The electronic spectra of diphenylmethane dyes

JOURNAL

OF

MOLECULAR

SPECTROSCOPY

The Electronic

Spectra

4, 359-371

(1960)

of Diphenylmethane

Dyes*

FRANK C. Amivt University

of Washington,

Seattle,

Washington

The absorption and emission spectra, along with their relative polariaations have been obtained for several diphenylmethane dyes. When the transitions are correlated with those of the triphenylmethane dyes, such as Crystal Violet, it is found that the number of observed transitions decreases, as do the number of low-energy resonance structures. The absorption intensities are calculated for the low-energy transitions, and good agreement is found with those observed. I. INTRODUCTION

The present investigation concerns the spectroscopic properties of four diphenylmethane dyes: Michler’s Ketone (MK) , Auramine (Aur) , N-Acetyl Auramine (AA) and Michler’s Hydrol Blue (MHB). Each can be represented by the general formula shown (I). X

0% e3

MeeN

NMe,

(1)

0 II -NH2 , -8H-C-CHs and -H, respectively. There are where -X is -o(-), a number of other possible substituent -X groups, but the most important are those with a third substituted phenyl group. The above four diphenylmethane (DPM) dyes can be considered as special cases of this latter triphenylmethane (TPM) group which is typified by Crystal Violet, with -X being NMez

-0

* This research was supported by the U.S. Air Force through Air Force Office of Scientific Research of the Air Research & Development Command under contract AF 18(600)-375. Reproduction in whole or in part is permitted for any purpose of the U.S. Government. t Present address: Department of Chemistry, University of Alberta in Calgary, Calgary, Alberta. 359

360

ADAM

The similarity in structure is reflected in the spectra and there are many features which are common to the absorptions of both the TPM and the DPM dyes There are typically two intense optical transitions in or near the visible region of the spectrum, and several weak absorptions in the ultraviolet above 200 rnp. Since the color of the dyes varies from colorless to deep blue, a comparison of the spectra of the various dyes will show that these part#icular transitions vary considerably with respect to energy and intensity from dye t*o dye. On the other hand, some of the ultra~riolet transitions have much the same spectral characteristies, regardless of which dye is considered (1) . This differing sensitivity to molecular structure serves to distinguish two kinds of excited states, much like those suggested by Simpson (9). The sensitive absorptions, those which vary from dye to dye, are usually found at low energies, and are transitions between the socalled “N-states.” These states result from the resonance between the “normal” valence structures which are usually drawn to represent the particular molecule. On the other hand, the insensitive ultraviolet absorptions arise from transitions between the ground state and the “E-states,” which are those states derived from resonance between various excited valence structures. Since the two types of states may, to good approximation, be considered independently of each other, our att,e~lt,ion will be focused on the low-energy “N-states,” since these states are particularly amenable to treatment by pertllrbat,ion theory, in a framework based on t,he valence structures. Each TPM or DPM dye molecule may be represented by the i’norn~al’, xtructurcs shown in Diagram II [(I), (2), and (4)]

with (3), which is the mirror image of (2). The wave fun~t,ions corresponding to these four structures are combined to describe the four N-states. Three transitions are expected between these N-states, and each has been observed experimentally for the TPM dyes (:3). In the particular case of Crystal Violet, (CV), where structures (I), (2), and (3) are equivalent, there occurs an essential degeneracy in the excited states, which is not found when --X is different along

-NMes as for the other TPM dyes (,G). This two-fold degeneracy i) is removed in a manner dependent upon whether the group substituted is more or less electrophylic than that of Crystal Violet. The situation is shown diagrammatically in Fig. 1. For instance, t*he degeneracy of CV is split to the 6( + ) side by a substitutional perturbation in which a more electrophylic group, such from

SPECTRA OF DIP~ENYL~~~THANE AB

*\ _-_

-3 ..=-_- _____ .

361

. .- A

-- ___________

+--__

B

c’

*.

