involving polar neutrals, binding in Mnaph'mB, where the charge on the ion is delocalized, ..... Therefore, we defined the .... metal-to-ligand charge-transfer (3MLCT) excited states, with CN- and i-biq playing the role of "spectator" ligands. The.
1088
J . Phys. Chem. 1987, 91, 1088-1095
Ionic Charge-Transfer Complexes. 3. Delocalized .Ir-Systems as Electron Acceptors and Donors. Dimer Cations of Naphthalene Derivatives M. Samy El-Shall*+and Michael Meot-Ner (Mautner)t Department of Chemistry, Georgetown University, Washington, DC 20057, and Chemical Kinetics Division, Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: August 25, 1986)
Dissociation energies (AH",) of the complexes of the charge-delocalized 1-methylnaphthalene+(Mnaph') cation with Mnaph, mesitylene, and benzene were measured by pulsed high-pressure mass spectrometry as 18.8, 13.9, and 9.0 kcal/mol, respectively. AH", decreases with increasing difference in the ionization potentials (IP) of the components. This suggests a significant Mnaph-Mnaph' in the symmetric dimer where AIP = 0. In contribution of charge-transfer resonance Mnaph'mMnaph this dimer, AHo,, is estimated as about 5 kcal/mol. This is similar to AH',, in benzene,' and aniline,', despite the different sizes and charge delocalizations of the *-systems. In mixed dimers, AH",,, decreases with increasing AIP more rapidly in the delocalized Mnaph+-Bseries (B is any of the neutral molecules used) than in the An'SB (An = aniline) series. In complexes involving polar neutrals, binding in Mnaph'mB, where the charge on the ion is delocalized, is weaker by 2-5 kcal/mol than in An+.B, where the negative pole can interact with a localized charge center. To elucidate the nature of the interactions in delocalized *-complexes such as Mnaph,', extended Huckel theory calculations are applied to benzene2+,naphthalene2+, and anthracene,'. In all cases, electronic interaction is strongest in the eclipsed geometry but other electronic energy minima are also calculated at slips 6(x) = 3 %, and 6b)= 3.2-4.2 A along the molecular axes. Calculations show that AHo,,/ring decreases and AH",,, increases and reaches a constant limit in dimers of increasingly large polycyclic aromatics.
-
Introduction
Bonding between radical cations and neutral molecules is of obvious interest in gaseous ion chemistry,' mass spectrometry,, and radiation chemistry3 and also in condensed-phase chemistry. For example, ion-neutral interactions play a role in determining ionization potentials in the condensed phase; ionized states, in turn, contribute to conductivity in organic semiconductor^.^ Therefore, ionic complexes formed between organic radical cations and neutral molecules are interesting both from practical and theoretical points of view. In preceding work we studied complexes involving benzene derivatives,5 polycyclic aromatic hydrocarbons,6 and aniline derivatives.' In these ionic complexes, the electron acceptor was a molecular radical ion, which is a highly electron deficient Aacceptor, and the neutral aromatic molecule was a *-electron donor. The complexes are of a parallel plane sandwich type, and bonding energies are 10-20 kcal/mol. Estimated charge-transfer resonance contributes up to 6 kcal/mol to AH",. Resonance charge transfer decreases with increasing ionization potential (IP) difference between the components (AIP), as the component with the lower IP increasingly retains the charge. The bonding in these complexes results from the combination of electrostatic, polarization, dispersion, and short-range repulsion interactions, in addition to charge-transfer resonance.* Electrostatic and charge-transfer resonance interactions strengthen the bonds in (A-B)' ionic complexes compared with that in (AaB) neutral complexes. However, the detailed variation of the bonding components depends on the structure of both the ion and the neutral molecule. Therefore, information on the nature of the bonding requires a comparison of association reactions of different neutrals with the same ion or of the same neutral with different ions. In the present work we study the ionic complexes formed between the charge-delocalized radical cation CIIHlo+,l-methylnaphthalene (Mnaph'), and some neutral organic molecules. To examine the effects of charge delocalization in the ion, these will be compared with complexes involving the smaller partially charge-localized electron acceptor aniline+ (An'), whose dissociation enegies (AH",)were determined previously.' To examine 'Georgetown University. Present address: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024. INational Bureau of Standards.
0022-3654/87/2091-l088$01.50/0
the effects of electron delocalization in the neutral component, we shall compare complexes of naphthalene derivatives to complexes of other electron donors with (An') or the electron acceptor. These studies should provide increased understanding of the effect of the variation of electron delocalization and molecular size on the electrostatic and resonance interactions in ionic complexes. In the present work, Mnaph,+ serves as an example of a symmetric delocalized aromatic ionic complex. To better understand the binding in this and similar complexes, we shall present qualitative molecular orbital theory predictions concerning the geometries and the electron interaction energies of aromatic ionic complexes. Experimental Section
The measurements were performed by using the NBS pulsed high-pressure mass spectrometer. This mass spectrometer has been described in a number of publication^.^-" It consists of a temperature-controlled reaction chamber in which a suitable reaction mixture, at 0.1-1.5 Torr total pressure, is irradiated by short high-energy electron pulses. The ions are created by a 500-1 000-ms pulse of 500-1000-eV electrons from an electron gun. The ions react and the reactions reach equilibrium as the ions diffuse toward the walls of the ion source. Ions diffusing to the vicinity of a small pinhole are carried by gas flow into an evacuated region pumped by a high-capacity pumping system. (1) Ferguson, E. E. Atmospheres of Earth and the Planets; McCormac, B. M., Ed.; Reidel: Dordrecht, 1975. (2) Franklin, J. L.; Harland, P. W. Annu. Rev. Pbys. Chem. 1974,25,485. (3) Ausloos, P.: Lias, S. G. Ion-Molecule Reactions and Their Role in Radiation Chemistry; American Chemical Society: Washington, DC, 1975. (4) Gutman, F.: Lyons, L. E. Organic Semiconductors; Wiley: New York, 1967; p 331. (5) Meot-Ner (Mautner) M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J .
