4214
J . Phys. Chem. 1984,88, 4214-4218
Absolute Fluorescence Quantum Yields of Large Molecules in Supersonic Expansions Mark Sonnenschein, Aviv Amirav,* and Joshua Jortner* Department of Chemistry, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel (Received: July 18, 1983; In Final Form: February 28, 1984)
In this paper we report on the determination of absolute quantum yields from the electronic origin, Sl(0),of the first electronically excited singlet state of isolated large molecules cooled in pulsed, planar, supersonic expansions. We have measured the absorption spectra, the fluorescence excitation spectra, and the fluorescence quantum yields from the photoselected Sl(0) states for pairs of large molecules seeded in planar supersonic expansions by using a pulsed xenon lamp and a monochromator. The relative quantum yields thus obtained were calibrated by using a reference molecule to give absolute quantum yields, Y,from the vibrationless SIstates. Yvalues from the Sl(0)state of 13 aromatic molecules and their derivatives were determined. Information is inferred on the mechanism of intersystem crossing mediated by a nearby triplet state and on solvent effects on this process.
Introduction The elucidation of excited-state intramolecular dynamics, e.g., electronic relaxation and unimolecular photochemical rearrangement in collision-free large molecules, requires the determination of fluorescence quantum yields from photoselected states.' Photoselective excitation of well-specified rotationalvibrational electronic states of isolated large molecules can be accomplished in seeded supersonic expansions.2 Relative fluorescence quantum yields from different groups of rotational states in the electronic-vibrational origin of the first excited singlet state of aniline in planar jets were reported by Amirav et aL3 Relative fluorescence quantum yields from photoselected vibrational states of jet-cooled large molecules were explored by Zwier, Carrasquillo, and Levy4 for trans-stilbene and by Amirav and Jortners for anthracene, 9,1O-dichloroanthracene,and transstilbene. In this paper we report on the determination of absolute fluorescence quantum yields from the electronic origin, SI(0), of the first electronically excited singlet state of isolated large molecules cooled in pulsed, planar supersonic expansion^.^ Relative fluorescence quantum yields from the Sl(0)states for pairs of large molecules in planar jets were determined from the simultaneous measurements of the fluorescence excitation spectra and of the absorption spectra. The relative quantum yields thus obtained were calibrated by using a reference molecule to give the absolute quantum yields, Y, from the vibrationless SI states. Y values from the S,(O)state of the following 13 aromatic molecules and their derivatives were measured: aniline, anthracene, anthracene-&, 9-methylanthracene, 9-cyanoanthracene, 9-bromoanthracene, 9,1O-dibromoanthracene, 9,10-dichloroanthracene,tetracene, perylene, fluorene, trans-stilbene, and 4-chloro-trans-stilbene. From the experimental point of view, our approach, which rests on the use of a lamp and a monochromator setup for the simultaneous measurement of the absorbance and fluorescence from narrow (1-2 cm-') rotationally broadened vibronic spectral features, is free from slit correction factors. From the point of view of general methodology, our method makes low quantum yields amenable to experimental determination, allowing of Y = for the interrogation of fast intramolecular relaxation processes in the picosecond time domain. Experimental Section The details of our technique are published el~ewhere.~ Briefly, we use a pulsed nozzle slit, whose dimensions are 0.27 mm in width and 90 mm in length. The repetition rate of the jet produced was 9 Hz with a temporal pulse width of 300 ps. The vacuum chamber (1) For a recent survey see J. Jortner, R. D. Levine, and S.A. Rice, Eds., Ado. Chem. Phys., 47, 1-114 (1982). (2) D. H. Levy, Annu. Rev. Phys. Chem., 31, 197 (1980). (3) A. Amirav, U.Even, and J. Jortner, Chem. Phys. Lett., 83, 1 (1981). (4) T. Zwier, E. Carrasquillo, and D. H. Levy, J. Chem. Phys., 78, 5493
(1983). ( 5 ) (a) A. Amirav and J. Jortner, Chem. Phys. Lett., 94, 547 (1983); (b) ibid., 95, 295 (1983).
