J. Phys. Chem. 1982, 86, 2049-2052
leaving one at a loss to explain how both shifts can change with water content in nearly the same way until the weight fraction of water reaches 0.45. In the face of these difficulties, it is necessary to reexamine very critically the attractivelo assumption that water and soap form inverted micelles in a continuous alcohol phase, with added oil serving only as a dilutant having little or no effect on the micelle structure. At the highest water concentrations, the solutions represented by C in Figure 1 have compositions similar to those of systems described as o/w by Mackay and co~ o r k e r s . ~ J lHere at least S and E have significantly different environments. Since the signals with the higher value of 6 can be assigned to S, one may suppose that the structure is indeed o/w, with a little of the S having entered the aqueous microphase, an effect which increases with rising water content and which is absent or much smaller for E. However, although the traditional model calls for these solutions to be o/w at water concentrations near 5070, this is no longer plausible at the lowest water contents, near 5%, where the structure should be w/o. It is then somewhat puzzling that the curves show no discontinuity or sudden change of slope indicative of a transition point between the two types of structure. A similar lack of evidence for an abrupt structural transition was also reported for systems containing less exotic ingredients and (10) S.Friberg and I. Buraczewska, "Micellization,Solubilization, and Microemulsions",K. L. Mittal. Ed., Plenum, New York, 1977, Vol. 2, p 797. (11) R. A. Mackay, K. Letta, and C. Jones ref 10, p 801.
2049
studied by proton magnetic resonance and other methods.12 It thus seems that the traditional model may be approximately correct for these systems when the composition falls in some regions of the phase diagram, but it becomes progressively more unsatisfactory as the amount of water or oil is reduced. It could be argued instead that the difficulties reported above merely reflect peculiar behavior of trifluoroethoxy compounds and that they would vanish if attention were restricted to more conventional oils and surfactants. Further work is needed to rule out this possibility definitively, but it should be noted that a number of the observations presented here, such as those for S and E in aqueous micellar solutions, suggest that these compounds do not really have especially eccentric properties. Moreover, the implications of the present work are in close accord with the conclusions reached by the Swedish workers6 from studies involving quite ordinary materials: '"Classical' microemulsion systems of ionic surfactant, short-chain alcohol, hydrocarbon, and water show no distinct separation into hydrophobic and hydrophilic domains... and have flexible and highly dynamic disorganized internal interfaces." It is planned to examine a much wider variety of microemulsions by fluorine magnetic resonance to try to define conditions under which well-developed o/w or w/o structures are formed and to provide clues concerning the nature of the structures which occur when this is not the case. (12) A. M. Bellocq, J. Biais, B. Clin, P. Lalanne, and B. Lemanceau, J. Colloid Interface Sci., 70, 524 (1979).
Structural Effects on Photophysical Processes in Saturated Amines. 6. Excited-State Interactions in Piperazine Derivatives Arthur M. Halpern,' 8. R. Ramachandran, and Shobha Sharma Department of Chemlstty, Northeastern University, Boston, Msssachusetts 021 15 (Received: December 7, 198 1)
The spectroscopic and photophysical properties of a series of N,"-bis(alky1)piperazine derivatives are reported. On the basis of the photoelectron spectrum of N,"-dimethylpiperazine (I) and the UV absorption spectra of I, N-methylpiperidine (II), 12,14-diazadecahydroanthracene(III), and 13,14-dimethyl-13,14-diazatricyclo[6.4.4.42*7]tetradecane (IV), it is concluded that interaction between the N-n orbitals is nearly absent in the ground state. For I and 111, however, fluorescence spectral and lifetime data, when compared with monoamine 11, indicate the presence of N-N interaction in the lowest excited state. A, for I and I11 are 314 and 315 nm, respectively, while A, for I1 is 292 nm. Lifetimes for I and I11 are significantly longer relative to I1 (156 and 148 ns vs. 25 ns). Photophysical data for IV imply that upper state N-N coupling is much less significant than for I and 111. A comparison of photophysical data for IV and 1,4-dimethylhexahydro-l,Cdiazepine(V) is used to suggest that the piperazine nucleus in IV is twisted, resulting in a less efficient through-bond coupling of the locally excited state with the other N center. The fluorescence properties of substituted piperazines, it is suggested, can be used to infer the conformational structure of the cyclohexane ring.
