CT Bands of Methiodides of Nitrogen Heteroaromatics
The Journal of Physical Chemistry, Vol. 83, No.
5, 1979 029
Frequencies of Multiple Charge-Transfer Bands of Methiodides of Some Nitrogen Heteroaromatics Sanjib Bagchi Department of Chemistry, Presidency College, Calcutta-700073, India
and Mihir Chowdhury" Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta-700032, India (Received January 6, 1978; Revised Manuscript Received August 16, 1978)
The multiple charge-transfer (CT) absorption and emission bands of the methiodides of the diazines, quinaldene, and acridine have been studied. The frequency of the first charge-transfer band correlates well with calculated electron affinity of the cation and ionization potential of the anion, if interaction with the solvent is taken care of. The observed variation in the separation of the two CT bands of solution spectra can be explained if the interaction of the 2P1/2and 'P3/2 states of the I atom with the organic part in the excited CT state is taken into account. The multiplicity in maxima in emission spectra needs further investigation.
Introductiam Interest in charge-transfer (CT) bands of methiodides lies in solvat,ochromism,1-3in thermochr~mism,~ and in multiplicity of CT absorption maxima5 exhibited by some of these compounds. They also offer a suitable system for correlation of electron affinity with CT absorption maxima, studies on which are indeed very few. The thermochromism aspect has recently been studied by us in detail.4 This paper is concerned with the correlation of the CT absorption maxima (single or multiple) with the structural aspects of the complex. We have chosen a series of closely related singlie-ring cations as acceptors for studying the correlation of the CT,v with the electron affinity of the cation. The donor is also changed to see the effect of ionization potential. These compounds exhibit multiple absorption maxima, commonly ascribed to the final donor states 2Pljzand 2P3j2:5p24 The separation between the two maxima should be fixed if the final donor state is a free atom but will vary if there is partial bond formation in the CT excited state. The purpose of this work is to study the separation between the two maxima as a function of the acceptor cation and thus find the degree of D,A interaction in the excited CT state. Other probable reasons for the appearance of more than one maxima in absorption and in emission, such as involvement of multiple accepting orbitals,24vibrational levels, and different conformations, also have been discussed.
TABLE I: Electron Affinities of the Acceptor and the CT Band Maxima ( L J ~ , of~ Methiodides ~ ~ ~ lowest
-
compd pyridine methiodide pyridazine methiodide pyrimidine methiodide pyrazine met hi o di de
71 -Ip
lowest
vmaXzg8 (obsd) in CH,Cl,, cm-
of the neutral azine: eV
of the cation,b eV
26 600
9.73
-4.7083
22990
10.61
-5.3883
24100
10.41
-4.7572
21 050
10.18
-5.4536
EAV
a As obtained from the photoelectron spectra. lated by.