0022-3654/84/2088-4214$01.50/0
was pumped by a 4-in. Varian (VHS-4) diffusion pump backed by two Edwards rotary pumps (200 and 350 L/min). Pressures torr were achieved a t stagnation pressures between of -3 X 60 and 120 torr of Ar. Under these conditions, effective rotational-vibrational cooling of large molecules is ac~omplished.~J Light from an EG&G FX193U short arc Xe flashlamp was focused and resolved by using a 0.3-m McPherson 218 monochromator equipped with a grating of 2400 lines/mm. The spectral resolution was 0.4 8,(for 30-pm slits). The outgoing light pulses were then focused parallel to the supersonic expansion at a distance of 10 mm from the nozzle slit. Temporal coincidence of the light pulse with the gas pulse was achieved by using a homemade time delay unit triggered by the pulsed nozzle. The fluorescence light intensity I , was imaged onto a Hammamatsu R296 or IP28 photomultiplier perpendicular to the expansion plane. The light beam was split by a mirror and monitored by two vacuum photodiodes (Hammamatsu R645 or R727) both before and after crossing the planar jet. The absorption AZ/Io was normalized to the incident light intensity, Io. The electronic origin was identified as the most intense pressure-independent and lowest energy spectral feature in the electronic spectrum. The relative quantum yield was determined from the ratio IF/AI a t the electronic origin. The absorption and the fluorescence signals were measured simultaneously at a fixed wavelength. The total measurement time was 100 s, while the time constant of the measuring system was 10 s (90 pulses); each measurement was repeated several times. The base line was taken when the gas pulse from the nozzle was out of temporal coincidence with the light pulse, while the signal was recorded when temporal matching of the gas and light pulse occurred. Relative quantum yields for pairs of molecules were determined in tertiary mixtures, seeding two sample molecules in the supersonic beam of argon. Care was taken to ensure that the vibrational manifold of the molecule, whose S,(O) is located a t a lower energy, did not inadvertently lend significant intensity to the transition of the molecule, whose Sl(0) is located a t a higher energy. An additional precaution to avoid spurious signals was taken by placing a cold trap opposite the nozzle to prevent absorption by hot scattered molecules. The spectral response of the photodiodes and of the photomultiplier at the two wavelengths, corresponding to the location of the Sl(0) for the two molecules, was corrected by using the manufacturer's characteristic sensitivity curves. These correction factors were usually of the order of 10% for molecules with widely separated Sl(0) energies. The relative fluorescence quantum yields for S,(O) were obtained from the ratio ( I F / A I ) I / ( I ~ / A Iwhere ) ~ , the subscripts 1 and 2 label the two molecules. Cross correlation between pairs of quantum yields were f5%. The high-purity molecules used in our experiments were obtained from commercial sources and used without further purification. Anthracene, fluorene, and perylene were obtained from Fluka; 9-bromoanthracene, 9,lOdibromoanthracene, 9-methylanthracene, 9-cyanoanthracene, anthracene-dIo,stilbene, and tetracene were obtained from Aldrich; 9,lO-dichloroanthracene was obtained from ICN. Two molecules 0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4215
Absolute Fluorescence Quantum Yields
5'
-.
DB A
4
ANTRACENE-DIO
0
0
'
(DCA)
D
D
D
D
H
H
H
:I$$$::
CI
+
D
;$I$$;
CI
Br
H
(DBA) Br
H
H
ANTHRACENE
-IQ I
DBP
1
7
hz38645i P=072
H 10 0-0 k=3610.5A
D 10 0-0 A=3601.6;
Qi0.67
9=0.13
W 0
P=lOO
z
W 0 v)
W
11:
0 3
J L
-J LL
I
3626 3610 3875
3855 3845 W A V E L E N G H (%)
3865
-
Figure 1. Absorption spectrum (upper curve) and fluorescence excitation spectrum (lower curve) in the region of 3845-3875 A for the 0-0 So SI transitions of a binary mixture of 9,lO-dichloroanthraceneand 9,lOdibromoanthracene cooled in a planar supersonic expansion of Ar. The nozzle temperature was 160 OC and the stagnation pressure of the Ar was 90 torr. The insert on the right-hand side of the figure shows the raw data for the simultaneous measurement of fluorescence (dashed line) and absorption (solid line) taken at the S,(O)excitation energies for the molecules. The measurement time, which corresponds to the horizontal scale, is 100 s for each measurement. The rise (fall) of the absorbance and fluorescence signals occurs when temporal synchronization between the pulsed lamp and the pulsed jet is switched on (off). In this measurement the absorption and the fluorescence relative gain was adjusted to give about an equal signal in 0-0 So SI transition of 9,lO-dibromoanthracene and was not changed while tuning to the 0-0 So SI transition of 9,lO-dichloroanthracene. The relative fluorescence quantum yields are denoted by Q. The peak of the absorption signal of 9,lO-dibromoanthracene corresponds to A I / I o = 0.1. The fluorescence excitation spectrum is displayed in arbitrary units.
