J. Phys. Chem. 1002, 86, 3371-3374
every act of energy transfer from benzophenone is "magnified" through 1,CDBN phosphorescence. The analysis of the above results was then plotted in Figure 6, from which an overall energy transfer rate constant of 1.2 x 109 M-' 5-1 was obtained for this system.
Conclusion The aggregation number of SPFO micelles is deduced to be very much smaller than that of conventionalmicelles. However, the "mini" micelles of SPFO are capable of solubilizing organic substrates such as benzophenone, nitrobenzene, and 1,4-dibromonaphthalene. In addition to solubilization, other features of conventional micelles are exhibited protection from quenching by species possessing the same charge as the micelle surface, and enhanced quenching by quenchers solubilized in micelles containing energy donors. Should an aggregate of less than
3371
10 units be termed a micelle at all? We think that the essential empirical characteristics of a micelle are its ability to enhance solubility of hydrophobic substance in aqueous solutions and to organize the solubilized species into a microscopically small space of the order of the dimensions of a sphere whose radius is roughly that of a detergent unit. Our results indicate that solutions of SPFO micelles in water display these characteristic^.^^ Acknowledgment. The authors thank the National Science Foundation for ita generous support of this work. They also thank Dr. Ian R. Gould for technical assistance. (35) A referee has pointed out that the Run complex could be cawing aggregation to form a 7:l SPFO/Run complex. If this were true, our aggregation number would refer only to the complex and not to the "native"micelle in the absence of Run.
Origin of Multiple Phosphorescence Emission from p -Chiorobenzaldehyde in Polycrystalllzed Methylcyclohexane-d,, at 4.2 K Mlchel Lamotte Laboratdre de Chimk, Physique A, Universite de Bordeeux
I, 33405, Talence, France
and Lionel Goodman' Depamnent of Chemistfy, Rutgers, The State Universw of New Jersey, New Brunswick, New Jersey 08903 (Received: February 1, 1982; I n Final Form: Aprll8, 1982)
The dual phosphorescence spectra of p-chlorobenzaldehyde in polycrystalline methylcyclohexane is shown to originate from different guest sites in the crystalline matrix, by narrow band excitation and lifetime experiments.
Introduction Coupling between closely spaced molecular electronic states, although widely studied1-" with extensive attention having been payed to molecules with na* and aa* states, is still controversial. The different sensitivity of aa* and na* states to substituent and environmental perturbation allows modulation of the state spacing and further allows state switching. Recently, a solid solution of p-chlorobenzaldehyde (PCB) in polycrystallized methylcyclohexane (MCH) was shown to constitute a favorable system for studying coupling effects on the triplet states since the solute triplet state sequence and spacing depends on the cooling rate of the solution and also on deuterium labeling of the matrix."6 In perdeuterated MCH (MCH-d14),PCB phosphorescence exhibited two types of spectra related to the existence of two crystalline phases of the host? the a phase, which occurs for moderate cooling rates (e.g., from 300 to 77 K in less than 10 min) and the j3 phase which is uniquely observed only for very slow cooling (e.g., from 300 to 77 (1)I. dzkan and L. Goodman, Chem. Phys. Lett. 64, 32 (1979). (2)I. dzkan and L. Goodman, Chem. Reu., 79,277 (1979). (3)A. Deepres, V.Lejeune, E. Migirdicyan, and W. Siebrand, Chem. Phys., 36, 41 (1979),and references therein. (4)S.Dym and R.M. Hochstraseer, J. Chem. Phys., 51,2458 (1979). (5) 0.S Khalil and L. Goodman, J. Chem. Phys., 64,4061 (1976). (6)0.S.Khalil and L. Goodman, J . Am. Chem. SOC.,99,5924(1977). 0022-365418212086-3371$01.25/0
K in more than 1h). In nondeuterated MCH, the j3 phase is not observed in either intermediately or slowly cooled samples. Phosphorescence of PCB in the j3 phase yields a single phosphorescence spectrum. However, phosphorescence of PCB in the a phase in both MCH and MCH-d14gives rise to multiphosphorescence emission. The spectrum has been interpreted as either an intramolecular dual phosphorescence originating from two independent 3na* and 37ra* states only weakly coupled by phonon interaction, or dual emission due to different sitesS6 In this paper we report narrow band phosphorescence spectra and phosphorescence lifetime experiments which definitively prove that the dual emission of PCB and PCB-d in the MCH a phase is due to a multisite mechanism. Moreover, phosphorescence excitation spectra monitored by narrow band observation provides compelling evidence for this interpretation. Experimental Details p-Chlorobenzldehyde and its deuterium derivative, PCB-dl (aldehyde hydrogen deuterated), had the same origin as in our previous studies. Perdeuterated methylcyclohexane (MCH-d14Merck Sharp and Dohm, 99.15% D was used without further purification. The degassed samples were sealed in 4-mm i.d. Pyrex tubes and cooled by being lowered into a dewar containing liquid nitrogen. 0 1982 American Chemical Society
3372
The Journal of physlcal Chernlstty, Vol. 86, No. 17, 1982
Lamotte and Goodman
PCB/ MCH-dl4 01
I
01
, 03
II,L ,-ALL 100
405
410
415
620
625
430
B
nm
Figure 1. 3371-A laser exdtation of pchlorobenzaldehyde phosphorescence spectrum in ~thylcyclohexane-dl4sbwly cooled to 4.2 K to produce only the a phase. Lines Vl-V, represent the first members of the vibrational progression in Y, buiit on origins 01-03.