A m . Chem. SOC.1978, 100, 5466. (6) Meot-Ner (Mautner), M. J . Phys. Chem. 1980, 84, 2724. (7) Meot-Ner (Mautner), M.; El-Shall, M. S. J . A m . Chem. SOC.1986, 108, 4386.
(8) Morokuma, K. Ace. Chem. Res. 1977, 10, 294. (9) Meot-Ner (Mautner), M.; Sieck, L. W. J . Am. Cbem. SOC.1983,105, 2956.
(IO) Sieck, L. W. Anal. Chem. 1979, 51, 128. ( 1 1) (a) Sieck, L. W.: Searles, S. K.; Ausloos, P. J . A m . Chem. SOC.1969, 91, 7627. (b) Sieck, L. W.; Searles, S. K. Ibid. 1970, 92, 2937. (c) Sieck, L. W.; Meot-Ner (Mautner), M. J . Phys. Chem. 1984.88, 5324. (d) El-Shall, M. S. Ph.D. Thesis, Georgetown University, Washington, DC, 1985.
0 1987 American Chemical Society
Ionic Charge-Transfer Complexes Here the ions are captured by electric fields, accelerated, and subjected to quadrupole mass analysis. The ion signal corresponding to each reactant ion is followed as a function of reaction time on a multichannel scaler. The reactions are observed to 2-10 ms, in channels of 10- or 20-ps widths. Since the concentration of the neutral molecules is constant, the intensity ratios of ions at equilibrium become constant. The ions are thermalized by 10-10000 collisions with an inert third gas, C6H6 in this study, between reactive collisions. The complete instrument consists of three operationally separable components: (1) a reaction chamber associated with a gas handling and pressure control system, (2) electron source, and (3) a mass analysis and detection section. Schematic diagrams and detailed descriptions of these components are given in ref 1Id and brief descriptions are given below. The ion source (reaction chamber) is fabricated from a single block of stainless steel 12 cm in length and 5 cm in diameter. The source has been described in detail elsewhere.llc Gas samples are admitted to the ion source from a heated 3-L bulb at selected ion source pressures in the 0.2-1 .O-Torr range via an adjustable needle valve. The pressure is measured with a capacitance manometer coupled to the source cavity by a 1-cm-diameter stainless steel tube. A 0.1-mm electron entrance hole is placed perpendicular to the axis passing through the inlet tube. Ions exit the source through a 0.06-mm-diameter hole (which is placed along the same axis passing through the inlet tube) to a low-pressure region. Ions freely diffusing from the reaction chamber through the Torr as ion exit hole exit to a low-pressure region (p C 5 X measured -30 cm from the source) where they are accelerated through a potential of 8 V/cm, obtained by a +lo-V potential applied between the ion source and the first aperture of an Einzel lens system. Mass analysis is achieved by a quadrupole mass filter. Ions are detected by an electron multiplier operating at a gain of lo6. Corrections were made for a slight mass dissociation by the quadrupole mass filter and the detector. The chemicals were purchased from commercial sources at stated purities of 98% or higher and used as supplied, except aniline which was used after vacuum redistillation. Liquid mixtures of 0.01-0.04% of Mnaph in benzene and 1-10% of B in benzene (where B is any of the neutral molecules used) were injected into a heated bulb. The mixture was allowed to flow to the ion source and benzene served as both the carrier gas and the source of the reactant ion C6H6+. Ion source temperatures of 250-450 K and pressures of 0.2-1 .O Torr were used. Checks were made to ensure that the equilibrium constants were independent of pressure and mixture composition. Conditions (concentration, pressure, and temperature) were selected to keep associated ions to less than 40% of the total ion intensity to minimize problems resulting from dissociation outside the ion source. In some experiments impurity ions were observed, but the buildup of impurity ions was slow compared with the establishment of equilibria. Therefore the presence of impurities did not affect the measured equilibrium constants. The temperature of the source was measured to an accuracy of f l "C. As was noted before,I2 the largest source of error in measuring gas-phase ion equilibria is the determination of the pressure of involatile compounds in the source. This is estimated (in the worst case) to be accurate within a factor of 2. This introduces an error of f 0 . 4 4 kcal/mol in the measurement of ACoD at the median temperature of these measurements, 320 K. The precision of measurements of AGO, was also estimated from the average of the results of 15 pairs of duplicate measurements as *0.41 kcal/mol. Therefore the magnitude of the error is comparable with the precision of measured A G O D values. The average of the standard deviations of the slopes and the intercepts of the van't Hoff plots give an error estimate of f0.5 kcal/mol for A H O D and f1.4 cal/(mol K) for ASoD. To allow for the possibility of unrecognized systematic errors in the measurements, the quantities given in Table I are assigned uncer(12) Meot-Ner (Mautner), M. J . Phys. Chem. 1980, 84, 2716.