-
-
I
3600" 3590
WAVELENGTH ( A 1 Figure 2. Absorption spectrum (upper curve) and the fluorescence excitation spectrum (lower curve) in the region of 3595-3620 A for the 0 So SItransitions of a binary mixture of anthracene-hIoand anthracene-dlo cooled in a planar supersonic expansion of Ar. The nozzle temperature was 130 OC and the Ar stagnation pressure was 100 torr. The insert near the right-hand side of the figure shows the raw data for the simultaneous measurements of fluorescence (dashed line) and absorption (solid line) taken at the respective S,(O) excitation wavelengths of the two molecules. The peak of the absorption signal of anthraceneh,,, is N/I0= 0.1, while the fluorescence excitation spectrum is displayed in arbitrary units.
-
W
u z w
0 VI W
LT
a t a time were placed in the sample chamber. The nozzle was heated to a temperature corresponding to a b u t 0.1-1.0-torr partial pressure of the sample molecules. The stagnation pressure of the Ar carrier gas was 60-120 torr. Under these conditions, efficient vibrational cooling was achieved and the contributions of hot and sequence bands are small. The Ar stagnation pressure and the vapor pressure of the aromatic molecule (M) are sufficiently low so that the contribution of M-Ar, van der Waals molecules and M, aromatic clusters to the spectrum is minor.
0 3 1 LL
0 I
L
a
z 0
Results We have measured simultaneously the absorption spectra and the fluorescence excitation spectra of pairs of molecules seeded in pulsed planar supersonic jets of Ar. Typical examples for the electronic origins of 9,1O-dichloroanthracene/9,1O-dibromoanthracene, anthracenelanthracene-dlo, and 9-cyanoanthracene/9,1O-dichloroanthraceneare portrayed in Figures 1-3. The width of each spectral feature is 4 cm-' (fwhm), which originates from instrumental resolution, so that no information on the rotational envelope of the vibrational So SI transitions can be inferred. Quantum yields for emission from electronic origins were obtained by a simultaneous measurement of absorbance and fluorescence at fixed wavelengths. Typical raw data are presented in the inserts of Figures 1-3. In this way, relative quantum yields from the Sl(0)of 13 molecules were measured. Typical relative quantum yield data are displayed in Table I. The numbers in parentheses in Table I represent the numerical correction factors for the differences in photomultiplier and photodiode response,
-
!
kLL
0 v,
m
a
v
31 IO
3835 WAVELENGTH
(i)
3810
-
Figure 3. Absorption spectrum (upper curve) and the fluorescence excitation (lower curve) in the region of 3810-3865 A for the 0-0 So SI transitions of a binary mixture of 9,lO-dichloroanthracene and 9cyanoanthracene cooled in a planar supersonic expansion of Ar. The nozzle temperature was 160 "C and the Ar stagnation pressure was 100 torr. The insert near the top of the figure shows the raw data for the simultaneous measurement of fluorescence (dashed line) and absorption (solid line) at the Sl(0) excitation wavelengths of the two molecules.
which were applied to the raw data. An absolute scale of quantum yields, Y, was established by assigning the value of Y = 1 to the S,(O) of trans-stilbene and
4216 The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 TABLE I: Relative Ouantum Yields for Pairs of Molecules in ANIL ANTH 9BA 9,lODBA aniline (1.06) 0.0035
anthracene
Sonnenschein et al.