7

-8(-1

DYES

7

ci Perturbation

%(+I-

FIG. 1. Energy level diagram for Crystal Violet and dyes related to it through a simple substitutional perturbation in one ring. Straight broken line at top of figure represents the lowest E-state. The other levels are M-states.

as -0Me (in Methoxy Malachite Green), replaces one of the -NMe2 groups of CV. It was found that any of the TPM dyes given by the above general formula could be treated as perturbations of CV. By using the method of structures to be outlined shortly, it was possible to calculate the absorption intensities with reasonable quantitative accuracy (5). Inasmuch as a similar plan will be followed for the DPM dyes, the first problem will be in deciding whether the removal of a benzene ring does or does not correspond to an electrophilic substitution, and whether there is any actual correlation between the absorptions of the DPM dyes and those of the TIM dyes which were studied in the earlier papers. It remains to be seen whether such large perturbations may be treated by this method, and the reliability of the assumptions and procedure will be measured by the agreement between the expe~men~lly observed absorption intensities and those calculated by the method of structures. II. EXPERIMENTAL

The methods and techniques used for the study of bis-dimethylamino fuchsone (BDF) in Ref. 1 have been used essentially unmodified in the present work, so that the preparations will be cited, followed by the results. (a) Auramine Hydrochloride (Eastman Practical Grade) was dissolved in water, precipitated as the perchlorate, and recrystallized from methanol until

362

ADAM

solutions of successive crystallizations (O.D. at emax z 1.0) gave no absorption compared one to the other. Intensity measurements using the perchlorate salt were checked within experimental error against samples of Auramine Acetate, crystallized from glacial acetic acid. C,b) Michler’s Ketone (Eastman White Label) was recrystallized from methanol. ( c) N-Acetyl Auramine was prepared by R. B. Lund from Auramine Hydrochloride ( 5 g) in dry pyridine (10 ml) and acetic anhydride (10 ml), heated on steam bath (2% hours), neutralized with 1% methanol, extracted, washed, dried, and recyrstallized from a benzene-ligroin mixture. The dye was prepared by adding 20 % perchloric acid to a methanol solution of t’he carbinol, and recrystallized from methanol. The room temperature absorptions of the Aur, MK, and AA in glacial acetic acid appear in Fig. 2. The low-temperature absorption spectra, emission spectra, and polarization ratios for Aur and AA are shown in Figs. 3 and 4. These are representative of a number of determinations with different rigid glasses and with varying values of pH. The curves for MK are similar, but there is more overlapping of the transitions. In this particular case, the low-temperature spect#ra are better resolved and have been used as a guide in determining the relative absorption of the two overlapping transitions seen in Fig. 2 of the room-temperature spectra. X(mpL) 700

500

400

300

a

0.6

n

* ‘0 x w

0.4

0.2

20

30

4

YCKK)

FIG. 2. Absorption

Ketone

spectra of N-Acetyl (3) in glacial acetic acid.

Auramine

(I), Auramine

(2), and Miohler’s

Mm@ I

800 II

600 1

500

400

300 I

250 3.0

2.0 P

1.0

20

IO

30 V(KKI

40

1 D.5

FIG. 3. Absorption, emission, and polarization curves for IV-acetyl Auramine in glycolwater mixture at -180°C.

800

I

600

I

500

I

400

I

.

300 I

250 I 3.0

2.0 P

1.0

IO

20

30

40

t

3.5

V(KK)

FIG. 4. Absorption, emission, and polarization curves for Auramine in glycol-water mixture at -180°C.

364

ADAM TABLE

I

CHARACTERISTIC FEATURES OF THE ABSORPTIONS OF DIPHESYLMETHANE DYES. POLARIZATION IMPLIES A-R ABSORPTION, WHILE ,vIMPLIES d-J Wavelength

Compound

Trans. moment length (A)

~.____

Michler’s

Ketone

382 346 335 313

Ratio

(372) (333) (325 ) 302 255

Total

2.3

Auramine

1.48

Total

585 420 (360) 300

Total

Hydrol

Blues

!/

*V

Isopentanc Isopropyl Alcohol (- 170°C)

s

N

Gl. acetic

aci(1

!I

s

Gl. acetic

acid

Gl. acetic

acid

615 385

Gl. acetic

acid

E

E

Y J,Y

AV

1.68 0.79

x

N x

0.3

Y

N

?I

JY

u z

E

E

1.2

z

E

2.01 0.92

%

N N

Weak Total

N N

s

Weak

0.74 1.0 0.70 2.25

Weak

Solvent’

Type

0.44 0.77

Weak

255 Michler’s

s !I s

(354) 316 (270 j (245) N-Bcetyl

______

440.5 372

Auramine

Polarization

THE s

Y X !J

E E

.r

E

X -_

Y,E

N

B Data taken from Ref. S.