Introduction The interaction between reactive centers imbedded in a molecular framework and the attendant consequences on spectroscopic and chemical properties has long been a topic of intense and wide-ranging interest. For example, the orbital interactions between certain atoms which are brought about or facilitated by an appropriate molecular structure is of central interest in many theoretical studies.' In addition, the spectroscopicproperties which result from such interactions and which therefore provide a quanti-
tative basis with which these theories must correlate are also actively studied.2 A classic example of this relationship is provided by the symmetrical diamine, 1,Cdiazabicyclo[2.2.21 octane, or Dabco. The arrangement of the two nitrogen atoms vis(1) (a) R.Hoffman, Acc. Chem. Res., 4 , l (1971); (b) R.Hoffman, A. Imamura, and W. J. Hehre, J.Am. Chem. SOC.,90,1499 (1968); (c) M. J. S. Dewar and J. S.Waason, ibid., 92,3506 (1970).
0 1982 American Chemical Society QQ22-3654/82/2Q86-2Q49$Q7.25/Q
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The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
Halpern et ai.
TABLE I : Absorption and Fluorescence Properties of Amines I-IV absorption amine
I I1 I11 IV V
Amax,
nm
200 200 200 195' 195
fluorescence
Emax,
M-' cm-I 8.0 X 5.0 X 8.9 X 3.5 X 1.0 X
Amax,
nm
if, ns'
l o 3 314 l o 3 292 l o 3 315
156 25.1 148 67 55.2
lo3
292 l o 4 294
gasb
k,,
0 . 4 8 3.1 0.69 2.7 0.41 2.7 0.61 9.1 0.47 8.6
s-' X X X X X
lo6
lo'
lo6
lo6 lo6
Lifetimes and quantum efficiencies were measured at X M ) n-hexane solutions in order t o minimize complications from self-quenching. Based on an absolute fluorescence efficiency of 0.17 for toluene. I' Shoulder at 234 n m . E = 1.88 x l o 3 . a
23 "C in dilute ( 1
a-vis the orientation of the u bonds of the three ethylene bridges gives rise to a substantial (2.1 eV) splitting in the two N-n-based orbitals: this, in turn, greatly affects Dabco's ground-state properties such as its low ionization potential, and is manifest, for example, by its photoelectron ~pectrum.~ Studies of the photoelectron spectra of Dabco and other diazabicyclic amines have nicely confirmed the theoretical predictions of Hoffmann and co-workers, and others, that the mechanism of the N-N interaction in Dabco arises from the "through-bond" coupling of the N-n orbitals via the three ethylene bridges. Dabco's spectroscopic and photophysical properties have also been shown to bear dramatic consequences of this unique molecular ~tructure.~ Pertinent to this report is the observation that, in Dabco, a low-lying parity-forbidden electronic transition exists as a consequence of the symmetry of the states produced by the coupling of the N-n orbitals, and that fluorescence from the lowest excited state (A) is anomalously long lived and red shifted relative to the monoamine model compound, l-azabicyc_l0[2.&2]octane.~ The symmetry forbiddenness of the A X transition in Dabco was recently directly confirmed by Parker and Avouris who measured the multiphoton ionization spectrum which results from two-photon resonances to the A state.6 This paper deals with the photophysical and spectroscopic properties of structural analogues of Dabco, namely, substituted piperazines. The piperazine nucleus can be thought of as a Dabco molecule in which one of the bridges is severed, allowing the diamine moiety to assume a cyclohexane-chair-type conformation. In an earlier paper in this series, it was pointed out that the fluorescenceof vapor phase N,N'-dimethylpiperazine (I) is anomalously long
-
b
N
i
-
-NqD