9,lODCL ANTH-dlo (1.14) 1.49
(1.0) 0.20
9MA (1.14) 0.87
F (1.0) 2.5 (1.11) 0.99
P
TS
(1.30) 1.47
(1.0) 1.49
T
(0.94) 284
9-bromoanthracene 9,lO-dibromoanthracene 9,lO-dichloroanthracene anthracene-dlo 9-methylanthracene fluorene perylene trans-stilbene
(1.0) 0.40
(0.88) 0.67 (1.0) 5.0 (0.98) 1.15 (0.90) 1.01 (0.77) 0.68 (1 .O) 0.67
(1.0) 0.57
(1.09) 0.97
(1.0) 1.75 (0.97) 1.47 (0.92) 1.03
(0.95) Q.41 1.03 0.68 (1.05) 2.44
tetracene
-
a The relative fluorescence quantum yields are corrected for the differences in photomultiplier-photodiode efficiencies, as given by the manufacturer’s spectral response figures. Absorption at the excitation wavelength fluorescence at 1400 cm-’ red shift accounting for the mean S,(O) So wavelength shift. Numbers in parentheses refer to the numerical correction factors which were applied to the raw data. Numbers appear as the quotient of the quantum yield of the molecule in the row (column heads of table) divided by the quantum yield of the molecule in the first column (left-hand margin of Table). bANIL = aniline; ANTH = anthracene; 9BA = 9-bromoanthracene; 9,lODBA = 9,lO-dibromoanthracene; 9,lODCL = 9,lO-dichloroanthracene;ANTH-dlo = perdeuterioanthracene; 9MA = 9-methylanthracene; F = fluorene; P = perylene; TS = trans-stilbene; T = tetracene.
of 9, IO-dichloroanthracene. Such a normalization is justified for several reasons. Firstly, the experimental decay lifetime for the Sl(0) of trans-stilbene in a supersonic jet has been measured and found to be T = 2.1 ns,6 T = 2.6 ns,’ while the pure radiative lifetime T, = 2.5 ns was calculated from the integrated oscillator strength,8 suggesting that 7 = T,, Le., Y = 1 for the Sl(0) state. Secondly, quantum yields of unity for 9,lO-dichloroanthracene and trans-stilbene have been measured at low temperatures in condensed phase^.^^^ Thirdly, we have found that the Sl(0) states of the reference molecules of trans-stilbene and of 9,lO-dichloroanthracene, as well as the Sl(0)of 9-cyanoanthracene, have equal fluorescence quantum yields, which are the highest among all the molecules studied by us (Table I). The absolute quantum yields obtained by using this normalization procedure are summarized in Table 11. We have obtained quantitative information on the fluorescence quantum yields from the vibrationless level of the SI manifold of large molecules. No direct information emerges concerning the rotational-state dependence of Yas in our experiments the spectral resolution of -4 cm-’ exceeds the intrinsic line widths of the rotational envelope for the individual vibrationless 0transitions. Former experimental data on the rotational-state dependence of the time-resolved decay of S,(O)of tetracene16s1’and the quantum yield of Sl(0)of aniline3 pointed to the independence of the quantum yield from an individual electronically-vibrationally (6) J. A. Syage, W. R. Lambert, P. M. Felker, A. H. Zewail, and R. M. Hochstrasser, Chem. Phys. Lett., 88, 266 (1982). (7) T. T. Majors, U. Even, and J. Jortner, J . Chem. Phys., in press. (8) R. Dyckes and D. S. McClure, J . Chem. Phys., 36, 2347 (1962). (9) J. B. Birks, “Photophysicsof Aromatic Molecules”, Wiley, New York, 1970. (IO) R. S. Becker, “Theory and Interpretation of Phosphorescence and Fluorescence”, Wiley-Interscience, New York, 1969. (11) R. Bennet and P. McCartin, J . Chem. Phys., 44, 1949 (1966). (12) E. C. Lim, J. D. Laposa, and J. Yu, J . Mol. Spectrosc., 19, 412 (1966). (13) D. Gegiou, K. Muszkat, and E. Fischer, J. Am. Chem. Soc., 90,3907 (1968). (14) E. J. Bowen, “The Photochemistry of Aromatic Hydrocarbon Solutions”,R. M. Noyes, G. C. Hammond, and J. N. Pitts, Eds., Wiley, New York, Adv. Photochem., 1, 23 (1963). (15) D. J. Birch and J. B. Birks, Chem. Phys. Lett., 38, 432 (1976). (16) A. Amirav, U. Even, and J. Jortner, J . Chem. Phys., 71,2319 (1979). (17) A. Amirav, U. Even, and J. Jortner, J . Chem. Phys., 75,3770 (1981).
