l~=+,ah~dro,, Letters ~~.49, Pp. 5131-5134, 1968. Perganon Press. Printed in Great Britain.

THE ROLE OF TBB "BASICITY" AND OF THE EXTEWHBOMOPHORIC COI'IJUGATION IlV TIIE

ABSORPTION OF SYMWTRICAL TRWETHINE CYABIIVES DERIVED FROM TEIAZOLE NUCLEI

Albert0 Gasco, Ermanno Barni and Gaetano Di Modica

Istituto Chimioo della UniveroitB, Torino (Italia)

(Received in UK 5 August 1968; accepted for publication 12 September 1968)

A symmetrical cyanine dye is formed by vinylene prolongation of the

amidinium ion system in which nitrogen atoms form part of equml heterocyclic

rings:

The qutilitative explanation of the absorption of these dyes in terms

of molecular structure was performed by L.G.S. Brooker and hia co-workers (1,3).

According to the point of view of the Authors above the Amax of a

symmetrical cyanine mainly depends on three factore: 1) Length of the conjuga-

ted chain, 2) Basicity of the nuclei, 3) Extrachromophoric oonjugation.

The influence of the length of the polymethine chain was studied al-

so with the aid of quantum methods (2) but the influence of the factors 2) and

3) was investigated only qualitatively (1, 3).

In order to evaluate the relative importance of the last.two factors

on the position of the absorption of some symmetrical cyanines derived fromthe

following condensed thiazole nuclei: 2-CH3-thiazolium, 2-CH3-benzothiazoliu,

2-CH3-naphtho[2,+d]thiazolium, 2-CH3-naphthob,2-~thiazolium, 2-CH3-naphtho

L-3 2 1 d thiazolium, 2-CH3-phenanthro[g, 10-q thiazolium (see Table I), some co_r

relations were performed.

The baeicity of the thiazole nudeue is related, according to L'.G.S,.

Brooker, to the lose of ring stabilization in passing from the positively char

ged form I to the net uncharged form II:

5132 NO.49

The same loss in stabilization followa the formation of the methyle

ne base from the corresponding cation (4): . .

x

jFi

I \

C-CH3- &// x

I LCH

I

../- *

I Using a simple HMO method, the energy differenceAEbbetweea quater-

nary salts of the 2 methyl derivatives above and their methylene bases, was

calculated. The coulomb and resonance integrals used for the heteroatoms were:

d,= o’+/j , dij= d+l,SJ3 , dkz d+2J3 , $_SO,ej3 , PC ijz 48~3, _ PC fi = ~3

The methyl group was treated as a heteroatom according to Streitwieser (5).

Coulomb integrals for the carbon atoms adjacent to the heteroatams were qxpres

sed in the following way:ol_CEOl+c$C.$3 where k=O,lrd,. In particular, for the me

thy1 group treated as a heteroatom, a value of C! =-0.2 was used. The substi- C-CH,

tuei& effects of the IV-alkyl &roup were disregarded. The results of these cal-

culations and the A values of the deviations (3) for the nuclei considered are

reported in Table I.

A plot ofAEbvalues versus A values is reported in Fig. 1. The eati-

sfacfory linear correlation between these sets of data is clear. It is intere-

sting to note that, with the aid of the quantum methods also, the appreciably

larger deviation of the 2-CH3-naphtho[l,2-d]thiazolium than the 2-CH3-naphtho

L-1 2 1 d thiazolium (3) is not entirely explained.

x catc

620

600

No.49

TABLE I

COMPOUNDS AEb’ A2

0

8

0 s\ 0

vc-cH3 I

4.2289

4.1592

41724

4.1872

925

555

365

720

650

745

1.7366

3.5947

5.2494

5.3094

5.3110

7.1002

x 4 max 6

5410 5435 18484 18399

5565 557.1

17969 17949

573.0 5792

17452 17266

599.0 589.1

16694 16975

595.5 588.3

16792 16998

617.5 625.0

16194 15999

5133

x I .

mrx

s

) Loss of the energy (p) in passing from the c&ion to the corresponding m*thylw base.

2) Deviation : values taken from referencr. (3 )

3) Resonance energy (-p)

4) Am&x (m4 ) in MeNO of the principal absorption of the symmetrical rrimethinecyrninr 6 =wave number (cm-l) Wues taken from referancr. (3)

5) x max and s calculated from quation (1).

5134 No.49

The exteneion of the scope of the poeeible charge migration in the

dye ion to carbon atome of the rings lying beyond nitrogen atome ie called by

L.G.S. Brooker extrachromophoric conjugation (1).

It appear8 reasonable to euppoee that, for the nuclei of Table I, the

exteneion of charge migration ie proportional to the delooalization energy.

The calculated delocalization energies are reported in Table I (Atr valuee).

If now 3 (eee Table I) of the first absorption peak of trimethine symmetrical dyes derived from the reported quaternary salte ie considered ae a linear ccm-

bination ofAEb andAEZ,i.e.i)=btbAfb+cAB r a rather positive regreeeicn surface

ie obtained. The values of a, b and c were calculated with the usu8l methode

(6) givingc = 48398 - 6888AEb- SOlAE, (R = 0.972) 1). TheAmax calculated

from equation 1) are satisfactorily in agreement with experimentalAm_ values

(see Table I and Fig. 2).

It can be deduced from equation 1)

the extrachromophoric conjugation and to the

that the euaceptibility of5 to

"baeicity" are in the Mtio 1:lO.

Financial'eupport for this work was obtained

Bicerche.

(1)

(2)

(3)

(4)

(5)

(6)

BEFEBEIVCES

from Coneiglio lazionale delle

L.G.S. Brooker and W.T. Simpson, Ann. Rev. Phya. Chem., $, 121 (1951).

H.Kuhn, Belv. Chin. Aota, & 1441 (1948); J. Chem. Phye., 10, 840 (1948); J. Chem. Phys., I$ 1198 (1949).

L.G.S. Brooker, A.L. Sklar, H.W.J. Cressman, G.H. Keyes, L.A. Smith, B.H. S-e, E. Van Lare, 0. Van Zandt, F.L. White and W.W. Williams, J. Am. Chem. Sot., $2, 1875 (1945) snd other papers of this seriee. -

L.G.S. Brooker, S.G. Dent Jr., D.V. Heaeltine and E. Van Lare, J. Am. Chem. Sot., 22, 4335 (1953).

A. Streitwieser Jr., Molecular Orbital Theory for Organic Chemiete, Wiley, Hew York-London (1961).

G.W. Snedeoor, Statietioal Methods, 4th. Edition, Iowa State College Prees, Ames, Iowa (1946).