0 2
400
4 04
nm
‘h u 400
nm
4 01
Flgure 3. Time-resolved origin band region of phosphorescence spectra of pchkrobenzaldehyde in the a phase of M C K d l 4 obtained by using 3371-A excitation. The amplifier gate delays are lo4 s in A and IO-* s in B. PCB-dl / MCH-d14
I
0 3
a- P H A S E
A LOO
105
110
415
420
425
130
nm
Flgure 2. 3371-A laser excitation of p-chlorobenzaldehyW, phosphorescence spectrum obtained under the same conditions as described in Figure l .
The cooling rate was set slightly faster than in ref 5 80 that PCB phosphorescence originating from MCH-d140phase was practically unobserved. Phosphorescence spectra were excited by using the 3371-A line from a pulsed nitrogen laser (Sopra 0803) and analyzed through a Jobin-Yvon monochromator (f/4.7, dispersion: 1,2 nm/mm). The resolution was -0.06 nm. Detection used an EM1 9756 QB photomultiplier, and phosphorescence spectra were recorded either in a continuous mode, the signal being send directly to a Sefram Servotrace recorder, or in a synchronous mode through a Tekelec Airtronic (boxcar) sampler averager. Phosphorescence excitation spectra were obtained with a Sopra dye laser by scanning the laser emission of PPBO (Exciton) pumped with a nitrogen laser. Narrow band 3 A) allowed observation of the phosphorescence (AXh* us to record excitation spectra corresponding to each individual phosphorescence line. Phosphorescence decay curves for the same lines were recorded by using the boxcar.
-
Time-Resolved Phosphorescence Spectra P& of the phosphorescence spectra obtained by direct Nzlaser excitation of PCB and PCB-dl in the MCH-d14 a phase are displayed in Figures 1 and 2. The spectra, which have been obtained with the continuous detection mode, are quite similar to the ones obtained with broad band xenon-lamp excitation reported in ref 5, except that, in the laser excited spectra, the 0phase emission is absent. Both phosphorescence spectra exhibit multicomponent structure with two main group of lines, 01-O3 and V1-V3.
I
4 00
404
nm
4 00
404
I
1
nm
Figure 4. Time-resolved origin band region of phosphorescence spectrum of pchlorobenzaldehyde-d, obtained under the same conditions as in Figure 3.
The high-energy group involves the origin region of the transition, while the similarly structured low-energy group involves the first member of the C=O stretching mode (u,) Franck-Condon progression. In our previous study of PCB in MCH: the total phosphorescence decay was resolved into two component lifetimes: 0.9 f 0.1 and 12 f 3 ms. In the present study of PCB and PCB-dl in MCH-d14, two seta of delays for sampling with the boxcar were used to separate the different lifetime components of the spectra. In the first set, a short delay (0.1 ms) between the laser pulse (pulse width 8 ns) and the gate opening (gate width 100 ps) was set to favor observation of the short-lived spectra. The spectra, thus obtained, are displayed in Figures 3A and 4A. In the case of PCB (Figure 3A), the O2 and O1components are greatly intensified with respect to Os,which is almost unobserved. In the case of PCB-dl (Figure 4A),the Oz component is greatly enhanced with respect to 03. The second set of parameters involved a much longer delay (10 ms). The resulting spectra (Figures 3B and 4B) were found to favor the O3 components in both PCB and
The Journal of Physical Chemlstty, Vol. 80, No. 17, 7982 3373
FCB Phosphorescence in MCH
TABLE I: Phosphorescence Decay Times (rp)and Assignment8 of Origin Region Phosphorescence Bands Observed in PCB and PCB-d, in the MCHd,, CI Phase at 4.2 K
4010.6 4018.0 4028.1
0, 0 1
0 3
24 921 24 881 24 819
12 2.2 r 0.2 11.3 * 0.5
Band designations in Figures 1, 3A, and 4A.
0 2
0 3
4011.6 4028.2
3n11*+ So
4.1 * 0.5 8.3 -+ 0.5
24 821 24 818
-f
3nn*
+
/
I
So
so
Band designations in Figures 2,3B,and 4B.
02 PCB/MCH-d,
o2
4.2K
4.2K
PCB-d, / M C H - d %
a- P h a u
a- Phase A
,
1
394
1
1
1
396
1
398
1
1
1
1
1
1
402
400
404 nm I
,
0.
B ob.. v,
p
B obs.
v3
>