TABLE 11: Absolute Quantum Yields from the S,(O) of Large Molecules in Planar Jets molecule ani1ine 2937.5 0.28 anthracene 3610.5 0.67 anthracene-dlo 3601.6 0.134 3742.0 0.0024 9-bromoanthracene 3864.5 0.72 9,lO-dibromoanthracene 9,lO-dichloroanthracene 3853.5 1.oo 1.oo 3821.0 9-cyanoanthracene 0.57 3711.5 9-methylanthracene 2960.0 fluorene 0.68 0.97 4154.5 perylene 1.oo 3101.0 trans-stilbene 0.65 4159.3 4-chloro-trans-stilbene 0.40 4463.5 tetracene
-
a Peak position of the (instrumentally broadened) spectral feature corresponding to the So S,(O) transition. bThe absolute quantum yields are scaled to the Sl(0) states of 9,lO-dichloroanthracene and of trans-stilbene, whose yields are taken as Y = 1.00.
excited state of large molecules on the specific rotational state. This conclusion is supported by theoretical evidence.Is
Discussion The present study again demonstrates the unique characteristics of seeded supersonic jets, where photoselection of an individual electronic-vibrational excitation of a large molecule can be accomplished. The determination of fluorescence quantum yields and decay lifetimes from the Sl(0) state of large molecules under low-pressure bulb conditions cannot be accomplished, as photoselection is precluded by vibrational sequence congestion. The only exception among the molecules studied herein is the 14-atom aniline molecule, where vibrational sequence congestion effects on the So SI electronic origin a t room temperature are minor and the rotational envelope of the electronic origin can be resolved a t 300 K.19 The absolute fluorescence quantum yield Y = 0.28 from the Sl(0) origin of aniline in the planar supersonic expansion,
-
(18) F. Novak and S. Rice, J . Chem. Phys., 71, 4680 (1971). (19) R. Sheps, D. Florida, and S. A. Rice, J. Chem. Phys., 61, 1730 (1974).
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4217
Absolute Fluorescence Quantum Yields TABLE I11 Pure Radiative and Nonradiative Lifetimes of the S,(O) Electronic Origin of Large Isolated Molecules in Supersonic Jets molecule Im r , ns 7 1 , ns Tnr, ns aniline 0.28 13 i 2b 46 i 5 18 i 2 anthracene 0.67 18 f 2c 30 i 3 60 i 6 20d 24' 17f
anthracene-d,,
0.13
fluorene 0.68 trans-stilbene 1.00 4-chloro-trans- 0.65 stilbene tetracene 0.40
d 4.48
31 i 3
4.6 i 0.4
22 i 2h 2.6 i 0.3' 1.6 i O . l k
32 f 4 2.6 i 0.3 2.5 i 0.3
69 f 8
20 f 2j
50 f 5
33 f 4
4.7 f 0.5
Present work, accuracy f5%. Reference 21. Reference 22. Reference 23. eReference 24. fReference 25. gReference 26. hReference 27. 'References 6 and 7. jReferences 16 and 17. Reference 7.
-
where the rotational temperature is 10 K,Zocompares favorably with the value of Y = 0.30 for S,(O)in the bulb, as reported by Sheps, Florida, and Rice.19 The close correspondence between the quantum yields for the S,(O)of aniline at 10 K and a t 300 K provides further support for the notion that the nonradiative decay time in this molecule is practically invariant with respect to the degree of rotational e ~ c i t a t i o n . ' ~ - ' ~ The combination of the present quantum yield data with independent measurements of the experimental decay lifetimes from the S,(O) of isolated ultracold large molecules allows for the experimental determination of the nonradiative decay times as well as of the pure radiative lifetimes. In Table 111, we present the experimental r values for Sl(0) determined in this laboratory7J6v2'922927 and by other w o r k e r s , 6 ~ ~together ~ - ~ ~ with the T,, and T , values calculated by the relations T~~ = ~ / ( 1 - Y) and T , = r / Y . The information obtained herein on the Y values from Sl(0) of anthracene and its derivatives (Table 11) provide detailed novel information on the mechanism of intersystem crossing. The nonradiative decay of Sl(0),or of any other vibronic state of SI, may proceed via two mechanisms: (A) Direct