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1089 TABLE I: Thermodynamic Quantities" for the Dissociation of Cationic Complexes radical cation B A H O D A P D b AcoDc ( r ) 1 Mnaph+ Mnaph 18.8 30.1 9.9 (296) 2 13.9 29.7 5.2 (295) mesitylene 3 24.0 2.2 (283) benzene 9.0 4 (14.7) (26) 6.9 (301) benzonitrile 5 26.3 5.2 (300) nitrobenzene 13.1 (11.3) (22) 4.6 (303) acetonitrile 6 7 22.3 4.4 (303) nitromethane 11.2 8 An*e 15.8 8.1 (322) Mnaph 7.1 (299) 9 1-Bromonaph 15.8 (1 5.4) 7.3 (324) 10 Naph
AIP~ 0 11.5
29.3 38.5 45.7
65.3 71.9 5.8
6.9 9.9
"All energies in kcal/mol, entropies in cal/(mol K). Mnaph = 1methylnaphthalene; Naph = naphthalene; An = aniline; 1-Bromonaph = 1-bromonaphthalene. ASo, values in parentheses are estimated from similar reactions. A H O D values in these cases calculated from AGOD - TASoD.Error estimate: 1 kcal/mol. eAG values given here. Temperature in degrees Kelvin. dAIP is the difference between the ionization potentials of the two molecules in the complex. IP values from ref 23. eSee ref 7. tainties of 1 kcal/mol, 3 cal/(mol K), and 0.5 kcal/mol for WD, A s O D , and A G O D , respectively.
Results Under the conditions described in the Experimental Section, ions were generated by the following typical sequence of chemical ionization processes:
- + + + + + Bz
Bz+
+e
Bz'
Mnaph
+ Bz Mnaph' + 2Bz
Mnaph'
or ( B Z ) ~ + Mnaph Mnaph+ Mnaph'
Mnaph Bz
2e
(Mnaph)2+
(Mnaph'eBz)
(1)
(3)
(4) (5)
The concentration of Mnaph was in the range of 0.01-0.05% in benzene (Bz), and reactions 1-5 were completed and observed within a few microseconds after the ionizing electron pulse. Other processes such as direct ionization of Mnaph by electron impact also occur to a small extent. However, because of the relative concentrations of the components of the reaction mixture, ionization of Mnaph was predominantly by reaction 3. In the study of (Mnaph+.B) complexes, where B is a neutral molecule, the concentration of Mnaph was less than 0.01% in Bz and [B] was in the range of 1-10% in Bz. In addition to reactions 1-5, the following reactions could be observed:
+B B+ + B B+ + BZ
Mnaph'
-
(Mnaph'eB)
(6)
(B)*+
(7)
(B+*Bz)
(8)
The interference of reaction 4 with equilibrium 6 was minimized by using only trace concentrations of Mnaph, usually below 0.01% of the total gas in the ion source. The presence of multiple equilibria should not, of course, affect ion concentrations at thermodynamic equilibrium. Generally, dimerization equilibria (4)were measured with only Mnaph and the reagent gas, Bz, in the ion source; thus equilibria 4 and 5 were obtained simultaneously. In cases where (4) was measured in both the absence and presence of B, the value of the measured equilibrium constant, K , was not affected by simultaneous presence of equilibrium 6. The ion intensities taken from the time-independent portions of the normalized ion intensities were used for equilibrium calcu-
El-Shall and Meot-Ner (Mautner)
1090 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
I9t
t
,&
I
c
TABLE II: Comparison between Dissociation Energies"of Different Complexes Having Comparable Values of AIPb*' reactant ion reactant neutral AHOn AIP 17.2 11.9 acenaphthylene 11 ani 1in e+ 14.6 11.9 12 azulene+ fluorene 13.9 11.5 mesitylene 13 Mnaph' 13.3 11.2 fluorene 14 biphenylene' 15.4 9.9 naphthalene 15 ani 1in e+ 12.0 10.0 toluene 16 mesitylene+ 12.4 9.3 17 toluene+ benzene 15.8 5.8 18 ani 1in e+ Mnaph 14.0 6.2 aniline 19 azulene+ 14.0 6.2 20 chlorobenzene' benzene 15.8 6.9 1-bromonaphthalene 21 ani 1in e+ 7.0 1,2,3,5-tetrafluorobenzene 11.2 22 benzene' 13.6 29.5 bromobenzene 23 aniline+ 9.0 29.3 benzene 24 Mnaph'
NO2
I
E
L
Q
E;
C
"All energies in kcal/mol. bAIP is the difference between the ionization potentials of the two molecules in the complex. EData from present work and ref 5-7. DDI
ECliP8Od ConfiQutrtion8
-Mi
c
1
30
%
'
"
3.2
'
"
1
34
34
"
3.8
'
4.4
-
103 K(K)
Figure 1. van't Hoff plots for the association equilibria l-methylnaphthalene+ + B + l-methylnaphthalene+.Bwith molecules B as indica ted.