lived and red shifted (i.e., T = 770 ns; A,, = 313 nm) as compared with N-methylpiperidine (11) (i.e., 7 = 60 ns; A,= 290 nm), although the electronic absorption spectra of the two compounds are ~omparable.~In fact, the absorption spectrum of I is consistent with a model that it (2)(a) E.Heilbronner and K. A. Muszkat, J.Am. Chem. Soc., 92,3818 (1970);(b) S.F. Nelsen and J. M. Buschek, ibid., 95,2013 (1973);( c ) ibid., 96,7930 (1974). (3) P. A. Bischof, J. A. Hashmall, E. Heilbronner, and V. Hornung, Tetrahedron Lett., 4025 (1969),and ref 2c. (4) (a) A. M. Halpern, J. L. Roebber, and K. Weiss, J . Chem. Phys., 49,1348 (1968); (b) T.M. McKinney, Spectrochim. Acta, Part A , 25, 501 (1969); (c) A. M. Halpern, Chem. Phys. Lett., 6 , 296 (1970);(d) A. M. Halpern and R. M. Danziger, ibid., 16,72 (1972). (5) A. M. Halpern, J.Am. Chem. SOC.,96,4392 (1974). (6)D. H. Parker and P. Avouris, J. Chem. Phys., 71, 1242 (1979). (7)A. M. Halpern and T. M. Gartman, J . A m . Chem. Soc., 96,1393 (1974).
200
24 0
280
320
360
1, N M Flgure 1. Left: absorption spectra, plotted as e X lo3 (M-' cm-' ) vs. wavelength (nm). Right: uncorrected fluorescence spectra: (- - .) I,(-) II,(----) III,(....) IV; (-.---.)V. Allspectraarerecorded at 23 O C in n-hexane solution. Solute concentration for the fluorescence spectra was kept below 1.0 X IO-' M.
-
is a noninteracting bonded dimer of trimethylamine. Thus the conclusion was reached that the lowest excited state in I is characterized by appreciable coupling between the two N atoms and that therefore the emission is essentially excimeric in nature. It was also suggested that a structural change occurs in the relaxed, excimeric excited state which results from, or facilitates, the electronic reorgani~ation.~ The present investigation was initiated with the aim of determining the extent to which the structural properties of the piperazine moiety affect its photophysical characteristics and hence inferring the degree of delocalization of excitation energy between the two N-atom centers. We report here the results of this study which is primarily focused on two structurally constrained piperazine derivatives: 12,14-diazadecahydroanthracene(111) and 13,14dimethyl- 13,14-diazatricyclo[6.4.1,12J]tetradecane (IV).
m Results and Discussion
LY
The situation regarding the ground-state interaction between the N centers in the Diperazines will first be discussed. Unlike the case in piperazine itself, where a moderate splitting in the N-n orbital energies is observed in the photoelectron spectrum (0.55 eV),2a,cI appears to retain quasidegeneracy in the N-n orbitals. This situation was discussed by Nelsen and Buschek who analyzed the photoelectron spectrum of I as being consistent with a diequatorial conformation of N-methyl groups and of piperazine in terms of a distribution of the axial-equatorial and axial-axial N-H conformations.2c As was already pointed out, the vapor-phase electronic absorption spectrum of I is consistent with the absence of a lower-lying transition such as would arise as a consequence of ground state N-n orbital coupling.' The electronic absorption spectrum, when taken alone, it should be pointed out, cannot provide conclusive evidence for the absence of such interaction because the presence of a weakly allowed, lower lying transition may be masked by the stronger absorption at shorter wavelengths (at ca. 200 nm). In the case of Dabco where the ground-state coupling is strong (vide supra), however, the absorption spectrum (in n-hexane) clearly reveals the presence of the weaker low-lying A X transition, Le., A,, = 231 nm; em- = 2.66 X lo3 M-' cm-I), whereas for I A,, = 200 nm; emax = 5.0 X lo3 M-' cm-I. We find that, in n-hexane solution, the absorption spectra of I-IV are all typical of tertiary monofunctional amines, having absorption maxima near 200 nm. Moreo+-
Photophysical Processes in Saturated Amines
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
2051