Sl-T, coupling. Here, S,(O)is isoenergetic and is effectively coupled with the dense manifold of the first triplet. (B) Mediated SI-T,-Tl coupling. Here, S1(0) is coupled to a nearby discrete manifold of vibronic levels of a higher triplet state (T, = T2or T3), which in turn are effectively coupled to the TI dense manifold. In the discussion of the general patterns of electronic relaxation in this class of compounds, it will be convenient to draw a qualitative distinction between two classes of molecules, Le., those characterized by low spin-orbit coupling (LSOC) and those characterized by high spin-orbit coupling (HSOC). The unity quantum yields from the Sl(0)states of 9,lO-dichloroanthracene and 9-cyanoanthracene imply that both decay channels (A and B) are closed, whereupon the direct Sl-Tl coupling is poor in these LSOC molecules, while the electronic origin of T,(O) is located above S,(O). The modest reduction of Y below unity for S,(O) of anthracene and of 9-methylanthracene cannot be reconciled in terms of mechanism A, which does not prevail in S,(O)of other LSOC molecules, but is rather attributed to mechanism B in these LSOC molecules. 1~12,28,29 Proceeding to the decay characteristics (20) A. Amirav, U. Even, J. Jortner, F. W. Birss, and D. A. Ramsay, Can. J . Phys., 61, 278 (1983). (21) A. Amirav, U. Even, and J. Jortner, Anal. Chem., 54, 1666 (1982). (22) A. Amirav, U. Even, and J. Jortner, to be submitted for publication. (23) W. R. Lambert, P. M. Felker, and A. H. Zewail, J . Chem. Phys., 75, 5958 (1981). (24) T. R. Hays, W. Henke, H. L. Selze, and E. W. Schlag, Chem. Phys. Lett., 77, 19 (1981). (25) S. Okajima and E. C. Lim, private communication and to be submitted for publication. (26) A. H. Zewail, private communication and to be submitted for pub-
lication.
(27) A. Amirav, U. Even, and J. Jortner, Chem. Phys., 67, 1 (1982).
of HSOC molecules, we note the large difference between the Y value for the Sl(0) states of 9,lO-dibromoanthracene ( Y = 0.72) and 9-bromoanthracene (Y= 0.0024), which demonstrates the dramatic implications of mechanism B.30*31The modest reduction of Y for Sl(0) of the H S O C 9,lO-dibromoanthracene molecule ( Y = 0.72) with respect to the S,(O) of the LSOC 9,10-dichloroanthracene molecule ( Y = 1 .O) reflects the enhancement of the direct coupling (mechanism A) in the latter case. Thus, in 9,lO-dibromoanthracene mechanism B is inefficient, Le., the T,(O) is located above Sl(0) in energy. On the other hand, because of the low Yvalue of S,(O)of 9-bromoanthracene, the T,(O) state is expected to lie below S,(O)and mechanism B is very effective for the decay of Sl(0), being magnified by the large spin-orbit interaction. A notable manifestation of the role of the mediating T, states was demonstrated by the inverse deuterium isotope effect in anthracene-hlo and in anthracene-dIo,the ratio of the quantum yields from the Sl(0) states of these two molecules being Y(Hlo)/Y(Dlo) = 5, while 7",(H)/r,,(D) = 13. The pure radiative lifetimes for anthracene-hLoand for anthracene-d,,,, which were determined for the combination of our Y data and recent decay lifetimes,22-26are found to be equal within experimental uncertainty (Table 111), inspiring confidence in our method for the determination of absolute quantum yields. A detailed analysis of the large inverse deuterium isotope effect on the electronic relaxation from the S,(O) state of anthracene will be presented elsewhere.32 Briefly, we ascribe this surprising effect to mechanism B, with the mediating sparse T, manifold involving a set of low-lying vibrationally excited T, states. The level structure of low-lying vibrational excitations in the T, manifold is very sensitive to deuteration. The sparse manifold is more congested for anthracene-dlo than for anthracene-hlo, resulting in the dramatic enhancement of the intersystem