lations. Equilibrium constants were calculated from the expression
P ,#"
0.1 #
E
sa
L e a
\
-
\ \ 09s
\
Z(Mnaph+-B) K =
I( Mnaph+)P(tot)P(B)
\
(9)
where Z(Mnaph+.B) and Z(Mnaph+) denote ion signal intensities at equilibrium, P(B) is the mole fraction of B, and P(tot) is the total pressure in the ion source in atm. For ( M n a ~ h ) ~ 'B, must be replaced by Mnaph. Temperature studies were applied to several systems to determine M O D and M O D . van't Hoff plots are presented in Figure 1 and the measured thermochemical values are given in Table I. For some reactions, AGOD was measured only at one temperature, and A H o D was obtained by using ASoD estimated from similar reactions.
Discussion 1. The Contribution of Charge-Transfer Resonance. It is generally a c ~ e p t e d ' ~that - ' ~ association ions of aromatic molecules have a sandwich structure; Le., the two molecules are parallel. In terms of resonance theory the resonance structures (A+.B) and (A-B+) will both contribute to the ground-state wave function of the association complex. These contributions depend on the energy difference between the two resonance forms. Electron delocalization, resulting in a lowering of the energy and hence stronger bonding, requires that the resonance structures have comparable energies. The effect will be maximized for association ions where (13) Field, F. H.; Hamlet, P.;Libby, W. F. J . Am. Chem. Soc. 1969, 91,
2839. (14) Lewis, I. C.; Singer, C. S. J . Chem. Phys. 1965, 43, 27 12. (15) Howarth, 0. W.; Fraenkel, G. K. J. Am. Chem. Soc. 1966.88.4514. (16) Badger, B.; Brocklehurst, B. Nature (London) 1968, 219, 263. (17) Jones, E. G.; Battacharya, A. K.; Tierman, T. 0. Int. J . Mass
Spectrom. Ion Processes 1975, 17, 147. (18) Hanschmann, G.; Helmstreit, W. J. Praktisch. Chem. 1980, 322, (6), 981
\ \ '
-0.3
\
',
B
\
b '\
-0.4
u
9.0
d
u
(A)
Figure 2. Frontier resonance energy, AEF,, as a function of interplanar distance, d , in the eclipsed conformation for the dimers as indicated.
the resonance structures (A+.B) and (A.B+) have identical energies. The increased bonding in (Mnaph)*+ is attributed to the MnapheMnaph'. The occurrence resonance Mnaph+.Mnaph of such resonance in the radical ion dimers is supported by EPR results which show that in (na~hthalene)~+ the charge is equally distributed between the two c~mponents.'~ The effect will diminish as AIP increases in association ions composed of molecules with different ionization potentials because the charge is expected to be localized on the molecule with the lower ionization potential. In the limit of very high AIP values only electrostatic interactions should contribute to the bonding. The resonance energy in (Mnaph),' can be estimated by subtracting A H O D of the even-electron, closed-shell dimer ion (Naph)2Ht, 14.1 kcal/mol,6 from that of (Mnaph),+, 18.8 kcal/mol. We assume that the sums of electrostatic, induction.
-
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Ionic Charge-Transfer Complexes
1091
0.3
0.2
. U P : ? I
"0.1
0.0
-0.1
- 0.2
- 0.3 -0.4
I
0.0
I
I
1.0
2.0
I 3.0
I
1 4.0
bo( 1 ;A" Figure 3. Frontier resonance energy, AEFrCs, as a function of slip along the x axis for the dimers as indicated. y = 0 in all dimers; x = 2.8 A in (Bz)*+, 2.7 A in (Naph)*+and (Anth),+.
dispersion, and repulsion forces are comparable in the two complexes. However, charge resonance is of course not possible in (Naph)zH+. This estimate yields 4.7 kcal/mol as the resonance energy in (Mnaph),'. This value is an upper limit since ion-dipole and dispersion interactions are higher in (Mnaph),' than in (Naph)2+H. 2. Structural Effects on AHo,,, in Symmetric Dimers. It is important to compare the resonance energies in several symmetric dimers in order to see if there is any structural effect on the resonance energy. In (Bz),+, (An),', and (Mnaph),', AHo,, is estimated to be 6 , 5.6, and 4.7 kcal/mol, respectively.' Within the accuracy of the estimates, Mor,is similar in these systems,
although in (An)2+the ionic acceptor is partially charge localized and the neutral donor is partially electron localized, while in (Bz),+ and (Mnaph),+ both components are delocalized. At the limit of AIP = 0, the resonance energy therefore appears to be insensitive to the charge localization or delocalization of the components. Another interesting question is the dependence of the resonance energy on the size of the *-system. Considering that AHo,, for (Mnaph),+, 4.7 kcal/mol, may be an overestimated value, one may conclude that the resonance energy decreases in going from (Bz),+ to the larger dimer (Mnaph),+. A theoretical investigation of this point, based on extended Hiickel theory calculations (EHT), will be given later.
