known that the addition of foreign gas such as cyclohexane induces the decrease in the fluorescence yield of the 'B1state of pyrimidine and the increase in the triplet The molecular collisions compete only with the slow decay component of fluorescence, and the decrease in the fluorescence yield is mainly due to the quenching of the slow fluorescence resulting from the rotational relaxation induced by collisions in the triplet state9 and therefore molecule reaches pure triplet states from mixed states. In the present case the experiment shows that from the 0' level of the complex, vibrational predissociation does not occur at least in SIbut fluorescence yield is greatly decreased. Two interpretations seem to be possible. (i) The complex as a whole is a statistical limit molecule and therefore slow decay fluorescence, which is characteristic of intermediate case molecule, does not appear. It should be important to note that the methyl substitution to pyrimidine causes the decrease of the slow decay fluorescence due to the increase of the effective density of triplet states coupled with the initially prepared singlet level.21 The complex formation would give the same effect on the nature of emission of pyrimidine as the methyl substitution. (ii) After intersystem crossing vibrational predissociation accompanied by vibrational relaxation occurs in the triplet state and consequently the slow fluorescence of the pyrimidine fragment is quenched. Evidence for vibrational predissociation in the triplet state has recently been given for the aniline-Ar complex from the measurements of the triplet lifetimes by Knee and Johnson.z3 This mechanism is an analogy of collision-induced intersystem c r o ~ s i n g ?In ~ this ~ ~ ~regard, ~ ~ vibrational predissociation is phenomenologically similar to molecular collisions and can be called a half-collision. When the complex is excited into the vibronic states studied in present work, the situation is quite different; vibrational predissociation does occur within the singlet state. The resultant vibronic levels of pyrimidine fragment are more fluorescent than the initially prepared level. Consequently we have large apparent intensities for the fluorescence excitation spectra. This phenomenon would be greatly indebted to the fact that vibrational predissociation is a competitive process with the fast fluorescence decay. This is remarkably different from vapor-phase collision^^^^ which quench only slow fluorescence. Actually at the limit of high pressure, even in solution, fluorescence of pyrimidine could be detected25with its lifetime of 1.5 ns,26which is comparable with that of the fast decay c o m p ~ n e n t .Therefore, ~ fluorescence should not be quenched completely even when the comparable fast nonradiative process within the singlet state, namely vibrational predissociation channel, is opened by complexation. It is worthwhile to note that the resonance fluorescence can be detected from 12l level of Ar complex. This means that the rate constant (23) (24) (25) (26)
Knee, J. L.; Johnson, P. M. J . Chem. Phys. 1984, 80, 13. Jouvet, C.; Soep, B. J . Chem. Phys. 1981, 75, 1661. Cohen, B. J.; Baba, H.; Goodman, L. J. Chem. Phys. 1965,43,2902. Kellog, R., referred in ref 22.
J. Phys. Chem. 1985,89, 3521-3526 of the vibrational predissociation is comparable with that of the fast decay component, that is, in the order of lo9 s-I. On the other hand, Knight et al. have obtained the rate constants of the collisional quenching of the pyrimidine fluorescence with rare gas partner, which is in the range of lo7 torr-' s - ' . ~ Therefore, one can make a condition in which the collisional quenching competes with the fast fluorescence decay if rare gas is added up to 100 torr. In such conditions, Knight et al. observed the collision-induced vibrational relaxation from the 12' level of pyrimidine. However, at the lower pressures, they could not detect the vibrational relaxation because the collision-induced electronic relaxation is dominant, resulting in the fluorescence quenching. Therefore, only when the rate of the collision is comparable with that of the fast decay can the vibrational relaxation be induced. From these considerations, it becomes clear that only competitive
-
3521
processes with the fast decay can result in vibrationa1 relaxation within the singlet state. The agreement between the rate,constant of the vibrational predissociation of the pyrimidine-Ar complex and that of the collision at the onset of the collision-induced vibrational relaxation is not accidental. This implies that the intrinsic intersystem crossing of pyrimidine which causes the fast fluorescence decay is not perturbed by collisionsg and even by complexation.
Acknowledgment. We thank Professor R. Shimada of Kyushu University for providing us with pyrimidine-d4. We also thank Drs.T. Ebata and N. Gonohe for their stimulating discussions. This work was partially supported by Toray Science and Technology Grants. Registry No. Ar, 7440-37-1; N2, 7727-37-9; pyrimidine, 289-95-2.
Fluorescent Probes for Silica and Reversed-Phase Silica Surfaces: 1,3-Di-I-pyrenylpropane and Pyrene David Avnir,*t Reinhard Busse,* Michael Ottolenghi,*x Edna Wellner,tt and Klaas A. Zachariasse*s Departments of Organic and Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel, and Max- Planck- Institut ftir biophysikalische Chemie, Abt. Spektroskopie, 0-3400 Gdttingen, Federal Republic of Germany (Received: January 2, 1985)
The fluorescence of the bifunctional probe molecule 1,3-di- 1-pyrenylpropane(Py(3)Py), adsorbed on silica (in the presence and absence of coadsorbed l-octanol) and on octadecylsilica, is investigated by utilizing photostationaryand single-photonaunting methods. It is concluded that dynamic (intramolecular) excimer formation, taking place after excitation, o&urs at low probe concentrations in the reversed octadecyl phase. A study of temperature effects yields a value of -40 kJ/mol for the effective activation energy of this process, indicating a relatively high viscosity for the reversed phase. Dynamic intramolecular excimers are also observed on silica surfaces in the presence of 1-octanol as coadsorbate. No excited-state rearrangements are observed for Py(3)Py on untreated (or partially C18silylated) silica, where the only path to excimers originates in ground-state intermolecular aggregates. Comparative experiments are also carried out with pyrene (Py) in the same systems. Py(3)Py appears to be a most convenient indicator for the degree of mobility freedom of adsorbed molecules.
Introduction The photochemistry and photophysics of adsorbed molecules have recently gained considerable attention.'-" This relatively novel field appears to be rapidly developing as a new tool, leading to a deeper insight into the nature of selected photoprocesses, as well as to better understanding of solid interfaces. The extensive use of pyrene (Py) in such studies is due to its unusually long fluorescence lifetime, to its ability to form excimers, and to the sensitivity of its structured fluorescence spectrum to the polarity of the environment? The wide variety of solid surfaces which have been studied with Py as a probe includes ~ i l i c areversed-phase ,~ silica! a l ~ m i n a ,titania ~ and other semiconductors,* calcium f l ~ o r i d eclays,l0 ,~ and zeoIites." In the present study we have applied the bifunctional pyrene derivative, 1,3-di-1-pyrenylpropane (Py(3)Py), as a probe for silica surfaces. The major advantage of this molecule is associated with its ability to form excimers intramolecularly. Consequently, exceedingly low concentrations may be used, avoiding experimental difficulties due to ground-state aggregation' which highly complicate the excimer formation mechanism in the case of Py. Especially relevant, in this respect, were previous studies in which the intramolecular excimer formation process in Py(3)Py was successfully applied to determine the local viscosity of heterogeneous systems such as micelles, biomembranes, and microemulsions.'* In his pioneering surface photochemistry studies, Leermakers detected the intramolecular excimer emission of 1,3-diphenylDepartment of Organic
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0022-3654/85/2089-3521$01.50/00 1985 American Chemical Society