crossing in anthracene-dlo relative to anthracene-hlo. The unity fluorescence quantum yield from the S,(O) state of the isolated trans-stilbene molecule implies that both the intersystem crossing channel and the isomerization channel are closed at this energy.5bs6*7The reduction of Y for the S,(O) state of 4-chloro-trans-stilbene (Y= 0.65) relative to that of trans-stilbene is attributed to intersystem crossing induced by the internal heavy-atom effect. These data are pertinent for the elucidation of the mechanism of trans-cis isomerization in isolated large molecules. b*6,7 We shall dwell briefly on the role of solvent effects on emission quantum yields by confronting the present data for Y from S,(O) of isolated molecules with the extensive information available on the quantum yields for the S1state of these molecules in condensed media. The fluorescence quantum yield from a level structure of a large molecule, which corresponds to the statistical limit,' is expected to be invariant with respect to medium perturbation provided that the following three restrictive conditions are satisfied. ( 1 ) The molecule is "very large" so that the rotational effects on electronic relaxation from a given vibrational level are insignificant. (2) The solvent is "inert". It does not modify the energy levels and the interstate coupling. (3) The quantum yields of the vibronic levels, which are accessible by medium-induced vibrational excitation, are independent of the excess vibrational energy. Condition 1 is satisfied for all the large molecules studied herein. Conditions 2 and 3 are very r e s t r i ~ t i v e .It~ ~is apparent that in molecules, where intersystem crossing can occur via mediated coupling (mechanism B), solvent shifts of energy levels and thermal vibrational excitation in solution grossly modify the magnitude of the intersystem crossing rates. Thus, for anthracene, 9,lOdichloroanthracene, 9-methylanthracene, 9-bromoanthracene, and 9, IO-dibromoanthracene in room-temperature solutions, large (28) T. F. Hunter and R. F. Wyatt, Chem. Phys. Lett., 6, 221 (1970). (29) R. P. Widman and J. R. Huber, J . Phys. Chem., 76, 1524 (1972). (30) K. C. Wu and W. R. Ware, J . Am. Chem. SOC.,101, 5906 (1979). (31) K. Hamanoue et al., J . Phys. Chem., 87, 813 (1983). (32) A. Amirav, M. Sonnenschein, and J. Jortner, Chem. Phys. Lett., 100, 435 (1983).
J. Phys. Chem. 1984, 88, 4218-4222
4218
TABLE IV: Comparison of S,(O) Quantum Yields from S1(0) Large
Isolated Molecules in Supersonic Jets with Solution Quantum Yields From Sa molecule in soh isolatedquantum molecule Im yield soh anthracene 0.67 0.31bJ hexane at 293 K 0.36C9k cyclohexane at 293 K 0.55 1.0"k ethanol at 77 K 9-methylEPA at 77 K 1.O'J anthracene 0.35fj ligroin at 293 K 0.0024 0.017ggk ethanol at 293 K 9-bromohexane at 293 K 0.0033 anthracene 0.72 0.095g,k ethanol at 293 K 9,lO-dibromoanthracene 0.043h,k hexane at 293 K 0.55 cyclohexane at 293 K 1.o 9,lO-dichloro1.0',k lucite at 77 K anthracene 'vk
nPresent work, accuracy *5%. *Reference 14. CReference 31. dReference 11. "Reference 12. /Reference 14. gReference 9. Reference 33. Reference 34. Y determined directly. Estimated from lifetime data. 'Reference 11.
'
differences between our Y value and the solution quantum yield data are exhibited (Table IV), manifesting the violation of conditions 2 and 3. Condition 3 can be satisfied in a condensed medium at low temperatures, where only the inhomogeneously broadened electronic origin is thermally populated. The unity quantum yield of 9,lO-dichloroanthracene in low-temperature glasses" is in g o d agreement with our isolated-molecule Y value, (33) D. Huppert, S. D. Rand, A. H. Reynolds, and P. M. Rentzepis, J . Chem. Phys., 77, 1214 (1982). (34) I . Berlman, "Handbook of Fluorescence Spectra of Aromatic
Molecules", Wiley-Interscience, New York, 1965.
