TAPAN K. MUKHERJEE
3442
The polarity of the metal Ptdn chelates had been qualitatively proven by structural studies and is now quantitatively defined by far-infrared refraction work. The results are consistent with dielectric studies at microwave frequencies and provide a model which clarifies the molecular dynamics of the chelates in electric fields. Extensive discussions of the most recent modern structural theories18 are compatible with this model and now stand more fully confirmed.
Acknowledgment. The authors are indebted to the National Science Foundation for a research grant in support of this work.
(18) K. Nakamoto and P. J. Mecarthy, “Spectroscopy and Structure of Metal Chelate Compounds,” John Wiley & Sons, Inc., New York, N. Y., 1968.
Charge-Transfer Donor Abilities of o,o’-Bridged Biphenyls
by Tapan K. Mukherjee Energetics Branch, Air Force Cambridge Research Laboratories, Bedford, Massachusetts
01730
(Received March IO,1969)
From the charge-transfer bands of their complexes with
T acceptors, the order of the donor strengths of o,o’bridged biphenyls has been determined to be carbazole > fluorene > dibenzothiophene > phenanthrene > dibenzofuran. Dibenzothiophene acts as a s donor. Carbazole, apart from being a possible n donor, undergoes reaction with strong electron acceptors. The behavior of dibenzofuran as a x donor is uncertain. The most important finding is that fluorene is a better donor than phenanthrene. Similarly, 1,2-benzofluoreneis a better donor than chrysene.
Introduction The charge-transfer theory predicts a nonlinear relationship between the energy of the charge-transfer band (hv,t) and the ionization potential (I,) of the donor mo1ecule.l Experimentally, however, a straightline relationship has been repeatedly obtained.2 Besides alternant hydrocarbons, the donors include alkyl and aryl halide^,^ aza-aromatic c o r n p o ~ n d saIcohols,j ,~ etc. The empirical linearity is so common that it has been extensively used to determine the I , values of donors for which direct measurements are not available.6 However, for reliable I, values, strict conditions of (a) identical experimental environments, (b) comparable electronic structures of the components without any sterically hindering factors, (c) similar types of complexes, and (d) clean charge-transfer bands, must be preserved. Although I p is a good measure of the donor strength, in those complexes where substantial changes in charge distribution with very little variation in I , are observed, the positions of the C-T band serve as a better tool for comparison. In view of the renewed interest in the excitation’ and emission8 energy levels of the donors belonging to the o,o’-bridged biphenyl system (I), a study of the complexing properties of the donors of this group seemed to be desirable. The Journal of Physical Chemistry
The report concerning the resonance energy transfer processesg from these donors (I) to 9-phenylanthracene (acceptor), and the observation of photoconduction10in dibenzothiophene (IC)and several of the (1) 5. H. Hastings, J. L. Franklin, J. C. Schiller, and F. A. Matsen, J . A m e r . Chem. Soc., 75,2900 (1953). (2) G. Briegleb, “Elektronen-Donator-Komplexe,” Springer-Verlag, Berlin, 1961, pp 74-88. (3) J. Walkley, D. N. Glew and J. H. Hildebrand, J . Chem. Phys., 33, 621 (1960). (4) S. K. Chakravarti, Spectroshim. Acta, 24A, 790 (1968). (6) M. J. Kurylo and N. B. Jurinski, Tetrahedron Lett., 1083 (1967). (6) G. Briegleb and J. Caekalla, 2. Elektrochem., 63, 6 (1959), is a somewhat outdated review. (7) (a) E. Merkel, Ber. Bunsenges. P h y s . Chem., 69, 716 (19653; (b) R. Gerdil and E. A. C. Lucken, J . A m e r . Chem. SOC.,88, 733 (1966); (c) S. Siege1 and J. S. Judeikis, J . P h y s . Chem., 70, 2205 (1966); (d) C. A. Pinkham and S. A. Wait, Jr., J . Mol. Spectrosc., 27, 326 (1968). (8) (a) R. N. Nurmukhametov and B. V. Gopov, Opt. Spectrosc., 18, 126 (1965); (b) K. B. Eisenthal, W. L. Peticolas, and K. E. Rieckhoff, J . Chem. Phys., 44,4492 (1966). (9) D, W, Wllis and B. 9.Solomon, {bid., 46, 3497 (1967).