1092 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
El-Shall and Meot-Ner (Mautner)
0.;
Qt
O.!
0.A
0.:
I
0.2
fi\
I I
:
\ \
I
.
\
I
U J
\
I
0.1
\d\
I I
0)
m L W
Q
\
\
0.0
- 0.1
- 0.2
-0.3
- 0.4
1
2
3
5
4
6
7
8
6 (Y) :i Figure 4. Frontier resonance energy, AEF,, as a function of slip along they axis for the dimers as indicated. x = O i n all dimers; z = 2.8 A in ( B Z ) ~ + , 2.7 8, in (Naph)2' and ( A I I ~ ~ ) ~ ' . '
3. Structural Effects in Mixed Dimers. In complexes 1-3 in Table I the naphthalene' moiety is the ionic electron acceptor. In this series, M O D decreases rapidly with increasing AIP, reaching a lower limit (9.0 kcal/mol) in complex 3 where AIP = 29.3 kcal/mol. A similar trend was observed before6 in complexes of radical cations with charge-delocalized a-systems. Thus in the Bz' and biphenylene' series, AHOD reaches a lower limit already at AIP N 17 and 16 kcal/mol, respectively. In comparison, the decrease of AHo, with increasing AIP appears more gradual in complexes of the charge-localized An' with hydrocarbon *-donors.' In order to separate the effects of AIP and ion structure on
Mom, we compare the dissociation energies of different complexes that have comparable values of AIP. This comparison is shown in Table 11. The complexes in Table I1 can be classified into five groups according to AIP, namely a (1 1-14), b (l5-17), c (18-20), d (21, 22), and e (23, 24). The most stable complex in any of these groups is that involving the charge-localized An+ radical cation. A similar trend was observed also in complexes of increasingly charge-delocalized carbonium ions C2H5+,i-C3H7+, etc. with n-donor bases C H Q , CH2Cl2,CF4, and S02F,, where the least delocalized ions gave the strongest c o m p l e ~ e s . ' ~ Increased charge localization on the cation should increase the ion-induced dipole interactions. This was illustrated before6 by
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1093
Ionic Charge-Transfer Complexes two models of (anthracene)2+. A hypothetical charge-localized model gave 8.55 kcal/mol for the ion-induced dipole interaction, while a more realistic charge-delocalized model gave only 4.4 kcal/mol. Increased charge delocalization in the ion should also affect the charge-transfer resonance interaction, as will be discussed below. 4 . Complexes with Highly Polar Neutral Ligands. In a preceding study,' we observed that cyano and nitro compounds formed strong complexes (AHo, = 14.4-19.4 kcal/mol) with An+ despite large AIP values (44-77 kcal/mol). This suggested that, in these complexes, the ligands are oriented in a linear geometry with the negative end of the dipole pointing to the -NH2' charge center in aniline+. In contrast, the highly polar cyano and nitro compounds do not form strongly bonded complexes with Mnaph'. AHo, for Mnaph+.B complexes is smaller by 1.5-4.7 kcal/mol than in An+.B for these highly polar ligands. It is clear that the ion-dipole interaction is more effective in An+ than in Mnaph' complexes with the same polar molecules. This results again from charge delocalization in Mnaph' since there is no strongly favorable site in the radical cation to interact efficiently with the negative pole. The larger AHOD values in complexes 4 and 5 compared to 6 and 7 are attributed to the induction interaction which remlts from the added polarizability of the phenyl ring in benzonitrile or nitrobenzene. Since strong induction and dispersion interactions are not possible in complexes 6 and 7, one has to assume that in these complexes most of the bonding is due to the ion-dipole forces. A perpendicular geometry seems to be reasonable for these complexes. Structures I and I1 represent possible geometries for C H ~
CH3
I
I
I
I1
complexes 6 and 7 . The measured low ASo, value for (Mnaph+CH,NO,), 22.3 3 cal/(mol K), is consistent with the perpendicular structure which allows free rotation about the ion-dipole bond. In comparison, in the more hindered sandwhich-type structures, such as (Mnaph)2+,ASOD was 30.1 cal/ (mol K). 5 . Qualitative Features of Resonance Energies in Symmetric Dimers from Extended Hiickel Theory Calculations. Mass spectrometric measurements yield information only on the energies of the complexes but not on geometries or energy components. Such information can be obtained from theoretical calculations. High-level ab initio calculations would be preferred, but these are not possible for the present large systems. Fortunately we foundz0 that certain aspects of ab initio STO-3G results on An+-An, An+.Bz, and Bz+.Bz are reproduced well by simple extended Huckel theory (EHT) calculations. In particular, the STO-3G energy vs. slip diagrams are reproduced well by EHT, which indicates that orbital overlap between the components, the main factor in EHT calculations, is crucial in determining the energy minima and maxima along the slip of the two monomers. To obtain further insight into the structural dependence of the resonance energy in hydrocarbon complexes such as 1-3 in Table I, we carried out E H T calculations on the symmetric dimers (ben~ene)~',(na~hthalene)~'.and (anthra~ene)~'.We are not attempting to use E H T to calculate the binding energies in these dimers. Our main concern is to find an expression for the resonance energy, in terms of the overlap between the H O M O s of the two fragments, and to see how this term varies with the conformations of the dimer.