The information from the above curves is summarized in Table I, along with that for MHB which properly belongs in the DPM series. Michler’s Hydrol Blue was studied earlier and the data has been taken from Refs. 3 and 6. Intensities in Table I are represented by the magnitude of the length of t,he transition moment vector, in A. III.

CORRELATION

OF THE

ABSORPTION

BANDS

As was mentioned in the Introduction, the initial problem is to determine the nature of the perturbation which changes the energies of the states of CV to those of the DPM dyes. If it can be assumed that the major contribution of the perturbation will result from the change in energy of structure (1)) then this is

SPECTRA

OF D~PHE~YL~~ET~A~~

DYES

365

equivalent to determining the polarization of the first low-energy transition of the DPM dyes in a scheme such as shown in Fig. 1. It is perhaps significant t,hat this transition is much more intense than the second or any higher energy transit,ions. Since the strength of an allowed dipole absorption is limited in magnitude by the dimensions of the molecule, it would appear that other things being equal, the intense transition is polarized in the direction where the electron system is longest, or in the z-direction. This thought is supported by crude “potential box” calculations, and by Hiikel calculations, which were carried out for CV, Aur, and MHB. The Hi.ickel results for CV are consistent with Fig. 1, while for both Aur and MHB there was found for the ground state a wave functions of symmetry A, and, for t,he lowest excited state, a function of symmetry B. Thus each of the considerations above is in agreement with an x-polarized assignment for the long wave transition.’ The Htickel calculations yield further information if it is assumed that the three lowest energy excited states predicted for CV are N-states, while the states higher in energy are E-states. In order of increasing energy, then, there is obtained the series of excited state types N,N,N,E, . . . , for CV. According to the calculations for Aur, only the first two excited states are N-states and the relative order for the DPM dyes is thus N,N,E,N,E, - ++ , so that the third and fourth states have “crossed over” in energy, and the choice of an A + 23, x-polarized assignment is completely consistent with the results of the polarization experiments for all of the lower energy transitio~~s. The above conclusions have been summarized for each absorption in Table I. Since the perturbation appears to be in the direction of 6( +> in Fig. 1, it would be expected by analogy to the TPM dyes, that the DPM dyes, MK, Aur, AA, and MHB should have spectra which show the two long wavelength absorptions lying at progressively divergent values of energy since this order of the molecules represents a successively greater electrophylic nature of -X, or greater 6( +) perturbations. This is easily seen to be confirmed in Table I. That the gross perturbation is of the 6( + ) type may also be tested by use of the structure method of calculations outlined in the following section. In this method, any plausable correlation of the transitions may be assumed, and the formal procedure of calculating the intensities carried through. Agreement should be closest with the correct assignments. Only one scheme gives reasonable results at all, and that is the correlation given above, corresponding to a 6( +) perturbation. Thus, by neglecting the single intervening E-state in the case of the DPM dyes, a good correlation may be established with the N-states of the TPM dyes. 1A is taken as symmetric, B antisymmetric to reflection (or rotation) in the yz plane (or about the y axis). The z axis is perpendicular to the plane of the molecule, thus a transition A--R is s-polarized while A-A is y polarized.

366

ADAM

The relat,ive energies and intensities of the correlated transitions are quite comparable, but what is more important, the relative polarization correspond in the precise manner required. This is not only the case for the N-state transitions, but is significantly true in the case for t#henearby E-state absorptions’ as well. IV. THEORETICAL

There exists a structure wave function, ql , etc., to each of the the four valence structures, and these $;‘s are combined to give the N&ate wave functions. An energy matrix, E’, is constructed so that diagonalization yields the observed term values of these N-states. This procedure is the most simple for CV, but for other molecules a perturbation matrix of the form shown in Diagram III

5l

0

0

E,q

0000 0000 E4l

:

0

0

644

\

may be added to the E’ for CV. The elements in this matrix are those which are ajudged the most important from a consideration of the different structures (1)) (21, etc., and how these structures are affected by the different -X groups. Hy using the term values for a second molecule, it is possible to evaluate cl1 , ~4 , and e44and to determine a new B’. The unitary transformations required to rediagonalize E’ determine wave functions which may be used to calculate the transit,ion moment (or intensity) of each of the N-state absorptions. Further details of the formal procedure may be found in Refs. 1 and 7. The above four-structure represent’ation is valid for some of the DPM dyes. However, MHB is best described using only three structures (Diagram IV)

(3)

(I) (IV)

_____

__~~~~~

-.

is assumed that the lower &states absorptions are benzenoid transitions (B?,, , BI,, , et,c.) modified by weak coupling between the rings. Each of the benzene levels is split into three with t,he TPM dyes, and into two with the DPM dyes. In either case, the low-energy process is expected to be polarized normal to the symmetry axis in the z-direction. * It

so that an additional set of calculations has been carried out for the DPM dyes, similar to those outlined above, but using instead only three structures, two term values, and a perturbation matrix of the form shown (V).

The restits of the four-structure and threw-structure calculations appear in Tables II and III. The following remarks about the numerical results are in order. When working with the four Btructures, trouble is immediately encountered when the formal TABLE

II

PERTURBATIION ~%TRIX ELEMENTS AND CALCULATBD TRhNsITroN MOMENTS FOR Two DFPFIENYIJ~ET~ANE DYES, USING A FOI&%RU~TURE PRESENTATION Matrix

elements

~orn~o~~ld t14

f44

0.376 0.785

-1.402 -0.992

0.333 0.340

moments

X12

---

Observed

Calcufated

--811

YJt4

Yi3

X12

-

1J13

Ml4 -

~

-Michler’s Ketona Auramine

Transition

-

1.36 1.54

0.60 (0.79)a

0.54 0.33

1.36 1.68

0.60 0.79

0.44 0.30

a Used ss hssis for calculation. TABLE

XII

PERTURBATION MATRIX ELE&IENTS AND CALCULATES TRANSITION MOMENTS FOR &CR D~P~~~NY~~~T~ANE DYE, USING A ~HR~~-~TR~~TU~~ REPRESENTATION Transition Mst~ix Compound

-. eu

IIMichler’s Ketone Auramine N-Acetyl Auramine Michler’s Hydrol

-0.8070 -0.1460 0.6650 0.9840

8 Used as bltsis for cafcufation.

moments

-___

elements Calculated

wz = w! X12 ~"~-0.4278 -0.3711 -0.1527 -0.1627

I.50 1.69 I.94 1.99

Y1a 0.80 (0.69)* 0.68 0.64

Observed X12 1.36 1.68 2.01 2.25

-,~ x/13 _111_-

0.60 0.79 0.92 0.3

368

ADAM

procedure is applied to the dyes MHB and AA. The difficulty is that the matrix contains a pure imaginary number for element) ~14. This number represents the interaction between structure functions ( 1) and (4)) and no trouble is encountered as long as it is real and nonzero. Presumably, when it assumes the value of zero, the structures (1) and (4) no longer interact, and resonance between the structures ceases. In the present case, this occurs concomitantly with an increase in the energy of structure (1) (increasing en), so that eventually a high-energy structure is obtained which doesn’t contribute materially to the energy of the lower N-states. Such a high-energy structure would be that of (1) for MHB in a four-structure representation. It should be noticed that the value of the perturbation matrix element cl4 for MK and Aur (Table II) decreases so that E:, becomes zero at a 6( +) perturbation smaller than that presented by AA. The conclusion is thus reached that whereas Aur and MK may be described by four structures, AA and MHB should properly be described by only three. That structure (1) is indeed rising in energy may he seen from the values of cl1 in Table II. When t’he above procedure is carried out, assuming that’ the DPM dye absorptions correlate with a 6( - ) perturbed TPM dye, there results for none of the dyes a purely real E’ matrix. This arises much in the same manner as above, for ,4il and MHB, in the four-structure representation. It is presumed by similar argument+ that this particular correlation should not he considered too seriously, since the other assignment gives much more realistic and meaningful results. The calculated transition moments for the four and three structure representation are also shown in Tables II and III. The y-electric moments of structures C2) and (3) are those of CV, but that of (1 j is of Aur. The gross reduction in size of molecule in the “y” direct’ion necessitated this variation in the usual procedure of relating all quantities to the experimental values for CV. v. DISCUSSION The figures in Tables II and III result from the application of numerical features to the qualitative arguments which have long been used by chemist,s. Indeed, it is those qualitative arguments that lead to the perturbation diagram of Fig. 1, and the realization that it might be possible to fit the DPM dyes into such a scheme. Such arguments using valence structures come into the construction of the pert,urbation matrix. The results in Table II and III show that energy levels of the DPM dyes may be treated quantitively, as well as qualit’atively. They may truly be considered as perturbations of the energy levels of CV. E’urthermore, these perturbations assume a simple form in a representation which uses these structures as a basis. Comparing the results in these tables, it can be seen that the best results are obtained are for MK and Aur, using a four-structure representation. However, the agreemem of the three-structure calculations with t#he experimental results