CHARGE-TRAXSFER DONOR ABILITIESOF O,O'-BRIDGED BIPHENYLS
3443 ~~~~
Table I : Position of Charge-Transfer Bands, mp (eV), for Together with Their 'L, Bands" Carbazole
UDQ TCIVQ TCNE cA DT4Fd DTF~ T4NF TNF DDF~
~
Acceptors with Locked Biphenyl Donors,
--
7
Acceptorb
T
'La 293 (4.23)
Fluorene 'La 262 (4.73)
630 (1.96) 578 (2.15) 600 (2.06) 528 (2.35) 630 (1.96) 550 (2.25) 525,' (2,36) 480, (2.58) 490 (2.53)
623 (1.99) 605 (2.04) 567 (2.19) 500 (2.48) 557 (2.22) 523 (2.37) 490 (2.53) 440 (2.81) 477 (2.60)
__--
__
DonorDibenzothiophene
Phenanthrene
Dibenzofuran
'La 287 (4.32)
'La292 (4.24)
'La 298 (4.16)
600 (2.06) 525 (2,36) 550 (2.25) 470 (2.64) 540 (2.29) 495 (2.50) 480 (2.58) 440 (2.81) 470 (2.64)
582 (2.13) 520 (2.38) 532 (2.33) 482 (2.57) 510 (2.43) 485 (2.55) 460, (2.69) 435 (2.85) 460 (2.69)
575 (2.15) 510 (2.43) 500 (2.48) 455 (2.72) 485 (2.55) 477 (2.60)
...
... 440 (2.81)
a "UV Atlas of Organic Compounds," Plenum Press, New York, N. Y. *Abbreviations for acceptors: DDQ, dichlorodicyano-p-benzoquinone; TCNQ, tetracyanoquinodimethane; TCNE, tetracyanoethylene; CAI chloranil; DTIF, 2,4,5,7-tetranitrofluorene-Aga-ma~ononitrile; DTF, 2,4,7-trinitrofluoreiie-~ga-malononitrile;T4NF, 2,4,5,7-tetranitrofluorenone; TKF, 2,4,7-trinitrofluorerione; DDF, 2,7T. K. Mukherjee, Tetrahedron, 24,721 (1968) and previous papers. dinitrofluorene-A9"-malononitrile. Complex highly insoluble.
complexes described in this paper provided additional motivation to this investigation.
Experimental Section The donors and the acceptors were purified by recrystallization from appropriate solvents until the melting points agreed with the published values. Carbazole (Ib), dibenzothiophene (IC),and dibenzofuran (Ie) were further purified by sublimation under reduced pressure. 1,2-Benzofluorene (Aldrich) was used as supplied. Spectral grade methylene chloride was used. The measurements were performed on a Cary Rlodel 14 recording spectrophotometer. The concentrations of the donor and the acceptor were empirically changed until well-resolved charge-transfer bands were obtained. With some weaker acceptors, phenanthrene (Id) and dibenzofuran (Ie) gave shoulders ; the resolution could not be improved by filling the reference cell with the donor or the acceptor solutions. I n such cases, the charge-transfer maximum was isolated by extrapolation to the short-wavelength side, followed by the half-band width method. Stability constants were determined at 19" by the Benesi-Hildebrand procedure. For the most accurate values of the stability constants," the donor concentrations were varied within the range 0.38-1.4 X lo-' M , while the acceptor concentration was held constant at 5.4 X AI. Results and Discussion I n Table I, the charge-transfer energy (hv,t) for a series of complexes along with the 'La band transition of the donor is recorded.I2 The absorption bands of the complexes of each donor show a steady bathochromic shift with increasing acceptor strength, indicating that the observed bands are not due to ionpair complexes. Further, absorptions due to the radial ions of stronger acceptors were not observed. As an additional test of charge transfer, when (hvot)of the complexes of the different acceptors with a fixed donor