ported by the photophysical and spectroscopic properties ver, the onset and strength of absorptivity (Le., below 250 of a related 1,4-diazacycloalkane, namely, 1,4-dimethylnm) also follow the pattern of monoamine analogues such hexahydro-l,4diazepine (V). As indicated in Table I, this as 11. Several absorption spectra are shown in Figure 1. While the absorption spectra of I and I11 fall off smoothly to the red of the maxima, as is common with tertiary -N monoamines, the spectrum of IV shows a distinct shoulder at 235 nm; this situation is reminiscent of the spectrum Y of trimethylamine (both in the vapor phase and in ndiamine, which also has a monoamine-like absorption hexane solution) which also reveals a shoulder (at ca. 225 spectrum, fluoresces at higher energy than I and 111. In nm). these data are summarized in Table I which also fact, the fluorescence spectrum of V, like that of IV,closely contains pertinent photophysical information to be disresembles the spectrum of the model monoamine, I1 (see cussed below. Figure 1). Interestingly, the radiative rate constants of IV Regarding the fluorescence spectra, lifetimes, and quantum efficiencies of the diamines I11 and IV, however, and V are very similar suggesting that the effect of the proposed distortion in the piperazine ring in IV, and thus we note some interesting and significant contrasts relative on the orientation of the N atoms and related u bonds, is both to monoamine I1 and piperazine derivative I. First, there is a striking similarity in the fluorescence spectra of similar to that which prevails in the cycloheptane ring I and 111, both with respect to the position of their maxima structure of V. Since the structure of piperazine is very close to that of cyclohexane as deduced from gas-phase and their shapes (see Figure 1). In addition, the fluoreselectron diffraction: it can be presumed that cycloheptane cence lifetimes and quantum efficiencies of these amines is an appropriate model for the structure of V. In the case (and hence their radiative rate constants, kR), are very similar. For both I and 111,these photophysical properties of cycloheptane, the lowest energy conformation is the twisted chair form,1° and in applying this information to are distinctly different from those of the monoamine 11. the case of the piperazine ring in IV, we conclude that a These observations indicate that the emitting states of I and I11 are similar. Because of the structural constraint twisting in IV's ring is the origin of its unique photophyimposed on the piperazine nucleus in I11 by the two fused sical properties (viz., its kR value). I t thus appears that the geometry of the intervening C-C bonds plays a critical cyclohexane rings, it can be presumed that the piperazine conforring is held in the diequatorial-substituted-chair role in determining the degree of excited-state coupling between the two heterocyclic nitrogen atoms. mation in both the ground and excited states. Therefore we reason that the structure of the excimeric or (N-N Conclusions coupled) state in I need not necessarily be associated with a change from the ground-state chair c~nformation.~,~ The fluorescence spectra and lifetimes of bis(alky1atThis may mean that the upper state coupling stems from ed)piperazine are, in general, very different from those of the intervention of higher energy (and thus more spatially monofunctional analogues such as N-methylpiperidine. extensive) orbitals associated with electronic excitation The excited state of piperazine derivativies in which the (e.g., 3s/3p on the N atoms and u* on appropriate C atoms) piperazine ring retains its cyclohexane structure and conrather than simply a change in the molecular geometry. formation can be characterized as excimeric in nature in It is reasonable to expect that both through-bond and which the excitation energy is delocalized between the two through-space mechanisms would play roles in the coupling N atoms. Deviations, however, from the normal cycloof the locally excited state (i.e., on one N atom) with the hexane structure such as ring twisting, which can be inother equivalent N center on the molecule via the interduced by appropriate ring substituents, have dramatic vening cyclic molecular framework. As we will show below, effects on the fluorescence spectra and lifetimes. Thus, there is some evidence that the geometry of the piperazine these photophysical properties can be used as a probe of ring, itself, affects the degree of upper state N-N coupling. the effects of substitution on piperazine-containing comUnlike the situation in 111,the photophysical properties pounds where there is no other interference on the native of IV are considerably different from those of I (see Table saturated amine emission. I). The fluorescence spectrum of IV is at higher energy Experimental Section and is narrower, closely resembling the spectrum of an N-methylated monoamines such as I1 (see Figure 1). In Synthesis of Diamines. 