SPECTRA

OF DIPHENYLMETHANE

DYES

369

is still quite good, especially when it is remembered that MK is a weakly absorbing, almost colorless, neutral dye molecule, while MHB is a cationic species with intense absorption near the other end of the visible spectrum. The perturbations which effect these changes are certainly not small compared to the zeroth order separation of the CV energy levels. There are a number of other features of the calculations that invite further consideration. The first of these is the way in which the calculated intensities follow the experimental values. The long wavelength transition shifts monotonically to the red with increasing intensity, while the second transition shifts first to the red with increasing intensity, then to the blue with decreasing intensity. This behavior is mirrored identically by the calculations. On the basis of the results for MK and Aur, the third transition shifts to the red with sharply decreasing intensity. In the calculati,ons this occurs concomitantly with a ,decrease in the absolute magnitude of El4,and it is not surprising that when El* is zero, the absorption is no longer detectable. Furthermore, it does not appear to return with further perturbation (E:, complex), so that, in effect, one energy state of the molecule has disappeared. This seems linked in a very satisfactory way to the fact that structure (1) for MHB in a four-structure representation represents a most unrealistic valence bond structure. It can be seen from Table III that the same trend is being established in the three-structure representation, since E:, is increasing, while El2 = I& is decreasing. Presumably, the hypothetical molecule which has E:z = 0 will be best described by only two structures. In view of the close agreement between the calculated and experimental absorption intensities, it is justifiable to assume that the correlation scheme which has been used is correct. This seems particularly valid when it is found that the other possible choices give, at best, only poor agreement. Although this consideration is perhaps sufficient reason for using the particular assignment and correlation which was chosen, the wealth of supplementary evidence indicated in an earlier section is in itself sufficient to lead to an identical conclusion. From the complete lack of contradiction, it is possible to gain confidence in the analysis, and the calculations, and in the correlation of the absorptions. Accordingly, a chart showing the history of the E-state and N-state transitions of CV and the DPM dyes has been drawn up and appears in Fig. 5. The line heights are proportional to the magnitude of the transition vectors. It can readily be seen that once the initial correlation between CV and MK is established, the bands change in energy and intensity only slightly from dye to dye through the perturbation series. The number of N-states changes through this same series, but an equivalent reduction in the number of resonance structures also occurs. There is predicted a reduction in number of E-states too, and while the data is not so clear cut, there is certainly no experimental evidence to contradict the supposition that two E-state absorptions are lost between CV and MK, as is expected theoretically.

370

BDAM

f

f

I I

I.

\ : :

Y,#' ; ‘\ ;j, II : ’ : ’

_-;--I-

x.x

i

:

__ -p

\,

MK

‘\

: : \ : ‘\ )I : a,’: ’ ‘\ , 4 : ‘, %\\ ‘, \ \ : , : : ‘\ 1 I I

:

AUR -... %\

-.