were plotted against the polarographic half-wave potentials l 3 of the acceptors, satisfactory straight lines were obtained. Presence of a heteroatom in the donor molecule (Ib, IC, Ie) requires that the nature of the chargetransfer complexes (n-r, r - ~ )be determined. In the case of the complexes with tetracyanoethylene, Cooper, Crowne, and Farrell14 did not consider this aspect. Since the excited states of the charge-transfer complexes are polarized, the solvent-effect technique, to distinguish n-T and r-r transitions is not useful. Further uncertainty is introduced by the failure to detect n-T transitions in carbazole, dibenzothiophene, and dibenzofuran.I5 Hence the formation of K-K complexes was verified by comparing the calculated and observed charge-transfer energies. If i t is assumed that these donors form r-r complexes similar to the aromatic hydrocarbons, then their charge-transfer energies are given by
where the parameters C1 and CZ have characteristic (10) T. K. Mukherjee, Abstracts, 156th National Meeting of the American Chemical Society, Atlantic City, N . J., Sepl 1968, No. PHYS 023. (11) W. B. Person, J . Amer. Chem. SOC.,87, 167 (1965). (12) There seems to be some uncertainty regarding the assignment of 'Laband in dibenzofuran. The long-wavelength side of the spectrum is more structured than that of carbazole, fluorene, and dibenzothiophene. The second high-energy peak near 298 mp (log E 4.0) is quite close to the first peak ai, 301 mp (log E 3.4). It is possible that some authors have missed this.i8pb The value of 297 mp (4.17 eV) for the 'L, band, as predicted from molecular orbital calculation, agrees well with the observation of the vapor-phase spectrum (298 mp, 4.16 eV) .id (13) E i / p d values were taken from: ( a ) G. Briegleb, Angew. Chem., 76,326 (1964); (b) T. K. Mukherjee, Tetrahedron, 24,721 (1968). (14) A. R. Cooper, C. W.P. Crowne, and P. G. Farrell, Trans. Faraday Soc., 6 2 , 18 (1966). (15) R. C. Heckman, J . M o l . Spectrosc., 2,27 (1958).
Volume 78, Number 10 October 1960
TAPAN K. MUKHERJEE
3444 Table 11: Calculated Position of t h e Charge-Transfer Band and Displacement from t h e
- hveXptl = AeV)
Experimental Value (hvoalo
Acceptor
-TCN+Donor (Ip)
Calod
Carbazole
(8.07) Dibenzofuran
(8.02) Dibenzothiophene
(8.14) Fluorene
(8.42) Phenanthrene
18.08) a
Reference 2, p 77.
(a) 2,24 ( b ) 2.16
(a) 2.20 ( b ) 2.12 ( a ) 2.30 (b) 2.23 ( a ) 2.55 (b) 2.49 ( a ) 2.25 ( b ) 2.17
--TCN&------r AeV
0.18 0.10 -0.28 -0.36 0.05 -0.02 0.36 0.30 -0.08 -0.16
Calcd
AeV
...
...
2.09
-0.06
... 2.05
...
... -0.38
...
2.16
-0.20
...
...
2 42
0.39
...
...
2.10
-0.28
-TNF--
C -ACalcd
AeV
2.55 2.48 2.50 2.43 2.62 2.54 2.88 2.79 2.56 2.49
0.20 0.13 -0.22 -0.29 -0.02 -0.10 0.40 0.31 -0.01 -0.18
Calod
AeV
... 2.76
... 2.71
.*. 2.82 , . .
3.07 ,
.
I
2.77
0.18
... ...
...
0.01
... 0.21 .,.
-0.8
' Reference 16.