*
0 0.08 *1°*
0.04
t
0.02t:
,I
111
-0.02y
-
\
-0.04 -
-
-0.06 -0.08 -0.10
1
"
1
1
1
1
1
"
0.0 0.5 1.0 1.5 2.0 2.5 0.0 3.5 4.0 4.5
5.0
d (A')
Figure 5. Frontier resonance energy, AEFImr vs. slip, 6, for (aniline'. naphthalene) along the x and y axes. Conformations are given in Figure 6.
In the monomers M and M+, a total of three electrons are in HOMO's of M and M+ (using Koopmans' theorem and equating the sum of the orbital energies with the total energy of the molecule). Each of these electrons is assigned the energy t . In the complex M2+, one electron is in the H O M O of M 2 and is assigned the energy e l , and two electrons are in the n-MO of M2 that results from the interaction of H O M O s of the two molecules; each of these is assigned the energy c2. Therefore, we defined the frontier resonance energy, hEFIs,for the dimer cation M2+,where M is Bz, Naph, or Anth, as hEFres =
(tl
+ 2 9 ) - 3c
AEF, was calculated as a function of the motion of one monomer with respect to the other along the x , y , and z axes, where the interplanar distance is taken along the z axis. The results are shown in Figures 2-4. The following features in Figures 2-4 should be noted. First, there is a minimum in AEFreB for each system at the eclipsed conformation which occurs at z = 2.8, 2.7, and 2.7 A for (Bz),+, (Naph),', and (Anth)z+,respectively. In order to interpret these unreasonably small separations, it must be remembered that these quantities were obtained considering only the charge resonance due to the overlap between the HOMO's with complete neglect of the overlap due to other orbitals. The inclusion of all doubly occupied orbitals (u n) resulted in optimized interplanar distances of 3.6, 3.9, and 4.0 A for (Bz),+, ( N a ~ h ) ~and + , (Anth),+, respectively. The important point is that the minima at the eclipsed conformations reflect an optimum phasing of AO's (between monomers) that compose the n-MO of the highest occupied level of the dimer. Second, the motion along the x axis is characterized by another minimum which appears around x = 3.0 A. Third, the motion along they axis (the long molecular axis in Naph and Anth) is characterized by a single minimum in each of (Bz),' and (Naph),+ around y = 3.1 and 4.2 A, respectively, and two minima in (Anth),' at 3.5 and 7.0 A. Finally, turning to somewhat less reliable numerical results one can infer that EHT predicts that relative values of AEFresin the most stable configuration (through the parallel geometry) increase slowly in going from ( B z ) ~ +to (Naph)2+ to (Anth)2+. However, the values of (AEFres/n),where n is the number of aromatic rings in the monomer, decrease as n increases. This means that the resonance energy increases slower than n. This suggests that the resonance
+
1094 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
El-Shall and Meot-Ner (Mautner) 0.3 9 5
0.228
N 0.115
-0274
0*37-~
0.1 3 7 0.188
- 0.29 5 -0.1 6 7
a392
0.1 92
- 0.1 6
-0296
-0.187
-0949
0.303
0.3 1 1
0.2 8 3 -0.122
a365
-0.07 6
-0.3 3 1
- 0.2'0 O30.0'4 48 - 0.1 4 5
-0.36
0 .0.0 24 30 6
0.1 3 7
6 ( ~ )= 6b) = 0, AEFres = -0.46
6b)= 3.9, AEFres
= 1.97
& ( x ) = 2 . 8 , AEFres = -0.55
Figure 6. The HOMO's of (aniline'mnaphthalene) complexes in the conformations corresponding to minima in AEFresvs. 6 plot (Figure 5 ) .