-
Calcu-
where C1 and C2 are constants for a fixed donor. The electron affinity of the cation is expected to be nearly equal to the ionization potential of the neutral azine. Since the electron goes to a 7r-accepting orbital, it is the 7r-ionization potential of the azine in which we are interested. These are available from the photoelectron spectra of corresponding diazines (unmethylated) and are given in Table In9 Although the accepting 7r orbital of the cation (methylated) will not be quite identical with the ionizing 7r orbital of the neutral unmethylated species, the ioniExperimental Section zation potential of the neutral azine may be expected to The preparation of the methiodides of pyridine and the provide a relative comparison of E A of corresponding three diazines, purification of solvents, and the spectromethylated cations. However, the parallelism of these photometric techniques have already been d i s c ~ s s e d . ~ values with the observed P,, is not a good one. We have, Quinaldine and acridine (Koch Light) were quaterinized therefore, carried out a CND0/2 all-valence-electron SCF by methyl iodide, and the resulting compounds were calculation on the methylated species themselves, with the purified by repeated crystallization from dry ethanol. standard program QCPE No. 29.'OJ1 The CNDO/2 scheme Crystals were grown from ethanolic solution and polished developed by Pople and Segalll has been chosen, for these by solvent. Our luminescence setup has been described are known to give acceptable relative electron affinities in an earlier communication.6 and charge densities. The calculated E A values, as shown in Table I and Figure 1 in fact parallel the P , , ~ ~for Results and Discussions different complexes. (1) Correlation of CT P,,, with Electron Affinity of The absorption spectrum of methylpyridinium bromide Cationic Acceptor and with Ionization Potential of in CHC13gives a shoulder at 35 200 cm-l (Figure 2) which Anionic Donor. ijmm for a particular compound in a given is presumably due to a charge transfer similar to the case solvent should be dependent on the vertical electron afof the iodide. However, this difference between the finity (EA) of R+ as73 position of the CT band maxima (-8600 cm-l) of the ~cP,,'~ C1- EA + C2/(C1 - EA) (1) bromide and the iodide complex does not correspond to 0022-3654/79/2083-0629$0 1.OO/O
0 1979 American Chemical Society
830
The Journal of Physical Chemistry, Vol. 83, No. 5, 1979 301
1
.:
Sanjib Bagchi and Mihir Chowdhury
TABLE 11: Relevant Data for Calculating the Relative Band Maxima of Methobromide and Methiodide
1
iodine
1
\
T?
'yi]
,
2
20 45
I,
1
I
s5
5'0
I EI~ (in eo)--
a
bromine
,a kcal/mol
70.99 77.91 (iodide) (bromide) E",b V 0.5355 1.0652 Br-J / Br 2 ( s) I--' / 2 I, (s) Ak? ,b kcal/(g 7.07 3.58 - atom) (sublimn of (vaporn of 1,) Br,) 46 bond dissocn 36 energy,b kcal/mol (iodine) (bromine) Reference 12. Reference 13b.
Figure 1. Wavenumber of the CT band maxima of various methiodides calculated vertical electron affinity of the acceptor.
WAVE NUMBER
vs. the
(Cm-) ZC.OC0
1 WAVE NUMBER(Cfi') 35710 3850
670
I
I
\
I
2778
\
\
1
\
300
I
350
,
'100 W4VE LENGTH ),,,,,(
4%
1 300
Figure 3. Charge-transfer band of various complexes: (1) pyrazine methiodide in CH2CI,, (2) pyrimidine methiodide in CH2CI,, (3) pyridazine methiodide in CH2C12.
280
240
-
320
A (in nm)
3(
Figure 2. CT absorption band of pyridine methobromide in CHC13.
Scheme I
- - step I
X-(solv)
step I1
'/2X,(C)
step I11
'/2Xz(g)
X(g)
the difference in the ionization potential (I,) of iodide and bromide12in the gaseous phase. Evidently, the interaction of solvent with the iodide is different from that with the bromide. One can try to estimate the electron-donating ability of X- (X- = Br-, I-) in a solution by means of Scheme I. c represents a condensed phase, solid for I2and liquid for Brz. The difference in energy for Br- and I- in step I will be given by the difference in the single-electrode potentials (EO) of the 1--1/2I2 and Br--1/2 Br2 system. The difference of energy in step I1 can be estimated from the difference of the heats of sublimation-vaporization of iodine-bromine and that in step 111, from the gaseous dissociation energies of the halogens. Step IV involves the solvation of a neutral species, and the energy associated with the process is not likely to differ greatly between Br and I. Actually the value is about 3.6 kcal/mol13" for C1, Br, and I in water at 25 "C. The relevant data are given in Table 11. Thus, the difference between the electrondonating ability of bromide and iodide in solution comes out to be of the order of 14 kcal/mol(-4900 cm-l). This value, although it corresponds to the difference of E,, of the charge-transfer-to-solvent (CTTS) spectra of bromide