values for each acceptor. Briegleb2 has determined the C1 and Czvalues for a number of acceptors. Becker and Chen, in addition to providing a few more values, used eq 1 to derive the ionization potentials for several hydrocarbon donors. Similarly, Kearns and coworkers17 distinguished the n-n and n--8 nature of the chargetransfer complexes of eight aza-aromatic donors. Since the direct measurement (by photoionization or electron-impact experiments) of the ionization potentials of most of the donors used in this work are not available, spectroscopically determined I , values from the equation18
were utilized in eq 1. In eq 2, vo represents the energy of the ' L a band of the donor, and the I , value corresponds to the gas-phase n-ionization potential. In Table 11, the energies of the charge-transfer bands and the corresponding displacements from the experimental values are shown. Considering the deviations inherent in the computations of I,, C1, and Cs, the agreement between the predicted and the observed values is excellent in phenanthrenelg and dibenzothiophene. The sulfur atom in dibenzothiophene, however, offers both n-type and s-type donor sites.20 Since no paramagnetic species were detectedzLin the solutions of dibenzothiophene with strong Lewis acids (vix., SbCL, A1C18, HIS04) the lone-pair electrons on the nonbonding orbital are not available for "stable" complex formation. The remaining pair is delocalized in the r-molecular orbitals of the adjacent ring.22 Thus, dibenzothiophene acts as a n donor toward the s acceptors. Table I also shows that dibenzothiophene is a slightly better donor than phenanthrene. On the basis of the ionization potentials (Ip), derived from the respective 'La bands, the reverse would be expected. Since the electronic structure of an excited molecule differs from the ground-state structure, it seems possible that the 3d orbitals of the sulfur atom have sinall contribution ~
The Journal of Physical Chemistry
to the n delocalization of the excited dibenzothiophene, which is reflected in the enhancement of the n-donor ability. Although conclusive experimental evidence is lacking, theoretical calculations using the SCF-MO method2a and the ultraviolet spectral studies of aromatic sulfur compounds24support the probability of some small contribution from the 3d orbitals in the through-conjugation of the sulfur-containing condensed heterocyclic systems.2s Due to the high ionization potential of the oxygen atom (0,13.6; S, 10.4),the 2p orbitals of oxygen are less effective in the overlap with the ring; consequently dibenzofuran (Ie) is less aromatic than dibenzothiophene. However, the discrepancy between the predicted and calculated C-T energy (Table 11),does not lend explicit support to n-n type of transitions in the complexes of dibenzofuran. Carbazole (Ib), as a secondary amine, is expected to behave primarily as an n donor, hence the large deviation. Since excited carbazole has greater tendency to hydrogen bonding,26 part of the shift may also be due to such interaction.27
(16) R. S. Becker and E. Chen, J . Chem. Phys., 45,2403 (1966). (17) D. R. Kearns, P, Gardner, and J. Carmody, J . Phys. Chem., 71, 931 (1967). (18) G. Briegleb, Angew Chem.,Int. Ed. Bngl., 3,617 (1964). (l9), I n the computation, using Becker and Chen'o CI, CZvalues, the deviations and correction factors have been neglected. Sinoe phenanthrene forms true T-T complexes with the aoceptora used in the present work, AeV values of other donors in lines (b) of Table 11 are to be compared with reference to AeV of the phenanthrene (line (b)). (20) The majority of the sulfur compounds are n donors: W. M. Moreau and K. Weiss, Chem. Rev., in press. We are indebted to Dr. Weiss for making the review available to us before publication. (21) M. Kinsoshita and H. Akamatu, Bull. Chem. SOC.J a p . , 35, 1040 (1962). (22) R. Zahradnik, Adoan. Heterocyclic Chem., 5, 1 (1965). (23) M. J. Bielefield and D. D. Fitts, J. Amer. Chem. SOC.,88, 4804 (1966). (24) A. I. Kiss, Acta Phys. Chem. Szeged, 45 (1960). (25) For a discussion of p r - d r bonding and the valence shell expansion of sulfur heterocycles, see W. G. Salmond, Quart. Rev (London), 22, 253 (1968).
PHOTOCHEMISTRY OF AQUEOUS XITRATESOLUTIONS
Table 111. Position of Charge-Transfer Bands, mp (eV), for 7r Acceptors with Chrysene and 1,2-Benzofluorene Donor-Acceptor'
Chrysene 'La 319 mr (3.38 eV)
1,Z-Benzofluorene 1L. 316 (3.92 eV)
DDQ TCNE CA TiNF TNF DTF
602' ( 2 . 0 5 ) 628 (1.97) 541 (2.29) 525 5" (2.36) 480 rt 10 (2.58) 576 (2.15)
738 (1.68), 510 (2.43) 643 (1.92) 555 (2.23) 545 (2.27) 492 jI 5 (2.52) 588 (2.10)
a For abbreviations, see Table I. *Reference 28. insoluble.