13,17-Dimethyl-13,14-diazaaddition, IV's radiative rate constant is considerably larger tricyclo[6.4.1.1 tetradecane (IV) was prepared following than that for I and 111, although it is smaller than kR for the procedure described by Johnson and Simm0ns.l' 11. These data suggest that the degree of N-N coupling N-Carbethoxyazepine, obtained from the thermolysis of in the excited state of IV, while not entirely absent, is ethylazidoformate in benzene,'* was dimerized by being considerably reduced relative to I and 111. heated in a sealed tube at 200 "C for 10 min. The bis(NAn examination of a molecular model of IV reveals that, carbethoxy)tetraene was catalytically hydrogenated to the in order to minimize the repulsions arising from the Ntetradecane and then reduced to the bis(N-methyl) comCH3. .H2C interactions, and also the transannular interpound by using lithium aluminum hydride in dioxane. IV actions of the ring H atoms, the central piperazine ring will was isolated via extraction with 2-methylbutane and dried undergo a distortion, i.e., twisting, from the normal cyover MgSO, and finally sublimed. The 13CNMR spectrum clohexane structure. It is likely, then, that it is this ring was consistent with the expected product: 6,25.8 (t, C4); twisting which disrupts the orbital orientations necessary for the optimal through-bond-derived excited-state cou(9) A. Yokozeki and K. Kuchitau, Bull. Chem. SOC.Jpn. 44, 2352 pling of the two N atoms. This conclusion is nicely sup-
&-
237]
-
(8) It should be pointed out that the relaxed upper state of the saturted amine is planar or nearly planar. Since the fluorescence spectra of I1 and other appropriate N-bridgehead analogues imply upper state N-centered planarity, some change from the cyclohexane configuration is expected in the compounds studied here.
(1971); see also F. A. L. Anet and I . Yavari, Tetrahedron Lett., 2693 (1976). (10) D.F. Bocian, H. M. Pickett, T. C. Rounds, and H. L. Straws, J. Am. Chem. SOC.,97,687 (1975), and references cited therein. (11) I. B. Berlman, 'Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1971. (12) (a) A. L.Johnson and H. E. Simmons, J.Am. Chem. Soc., 88,2590 (1966); (b) L.A. Paquette and J. H. Barrett, 88, 2590 (1966).
J. Phys. Chem. 1082, 86, 2052-2058
2052
33.4 (t, C3);41.7 (4, N-CH,); 61.2 (d, CJ, mass spectrum m/e 222 (M+), 165 (M+ - 37), 112 (M+ - 110),110 (M+ 112). Both 'H NMR and UV spectra showed no evidence of unsaturated impurities. 1,4-Dimethylhexahydro-l,Cdiazepine (V) was prepared by reacting homopiperazine (Aldrich) with an excess of both formic acid and formaldehyde at 80 "C for 10 h. The reaction mixture was acidified with HC1 and then neutralized with NaOH. After an excess of NaOH was added, the product was separated and distilled over CaH, under dry N2 at 1 atm (bp = 160 "C). 12,14-Diazadecahydroanthracene(111)was prepared via the dimerization of 2-(iodomethy1)piperidine(VII). 2(Hydroxymethy1)piperidine (Aldrich) was iodinated via reaction with HI (aqueous, 57%) and red P for 6 h at 100 O C . After hydrolysis and filtration, aqueous VII-HI was transferred to a 5-L flask containing ca. 2 L of H20. To the refluxing solution, aqueous NaOH was slowly added until the entire mixture was alkaline. The product was removed and isolated by steam distillation followed by extraction with isopentane and finally dried with MgS04. After removal of the isopentane, I11 was twice sublimed: mp 92 OC (lit. mp 90-92 OC;1495-96 OC15);mass spectrum m/e 194 (M+), 111 (M+ - 93), 98 (M+ - 96); 13C NMR (13)R. J. Cotter and W. F. Beach, J. Org. Chem., 29, 751 (1964). (14)K.Winterfeld and E. Will, Natunuissemchaften, 42,178(1955). (15) E. E.Glover and G. H. Morris, J. Chem. SOC.,3366 (1963).