and iodide in so1vent,14is only -60% of the experimentally observed difference in the present case. The agreement is not bad considering the number of approximations made in the simple model. (2) Multiple C T Maxima and Their Separation. Another interesting feature of the spectra of these complexes is the appearance of a second CT band (Figure 3) of similar intensity and solvent sensitivity. The generally accepted explanation of two CT bands is that the iodine atom produced in the excited state may exist in either 2P1,2or 2P3/zstate. In fact, for methylpyridinium iodide the separation is 7600 cm-l 5~15which amounts to the difference between 2P1/2and 2P3/2states in a free atom. However, for most other compounds we studied, the separation is greater than the free-ion value, while for pyridiazine methiodide the separation is slightly less than 7600 cm-l (Table 111). This variation of the separation of two maxima from compound to compound led us to consider the possibility of other mechanisms from which two CT bands might arise. First, it is well-known that benzene has a degenerate pair of accepting orbitals and the process of charge transfer might involve the lowest and the next lowest unoccupied ( T * ) orbital of the acceptor cation.24 The difference between the two lowest *-ionization potentials of azines (nonmethylated) as obtained from the photoelectron spectra is given in Table III.g Though the energy difference is of the correct order to explain the gap between the two CT bands, the relative order is not in agreement with our observation, Our CND0/2 calculation on the methylated species itself gives a value much too large to explain the observed separation; even the relative order is different. Second, an explanation in terms of multiple conformations may also be offered for the two CT bands. In fact X-ray studies of different methiodides have revealed the existence of two types of structure-one in which the Iis seated at the center of the plane of the ring without
CT
Bands of Methiodides of Nitrogen Heteroaromatics
The Journal of Physical Chemistry, Val. 83, No. 5, 1975, 631
TABLE 111: Separation between the Two CT Bands and Difference between the LUMO and Next Higher Orbital of the Acceptor Cation diff in first two -- 298 K,a n - I p ' s of Vmax A sb azines,c eV E, - E l $ eV complex cmcmpyridine methiodide 26 6 0 0 7 600 0.77 2.3 pyridazine methiodide 22 9 9 0 7 400 0.69 2.3 pyrimidine methiodide 24 100 8 000 0.98 1.5 pyrazine methiodide 21 050 1 0 000 1.59 2.8 4-cyanopyridine methiodide 20 6 2 0 1 0 000 4-carbomethoxypyridine ethiodide 2 2 200 9 900 A is the separation between the two CT a FmZgsK is the wavenumber of the CT band maxima at 298 K in CH,Cl,. As obtained from photoelectron ~ p e c t r a . ~ E , and E , are the energies of the first two excited levels of the bands. acceptor (cation). Calculated by us.
being bonded to a specific atom (i.e., tropylinium iodide), and the other in which the I- is closer to certain atoms than others (i.e., quinaldene ethiodide).16 Since most of our cations contain two N atoms, multiple conformations could certainly occur. However, if the multiple CT maxima observed were due to different conformers, one would expect the relative height of two peaks to be changed with the change of temperature due to a shift in equilibrium. This has not been found. Let us, therefore, go back to the conventional explanation of the two CT bands in terms of the 2P1/2 and 'P3/Z states of the I- atom. In order to explain the variation of 2P1/z-2P3 splitting, one has to look into the energy levels of the iodine atom in the presence of the organic part. The important interactions that modify the energy levels are spin-orbit interaction in the iodine atom and (2) the electrostatic-cum-covalent interaction of nonspherical symmetry between the two parts. One can have two situations, iriz. (a) when the bonding interaction greatly exceeds the spin-orbit interaction and (b) when the latter predominates. The nature of the energy states in different cases is shown schematically in Figure 4. In case (a), the electrostatic-culm-covalent interaction splits the three energetically degenerate p orbitals of the iodine atom to one C-type and two a-type orbitals. As discussed by JBrgensen, the