energy may reach a higher value and then remain nearly constant as n increases. In other words, the contribution to the resonance energy should be the same in all large polycyclic dimer cations. The variation of A H o Dof the radical ions with molecular size was found experimentally to exhibit an irregular trend.6 AHOD first remains approximately constant from c 6 (benzene) through CJ4(anthracene) compounds, then its value increases with increasing size. This irregular behavior may be understood in terms of the combined contributions to AHoD from electrostatic, dispersion, and resonance interactions. These contributions may vary with molecular size as follows: (i) The resonance energy, AE,,,increases slowly with increasing molecular size and probably reaches an upper limit. (ii) The induction energy (ion-induced), decreases with increasing molecular size. (iii) The combined dispersion, AEdSp,and repulsive interaction increases (more binding) slowly with increasing size.6 It is suggested that the decrease in is approximately balanced by the increase in both AE,, and AEdep and thus AHoD remains nearly constant. After AE,, reaches its upper limit and A&,d reaches its lower limit, the overall value of M O D increases due primarily to the increase in dispersion energies in the large dimer cations. These conclusions, based on the EHT results, are different from the previous, purely experiment-based interpretation, that AE,,, decreases with increasing molecular size.6 Extrapolating to the largest member of the polycyclic series, graphite, one can infer that resonance interactions between an ionized and a neutral layer of graphite can be approximated by calculating AEFr, for a model polycyclic dimer cation containing a large number of rings. It is also noted that the magnitude of U l n d decreases with increasing size and thus may be expected to become negligible in graphite. With AElndvanishing, AE,, leveling out at a small value, and AEdlSpincreasing, the former specifically ion-neutral interactions (AE,,, and AElnd)may be expected to make only a small contribution to the interaction between a singly ionized or protonated layer and an adjacent neutral layer in graphite. The previous electrostatic calculations6 and the present E H T results give only qualitative suggestions as to the variation of different energy components among the series of polycyclic dimer cations. More sophisticated calculations of these energy components are needed. 6. Application of Extended Hiickel Theory to (Aniline'. Naphthalene) Dimer Cation. In the preceding section, E H T was applied to symmetric polycyclic aromatic dimer ions, as models for (Mnaph),' and similar species. As we saw from the experimental results, complexes of aromatic donors with the localized ion An' are, for a given AIP, stronger than complexes o f delocalized ions. This can be related in part to orbital overlaps in the complexes. To illustrate this point we carried out calculations on the aniline+.naphthalene (An'aNaph) dimer cation. In this
system the difference in ionization potentials, AIP, between An and Naph is 9.9 kcal/mol. The frontier resonance energy, UF,,, was calculated according to
+
AEFres= [tHoMo(An.Naph) 2tMo(An-Naph)] [ ~ H o M o ( A-t ~ )2 ~ ~ o ~ o ( N a P h(1)1) l where tMo(An.Naph) is the energy of the MO of (An-Naph) that results from the interaction between the H O M O of An and the HOMO of Naph. The parallel conformation was adopted in which the two molecules were separated by 3.1 A, which is a reasonable assumed interplanar distance for aromatic dimer cations.*' The directions of varying the slip parameter, 6, along the x and y axes (the short and long molecular axes in Naph, respectively) are shown in I11 and IV. In Figure 5, AEFresis plotted as a function
I1I
IV
of 6 ( x ) and 6b). There are minima at 6 ( x ) = 6 ( y ) = 0, 6 ( x ) = 2.8 A, and 6b) = 3.9 A. The optimum interplanar distances at these minima are 3.4, 3.2, and 2.9 A and the calculated AEFres are -0.9, -0.6, and -2.1 kcal/mol, respectively. The experiment-based value is -3.5 kcal/mol.' The positions of these minima are reasonable in terms of orbital overlaps. It is clear that bonding occurs for all arrangements which allows .x overlap between the parallel rings. The minimum at S = 0 reflects a 6-coordinate dimer bond which occurs at inter-ring distance of 3.4 while the minima at 6 ( y ) = 3.9 8, and 6 ( x ) = 2.8 A reflect 2-coordinate and 1-coordinate dimer bonds, respectively. The conformations corresponding to these minima are shown in Figure 6 as they appear on the HOMO's of (AmNaph) dimer. In all the minima, the principal overlaps are between the ring carbons of the two fragments. It is suggested that the stable dimer may be a slippery variant of the 2-coordinate conformation (Sb)= 3.9 A) which results in a resonance energy of about 2 kcal/mol at the inter-ring distance o f 2.9 A. The conclusion from EHT calculations is that several structures of comparable energy may be expected for most symmetric and asymmetric aromatic dimer cations. Also, it seems that the origin (21) Berlinsky, A. J.; Carolan, J. F.; Weiler, L. Solid Sfate Commun. 1976, 19, 1165.
J. Phys. Chem. 1987, 91, 1095-1098 of bonding is charge-spin delocalization,22 which preserves aromatic character by allowing for intermolecular resonance stabilization. We are currently exploring this conclusion by applying ab initio calculi-ttions to some small aromatic dimer cations. ~~~~~
(22) Milasevich, S. A,; Saichek, K.; Hinchey, L.; England, W. B.; Kovacic, P. J. Am. Chem. SOC.1983, 105, 1088. (23) Levin, R. D.; Lias, S. G. Ionization Potential and Appearance Potential Measurements; National Bureau of Standards; U S . Department of Commerce: Washington, DC, 1971-1981; NSRD/NBS 71.
1095
Acknowledgment. We thank the National Science Foundation for support of M.S.E.-S. through Grant No. CHE8305045. We also thank Dr. M. Kertesz for helpful discussions on the theoretical calculations and Dr. D. E. Martire for his encouragement throughout the course of this work. Registry No. 1, 105991-19-3; 2, 105991-20-6; 3, 106006-62-6; 4, 105991-21-7; 5, 105991-22-8; 6, 105991-23-9; 7, 105991-24-0; 8, 105991-25-1; 9, 105991-26-2; 10, 105991-27-3; 11, 102978-84-7; 12, 75209-07-3; 14, 75209-00-6; 15, 105991-27-3; 17, 105991-28-4; 19, 105991-29-5; 20, 105991-30-8; 22, 105991-31-9; 23, 102978-89-2.