implying that both in solution and in the isolated molecule T,(O) is located above Sl(0), whereupon the mediating mechanism B does not prevail in both cases. Another dramatic effect of thermal excitation on the nonradiative decay involves the fluorescence quantum yield of transstilbene, which takes the Yvalue of 3 X in room-temperature solution, while the quantum yield in a glass a t 70 K is close to ~ n i t y . ~Thermal ~ ? ' ~ population of high vibrational states, which undergo reactive trans-cis isomerization, is switched off in lowtemperature glasses where the high emission yield concurs with our value of Y = 1 for Sl(0) of the isolated molecule. The methodology for the determination of absolute fluorescence quantum yields from photoselected states of isolated large molecules, advanced in the present work, was extended recently in two directions. First, the Yvalues for the Sl(0) state, reported herein, were utilized to obtain extensive information on the absolute fluorescence quantum yields from vibrationally excited states of isolated large molecules on the excess vibrational energy within the SI manifold.35 Second, this approach was applied for probing the intramolecular dynamics of large van der Waals complexes.35
Acknowledgment. This research was supported in part by the U S . Army through its European Research Office, and by the United States-Israel Binational Science Foundation, Jerusalem (Grant No. 2641). A. A. acknowledges the support of the Basic Research Fund of the Israel Academy of Sciences and Humanities, Jerusalem. Registry No. Aniline, 62-53-3; anthracene, 120-12-7; 9-bromoanthracene, 1564-64-3; 9,1O-dibromoanthracene, 523-27-3; 9,lO-dichloroanthracene, 605-48-1; anthracene-d,,,, 1719-06-8; 9-methylanthracene, 779-02-2; fluorene, 86-73-7; perylene, 198-55-0;truns-stilbene, 103-30-0;tetracene, 92-24-0; 4-chloro-truns-stilbene,1657-50-7; 9-cyanoanthracene, 1210-12-4. (35) A. Amirav, M. Sonnenschein, and J. Jortner, Chem. Phys., in press.
Hydrogen Evolution and Iodine Reduction on an Illuminated n-p Junction Silicon Electrode and Its Application to Efficient Solar Photoelectrolysis of Hydrogen Iodide Yoshihiro Nakato,* Yoshihiro Egi, Masahiro Hiramoto, and Hiroshi Tsubomura* Laboratory for Chemical Conversion of Solar Energy, Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: September 12, 1983; In Final Form: February 29, 1984) A p-type silicon electrode, highly doped with phosphorus from the surface (hereafter abbreviated as an n+/p-Si electrode), generated a photocurrent corresponding to iodine (or triiodide) reduction in a hydrogen iodide/iodine solution at potentials much more positive than those at platinum and p-Si electrodes. The photocurrent-potential curve did not change when the electrode was illuminated for 60 h at a sufficiently cathodic potential, but changed due to the formation of a thin silicon oxide layer at the surface when illuminated near the onset potential of the photocurrent. The kinetics of the photoevolution of hydrogen on the n+/p-Si electrode in acid solutions were improved by depositing a 0.5-3 nm thick platinum layer on the surface. The hydrogen-evolution photocurrent at the Pt-coated nt/p-Si electrode gradually decayed under prolonged illumination, but was restored to the initial value by keeping the electrode in an oxidative condition. This suggests that the decay is due to the formation of a reduced surface species. Photoelectrochemical cells constructed with a Pt-deposited n+/p-Si electrode and a Pt counterelectrode separated by a commercial cation-exchange membrane electrolyzed hydrogen iodide into hydrogen and triiodide ions under no externally applied voltage with an encouragingly high solar-to-chemical energy conversion efficiency (4Schem) of 7.8% under simulated solar AM 1 radiation.
Introduction Active studies have been made on semiconductor photoelectrochemical (PEC) cells from the point of view of direct conversion of solar energy into storable chemical Recently Heller (1) (2) (3) (4)
Nozik, A. J. Annu. Reu. Phys. Chem. 1978, 29, 189-222. Gerischer, H. Top. Appl. Phys. 1979, 31, 115-72. Bard, A. J. J . Photochem. 1979, 10, 59-75. Wrighton, M . S. Acc. Chem. Res. 1979, 12, 303-10.
et al. reported that hydrogen bromide or iodide could be photoelectrolyzed by using a PEC cell composed of an n-MoSe, or n-WSe2 photoanode and a metallized p-InP photocathode with considerably high conversion efficiencies (7.8% for the hydrogen (5) Tomkiewicz, M.; Fay, H. Appl. Phys. 1979, 18, 1-28. (6) Memming, R. Electrochim. Acta 1980, 25, 77-88. (7) Scaife, D. E. Sol. Energy, 1980, 25, 41-54. (8) Heller, A. Acc. Chem. Res. 1981, 14, 154-62.
0022-3654/84/2088-421 S%Ol.50/0 0 1984 American Chemical Society