:

’

L : I * :

I 300

I 400

I(?) 0:I

,cv

,-ic .s-

/#--

Y’k/“Y

I

\’

1 200

,/ I. x

;.I ‘*,

\

I 500

AA : ‘; \

1 600

MHB 700

Abqd

FIG. 5. Energy correlation diagram for Crystal Violet and related dyes. Line heights are proportional to transitional moment lengths.

diphenylmethane

It is interesting that the transitions above 200 rnp which are predicted by the simple theory are indeed found, and that each of these transitions has the expected polarization. From this it would appear t]hat the classification scheme of Ref. 2 is substantiated in t’he present work. In particular, the E-states do not interact st)rongly with the N-states, and may therefore be considered as independent, both qualitatively and quantitatively. Further, the assumption that the E-stat#es result from relatively weakly coupled benzene ring levels is apparently a sound notion, since t,he splitting of the erstwhile Bz, levels results in absorptions of similar polarization to t’hose transitions found by McClure (8 1 for diphenylmethane. Although the “splitting” of the interacting levels is much larger, the interacbion between the rings is probably much greater for the DPM dyes than for diphenylmethane. Since also t’he center of gravity and relative intensities of the transitions in this doublet vary with the different DPM dyes, it, must be assumed that the mode or magnitude of the coupling and t’he spatial configuration of the rings also change. This implies t,hat, the perturbation mat,rix should conOain more elements. Although such improvements are not, possible using the present semiempirical method, the classification scheme still yields a correct qualitative picture, and may be extended to give quantitative result,s of high precision for the transit)ions involving the lower energy N-states. ~w.YErVED: June 25, 1959 REFERENCIB 1. F. C. ADAM AND W. T. SIMPSON, J. Mol. Spectroscopy

2. W. T. SIMPSON, J. Am. Chem. Sot. 78, 3588 (1956).

3, in press (1959).

3. c. w. hxxa?XA~R ~.T.~~MP~~~,~.~~. ~~~n~.~~~.~~, 6293 (1954). 4. w. T. &ix~soN, J. hi. Ckem. &c. %,%I7 (1954). 5. W. T. SIMPSON ANDC. W. LOONEY,.I. Am. Chem. Sot. 76,0285 (1954). 6. C. W. bOONIX, the&, University of Washington, 1954.

7. F. C. ADANI,t~hesis,University of Washington, 1957. 8. D. MCCLURE,Can. J. Chew 36, 59 (1958).

The next Gorky ~es~~r&~ ~~~~re~ce on ~~~r~r~~ ~~ec~ro~~o~~ will be held at Kimball Union A~ad~rn~, Meriden, Kew Rampshire, between August 22-26, 1969. The purpose of the Gordon ~~~f~~~~e~ is to enabk s~~e~t~st,sto analyze the latest d~v~~o~rn~~ts through lectures and discussions conducted on an informal basis. The program for the ensuing conference is as follows: L. COUTURE-MATHIEU, Infrared and Raman Spectra in Crystals. J. FAHRENFORT, Infrared Reflection Spectra. H. J. HROSTOWBKI, Electronic Absorption of Bizarre Impurities in SiIicnn and ~~rmaniun~ in the Infrared Region. P. J~o~or;?so~r,Recent Advances in the near Infrared. L. GENZEL, Recent Advances in the Far Infrared. ~~e~~er fo be ~~~~~~~~~~~ Recent Infrared I~~estigatiol~ in Russia. A. D, B~c~~~~~~~~~~ Theory of Solvent Effects in Infrared Spectra. t7t’.B. PERSON, Infrared Intensities. A. C. JONES,Measurement of RaealRaman Intensities. M. A. EUASIIEVICH,Molecular Vibratians. J. OVEREND,Molecular Force Fields. L. J. BELLAMY,Infrared Spectra of Large Molecules. W. KLEMPGRE~,High Temperature Studies in the Infrared. C. II. TOVVNES,Progress Report on the Status of Infrared Masers, Attendance at the conference is by application (before end of June, X960) on standard hr~ns available from Dr. George Parks, Ike&or, Dept. of Chemistry, Tiniversity of ‘Rhode Island, Kingston, Rhode Island. The Co~lferen~e Committee usua& selects about 100 conferees, d~strib~~ti~~g the attendance a~~~o~~g institutions represented by the a~p~i~ati~ns. The resident conferees at eaeh conference will be charged ~~~.~ to cover registrations lodging, meals, and gratuities. For additional details please apply t,o the office of the Director at t*he address given above.