Temperature Dependence of the Ru(bpy),(CN), and R~(bpy),(i-biq)~+Luminescence F. Barigelletti,*'. A. Juris,lS*bV. Balzani,la.bP. Belser,'c and A. von Zelewsky'c Istituto FRAE-CNR, Bologna, Italy, Istituto Chimico "G. Ciamician", University of Bologna, Bologna, Italy, and Institute of Inorganic Chemistry, University of Fribourg, Fribourg, Switzerland (Received: April 24, 1986; In Final Form: October 3, 1986)
The luminescence behavior (emission spectrum and lifetime) of the uncharged cis-Ru(bpy),(CN)2 complex (bpy = 2,2'-bipyridine) has been studied in propionitrile-butyronitrile solution in the temperature range 84-3 10 K and compared with that of the Ru(bpy),(i-biq),+ cation (i-biq = 2,2'-isobiquinoline). For both complexes luminescence originates from triplet Ru bpy metal-to-ligand charge-transfer (3MLCT) excited states, with CN- and i-biq playing the role of "spectator" ligands. The red shift of the emission maximum observed in the temperature range 110-160 K is attributed to relaxation processes related to changes in the solvent matrix viscosity. The changes in emission lifetime with increasing temperature are accounted for by Arrhenius paths, which lead to the population of upper excited states, and solvent matrix effects, which involve either relaxation of metal-ligand vibrational coordinates or rearrangement of the solvation shell. The contributions of these processes to the overall rate constant of radiationless decay have been estimated from an analysis of the In 1 / vs. ~ 1 / T plots. The solvent rearrangement effect is much more important for Ru(bpy),(CN), than for Ru(bpy)2(i-biq)2+. The values of the kinetic parameters of the Arrhenius path that becomes important at high temperature suggest that Ru(bpy),(CN), and Ru(bpy),(i-biq)*+ exemplify two different limiting cases of radiationless decay behavior.
-
Introduction The photophysical properties of the members of the Ru"polypyridine family are the object of intense investigation2+ because of fundamental and applicative reasons. A number of studies'*'9 have shown that the luminescent metal-to-ligand (1) (a) Istituto FRAE-CNR. (b) University of Bologna. (c) University of Fribourg. (2) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. (3) Kemp, T. J. Progr. React. Kinet. 1980, 10, 301. (4) DeArmond, M. K. Coord. Chem. Rev. 1981, 36, 325. (5) Kalyanasundaram, K. Coord. Chem. Rev. 1982,46, 159. (6) Watts. R. J. J. Chem. Educ. 1983. 60. 834. (7j Balzani, V.; Juris, A.; Barigelletti; F.: Belser, P.; von Zelewsky, A. Riken Q. 1984, 78, 78. (8) Seddon, E. A.; Seddon, K. R. The Chemistry. of_Ruthenium; Elsevier, Amsterdam, 1984; Chapter 15. (9) Juris, A,; Barigelletti, F.; Campagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev., manuscript in preparation. (10) Van Houten, J.; Watts, R. J. J. Am. Chem. SOC.1976, 98, 4853. (11) Durham, B.; Caspar, J. V. Nagle, J. K.; Meyer, T. J. J. Am. Chem. SOC.1982, 104, 4803. (12) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (13) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (14) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J . Am. Chem. SOC.1984, 106, 2613. (15) (a) Wacholtz, W. M.; Auerbach, R. A,; Schmehl, R. H.; Ollino, M.; Cherry, W. R. Inorg. Chem. 1985,24, 1758. (b) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. H. Inorg. Chem. 1986, 25, 227. (16) Barigelletti, F.; Juris, A,; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1983, 22, 3335. (17) Juris, A.; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1985, 24, 202. (18) Barigelletti, F.; Belser, P.; von Zelewsky, A.; Juris, A,; Balzani, V. J. Phys. Chem. 1985,89, 3680.
0022-3654/87/2091-lO95$01.50/0
charge-transfer (MLCT) excited states formed upon light excitation deactivate following nonactivated and activated paths in which the solvent plays an important but not yet fully elucidated role. Recent investigations carried out in our laboratory have also shown that the viscosity of the solvent matrix may substantially affect the emission behavior.I6-l9 In particular, the rate of the radiationless decay process is enhanced on passing from a rigid to a fluid matrix. This occurs presumably because of the coming into play of large amplitude (low-frequency) vibrational modes (such as the Ru-N vibrations),18or by the lower energy separation between the excited and ground states due to solvent rearrangements and following an energy gap law dependen~e.'~ The analysis of the 1 / vs. ~ 1 / T plots has allowed in several cases an estimate of the increase in the rate of the radiationless decay process caused by matrix melting. In an attempt to obtain a better understanding of the role played by temperature and the solvent matrix state in the radiationless decay of the MLCT excited states, we have now investigated the temperature dependence of the luminescence properties of the R ~ ( b p y ) ~ ( c and N ) ~R~(bpy),(i-biq)~+ complexes in nitrile solution in the temperature range 84-310 K.
Experimental Section Ru(bpy),(CN)22a (bpy = 2,2'-bipyridine) and Ru(bpy),(ibiq),+ (i-biq = 2,2'-isobiquinoline) were prepared as described elsewhere. The experiments were carried out in a mixture of (19) Barigelletti, F.; Juris, A,; Balzani, V.; Belser, P.; von Zelewsky, A. J . Phys. Chem. 1986, 90, 5190. (20) Belser, P.; von Zelewsky, A,; Juris, A,; Barigelletti, F.; Balzani, V. Gazz. Chim. Ital. 1985, 115, 723.
0 1987 American Chemical Society