orbitals along the molecular axis (i.e., along the line joining the centres of the charges and perpendicular to the ring as assumed by Kosower3)will be more stable than the other two orbitals (a type). The separation of the bands according to this scheme depends only on the strength of electrostatic-cum-covalent interaction. OrgeP first suggested that the moderately intense band in the (X = C1, Br, I) electron-transfer spectra of CO(H~O)~X'+ is due to transition from two a orbitals of higher energy, and the very intense band is due to that from the u orbital. Yamateralg elaborated this idea quantitatively and explained the electron-transfer spectra in the transitionmetal-halide complexes where the gap between the bands was considerably greater than the energy difference between the 2P1/2.and'P3/2 states of the iodine atom. In these cases, the intensities of the two bands were different. However, in the case of methiodide the two bands appear with almost the same intensity. For diazines, the higher energy band is slightly more intense, which is presumably due to an overlap with the n-a* band of the cation. The second band could not be located underneath the stronger R-a* transition in the case of the quinaldinium and acridinium iodides. In the case of transition-metal-halide complexes, the electrostatic-cum-covalent interaction is very large in the excited state. But we cannot expect the same for methiodides where the excited state contains uncharged radicals and the electrostatic effect is due only
,
UNPERTURBED LEVEL
/
+
SPIN-ORBIT -t ELECTROSTATIC ENERGY
UNPERTURBED +ELECTROSTATIC LEVEL
ENFRGY
ENERGY
(b)
+
SPIN -ORBIT ENERGY
(a)
Figure 4. Schematic diagram
illustrating the effects of possible interactions on the energy levels of the iodine atom: (a) electrostatic interaction >> spin-orbit interaction, (b) spin-orbit interaction >> electrostatic interaction. to the distribution of charge in the neutral radical; also, the covalent interaction is likely to be small in view of the large separation between the organic part and the iodine atom. Thus, for iodine in methiodides, the spin-orbit coupling effects in the 5p shell are large and predominant. The case seems to be analogous to the splitting of the levels in the rare earth ions,,where the scheme (b) applies. In scheme (b), the energy state of the iodine atom splits due to spin-orbit coupling into a doublet 2P1/2and a quartet 'P3/2 corresponding to the J values of '1, and 3/2, respectively. In the presence of a field which deviates from spherical symmetry, the 'P3/2 state will further split into two doublets which in the axial field correspond to mj values f 3 / , and The states with mi= hl/, will have a greater percentage of u orbital than those with mj = f 3 / , and will be more stable (Figure 4). Thus, according ta this scheme, three bands should appear. The intensities of the bands, which depend upon the overlap between the organic part and iodine orbitals involved, should be different. The acceptor orbital is expected to have more overlap with those orbitals on the iodine atom which have a greater percentage of u orbital. Of the two levels formed from the 'P3/, state, the level with ml = fl/,will have a greater contribution from the u orbital. As a result, transition to states with m . = f3/, (J = 3/2) should be very weak and overshadowed in the solution spectrum by the band in-
632 The Journal of Physical Chemistry, Vol. 83, No. 5, 1979
volving transition to the states with mJ = & I / , ( J = 3 / 2 ) , particularly if the separation between two split components of J = 3 / 2 is small. The net effect of this would be an increase in the separation between the two CT maxima corresponding to the zP,,z and 2P3j2states of the iodine atom. In this model the magnitude of coupling between the radical and iodine atom in the excited state is small, which will result in a small separation between the two resulting states. This incidentally explains why no other CT triplet state could be located. Next, let us see why the separation varies with the nature of the cation. It is well-known that the LUMO of the para-substituted benzenoid aromatics shows a great difference from those of meta- and ortho-substituted derivatives.z0 We, therefore, compare the separations in compounds containing para-substituted cations only. It is seen from Table I11 that the extent of separation between the bands increases as )Z.cijmaxCT (=ECT)for a compound decreases. The excited state of the methiodides are given by $E
= $o(R.I.)