e
Very
Fluorene (Ia) shows the largest deviation (AeV, Table 11). There is no possibility of n-n transition in the fluorene complexes. On the basis of its high ionization potential and the presence of the insulating -CH2group, fluorene is expected to be the weakest T donor. However, the positions of the C-T bands show that it is a better donor than its conjugated analog, phenanthrene (Table I). The stability constant of the fluorenechloranil complex (K,,, 0.61 1. mol-'; E,,, 1236 1. mol-'
3445 cm-l) is found to be quite close to that of phenanthrene (K,,, 1.27 1. mol-'; E,,, 1365 1. mol-' cm-l). Srivastava and Prasad2*found that the fluorene-DDQ complex has a slightly higher stability constant than the phenanthrene-DDQ complex. As an extension of this study, the charge-transfer transitions of lJ2-benzofluorene were compared with those of its fully aromatic analog, chrysene (Table 111). Again, the fluorene derivative turns out to be a consistently better donor than chrysene. It is obvious that the polarizing effect of the hybridized carbon orbitals (sp3) of the C-H bond plays a profound role in the donor ability of fluorene and its bene analog. Such polarization causes the weakening of the C-H bond, as is evidenced from the facile formation of fluorenide anion in the presence of proton acceptors. (26) N. Mataga, Y. Torihashi, and K. Esumi, Theor. Chim. Acta, 2, 158 (1964). (27) Grayish precipitate mixed with white solid separated from DDQcarbazole and TINF-carbazole solutions, which are assumed t o be salts and were not further investigated. Carbazole-TCNE and carbazole-TCNQ solutions showed epr absorption indicative of electrontransfer reactions. (28) R. D. Srivastava and G. Prasad, Spectrochim. Acta, 22, 1869 (1966).
On the Photochemistry of Aqueous Nitrate Solutions Excited in the 195-nm Band by Uri Shuali, Michael Ottolenghi, Joseph Rabani, and Ziva Yelin Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
(Received March 11, 1969)
The photochemistry of aqueous nitrate solutions excited in the high-energy (195-nm) mr* band of the ion is investigated using steady-state and flash-irradiation techniques. The formation of hydroxyl radicals is demonstrated by observing the absorption of characteristic transients formed by the reaction of O H (or 0-) with C082-, CNS-, and 02. T h e formation of pernitrite is also investigated and found to be unaffected by [NO2-], [NOI-], [02], and [CH,CH20H]. The flash technique enables the determination of the dissociation constant of pernitrous acid, yielding pK, = 6.0 =t0.3. A third photochemical path, the evolution of 02 and the stoichiometric formation of NOz-, is also investigated. T h e effect of various solutes on the yields of molecular oxygen is analyzed, The results appear to be inconsistent with a mechanism involving intermediate oxygen atoms.
Introduction The photochemistry of the nitrate ion in aqueous solutions has been a subject of various The evolution of molecular oxygen and the formation of nitrite have been previously interpreted in terms of a primary act involving a dissociation, yielding oxygen atoms according to (NOa-)* 4 NO,0 (1)
+
The recent investigation of Daniels, et aL16in which the low-energy (300-nm) transition was excited, pro-
vided evidence for the occurrence of a competing dissociation (1) 0. Baudisch and E. Mayer, Ber,, 45, 1771 (1912); 0. Baudisoh and F. Bedford, i~'aturwissenscha~~en, 24,361 (1936), (2) E. Warburg, Sitzber. Preuss. Akad. Wiss. Phys. Math. Kl., 1228 (1918); 2.Elektrochem., 25,334 (1919). (3) W. T. Anderson, Jr., J . Amer. Chem. SOC.,46, 797 (1924). (4) D. S. Villars, ibid., 49,326 (1927). (5) H. hl. Papee and G. L. Petriconi, Nature, 204, 142 (1964). (6) M. Daniels, R. V. Meyers, and E. V. Belardo, J . Phys. Chem., 72, 389 (1968).
Volume 73, Number 10 October 196'9