(CDC13,Me4Si)aC 24.0 (C,); 25.6 (CJ; 29.7 (CJ; 55.4 (CJ. Anal. Calcd for C12HnH2:C, 74.2; H, 11.3; N, 14.4. Found C, 73.9; H, 11.7, N, 14.4. Spectroscopic Measurements. Absorption spectra were obtained with a Cary-14 spectrophotometer. The fluorescence spectra reported are uncorrected and were measured with a homemade dc fluorimeter described elsewhere.16 Fluorescence lifetimes were determined by using the time-correlated single-photon counting apparatus previously described.l' The fluorescence decay curves of all the amines reported were observed to follow single exponential decay over at least 3 decades. Lifetime values, as well as the criterion for single component decay, were determined by applying a nonlinear least-squares reiterative reconvolution procedure.l8 Acknowledgment. The authors gratefully acknowledge the support of the National Institutes of Health (GM20921), the National Science Foundation (CHE-77-27421), and the NIH, Division of Research Resources (RRO-7143). They also acknowledge Professor G. R. Underwood (New York University) for providing 13C NMR spectra and Professor D. Fonyth (Northeastern University) for helpful discussions. (16)A. M. Halpern and R. M. Danziger, Chem. Phys. Lett., 12, 72 (1972). (17) A. M. Halpern, J. Am. Chem. SOC., 96,7655 (1974). (18)A. Grinvald and I. Z. Steinberg, Anal. Biochem., 59,583(1974).
Three Novel Spin-Labeled Crown Ethers M. P. Eastman," D. E. Patterson, Department of Chemlsby, Llnlverstty of Texas at €1 Paso, €1 Paso, Texas 79968
R. A. Bartsch, Y. Liu, Department of ChemistrysTexas Tech University, Lubbock, Texas 79409
and P. G. Eller University of Callfornia, Los Alamos Natlonal Laboratory, Los Alamos, New Mexico 87545 (Recelved: December 29, 1981)
The following nitroxide spin-labeled crown ethers have been synthesized: 2,2-(sym-dibenzo-16-crown-5)-4,4dimethyloxazolidinyl-N-oxy1 (l),2,2-(sym-dibenzo-l6-crown-5)-4,4,5,5-tetramethylimidazolinyl-N-oxyl (2), and 3-carboxy-2,2,5,5-tetramethyl-3-pyrrolinyl-l-oxy hydroxy-sym-dibenzo-16-crown-5 ester (3). 1 and 2 are characterized by having the nitroxide near the crown cavity and rigidly bound to a ring carbon atom of the crown. 1-3 have been studied by EPR, and the proton hyperfine splittings have been determined by second harmonic out-of-phasedetection. The values obtained for the proton hyperfine splittings of 1 were compared to those determined by ENDOR and found to agree within experimental error. EPR studies of hyperfine splitting constants and Heisenberg spin exchange suggest that these spin-labeled crowns are poor complexing agents for alkali metal cations. The structure of 1 has been determined by X-ray diffraction,and the poor complexing ability of the crown is attributed to an unprecedented structural feature in which the hydrogen atoms of one methylene group of the propylene bridging linkage have turned inward with respect to the cavity of the crown. The proton hyperfine splitting observed for 1 is attributed to protons located at approximately van der Waals distances from the NO group.
Introduction The importance of crown ethers as cation complexing agents makes it d-irable and necessary to understand the solution properties of this group of compounds.1 One (1)Izatt, R. M., Christenson, J. J., E&. 'Synthetic Multidentate Macrocyclic Compounds"; Academic Press: New York, 1978.
0022-3654/82/2086-2052$0 1.25/0
possible approach to the investigation of these solution properties is through EPR spectral studies of nitroxide spin-labeled crown ethers. On the basis of previous studies of spin-labeled compounds, it could be anticipated that labeled crown systems with the nitroxide probe in proximity to the crown Cavity and rigidly bound to a carbon atom in the crown would yield EPR spectra containing 0 1982 American Chemical Society