:.m
WAVE NUMBER ( C k ) 16680
2ooQo I
I
+ X$l(R+l-)
where $o(R-I.) i s the wave function of the nonbonded structure and $l(RtI-) is that of the ionic structure. The constant X can be evaluated by the firstorder perturbation methodz1 where
Sanjib Bagchi and Mihir Chowdhury
400
500
600
h (in nm) -----c
Flgure 5. CT luminescence spectra of various methiodides in alcohol glass: (I) pyridazine methiodide, (11) pyrimidine methiodide, (111) pyrazine methiodide.
recently to play an important part in determining emission characteristics. In the present case, the insolubility of the compound in hydrocarbon solvent prevented us from clarifying further the role of the H bond with solvent.
SAD is the overlap between the donor orbital (4D) and the acceptor orbital ($A) involved in the CT process. The less Summary the value of ECT,the greater the percentage of RtI- in the excited state and the greater the splitting of the ‘P3/2 state, We may summarize our main findings as follows: if the splitting is assumed to be due mainly to electrostatic (1)The correlation of absorption maxima with electron interaction. Thus, the separation will increase with a affinity of an acceptor and ionization potential of a donor decrease in ECT value, although the case may become is fairly good if proper care is taken to consider solvent complicated if SAD’S are also different. interaction. (3) Multiple Maxima in Emission Spectra. Since the (2) The variation of the separation between two maxima multiple maxima in absorption are associated with two can be explained if the interaction of the I atom with the excited states and since the gap between the two excited donor in the excited CT state is taken into account. states is larger than hT, one expects only one CT emission (3) Multiple maxima in emission need further investimaxima. The compounds were found to luminesce in an gation. alcohol medium when cooled to liquid nitrogen temperReferences and Notes ature. The broad and structureless luminescence observed has been interpreted as charge-transfer f l u o r e ~ c e n c e ~ ~ ~ (1) A. Hantzsch, Ber. Dtsch. Chem. Ges., 52, 1535, 1544 (1919). (2) J. S. Brinen, J. G. Koren, A. D. Oimstead, and R. C. Hirt, J . Phys. (Figure 5). In the case of methiodides of the diazines more Cbem., 69, 3761 (1965). than one emission band was detected. We purified the (3) E. M. Kosower, J . Am. Chem. SOC.,80, 3253, 3261, 3267 (1958). (4) S. Bagchi and M. Chowdhury, J . Phys. Chem., 80, 2111 (1976). solvents scrupulously and repeated the experiment several (5) E. M. Kosower, “An Intoduction to Rysical Organic Chemistry”, Wiley, times (with and without degassing) to make sure that the London, 1968. extra band in emission was not due to any impurity. The (6) S. C. Bera, R. K. Mukherjee, and M. Chowdhury, J. Cbem. Phys., 51, 754 (1969). band positions were found to remain unchanged but the (7) S. F. Mason, J . Chem. SOC.,2437 (1960). relative intensities of the bands changed with temperature (8) M. W. Hanna and J. L. Lippert in “Molecular Complexes”, Vol. I, Roy (Figure 5). As discussed previously, strong internal Foster, Ed., Paul Elek, London, 1960. spin-orbit coupling within the I atom will make the (9) E. Heilbronner, S. P. Maier, and E. Haselbach in “Physical Methods in Heterocyclic Chemistry”, Vol. VI, A. R. Katritzky, Academic Press, concept of triplet state meaningless, and hence there is no 1974, p 1. scope for associating the extra band with triplet emission (10) G. Klopman and E. O’Leary, Top. Cuff. Chem., 15 (4) (1970). as is commonly done for organic molecules. The short (11) J. A. Pople and G. A. Segai, J . Chem. Phys., 44, 3289 (1966). (12) R. S. Berry, C. W. Reimann, and G. N. Spokes, J . Chem. Phys., s), observed by us oscillographically lifetime (
