CHEMISTRY

N. A. KITROSSKII and V. A. IZMAILSKII

ABSORPTION SPECTRA OF SOLUTIONS OF DIMETHYLAMINOSTYRYL DERIVATIVES OF ACRIDINE IN NITROBENZENE

(Presented by Academician B. A. Kazanskii, January 21, 1960)

Dimethylamino derivatives of quinoline and acridine (electron-donor components of the type (AK^)) can form colored molecular complexes of the type ([BK \cap AK]) with quinolinium and acridinium electrophilic components of the type (BK^) ((^{1-4})). Taking into account the ability of aromatic amines to give molecular complexes with nitrobenzene (component (BK)) ((^{5-7})), it could be assumed that analogous colored complexes with nitrobenzene should also be formed by dimethylaminostyryl derivatives of acridine and quinoline. To test this assumption, the spectra were studied of solutions of molecular complexes formed upon dissolution in nitrobenzene of a series of components (AK_1—AK_5) (I–V)—anhydro bases of acridine derivatives (Table 1**).

To shift the equilibrium (AK + BK \rightleftharpoons [AK \cap BK]) toward the complex, we used nitrobenzene ((BK)) as the solvent in large excess. In all cases, the interaction of the above (AK)-components with (\mathrm{C_6H_5NO_2}) ((BK)) led to a bathochromic shift of the anhydro-base curve, which we explain by the formation of a complex in solution.

An observation that was very important from our point of view was made: in the formation of the complex of (\mathrm{C_6H_5NO_2}) with 9-((p)-dimethylaminostyryl)-acridine,

* (AK) and (BK) are complex chromophoric components constructed from an electron-donor chromophoric component (A) (for example, (\mathrm{NMe_2})) or, respectively, an electrophilic chromophoric component (B) (for example, (\mathrm{C}=\overset{+}{\mathrm{N}}, \mathrm{NO_2})) with a (\pi)-electron conjugated system (K).

** (\lambda_{\max}) and (\varepsilon), determined approximately from the bend of the curve, are denoted by the sign (\sim). The values of (\varepsilon) for solutions of complexes are only conditional quantities ((^5)), which may be used for comparing the degree of complex formation.

((AK_1,\ \mathrm{I})) (2, Fig. 1) or with 9-((n)-dimethylaminostyryl)-3,4-benzacridone ((AK_4,\ \mathrm{IV})) (11, Fig. 2), a new band appeared, whose shape and region of maximum—

Fig. 1. Absorption spectra of molecular complexes of (AK_1), (AK_2), and (AK_3) with nitrobenzene. The numbers of the curves correspond to the numbers of the solutions in Table 1. (3—\lambda_{\max}\ 616\ \mathrm{m\mu}) ((\varepsilon = 29810)) (2)

Fig. 2. Absorption spectra of molecular complexes of (AK_4) and (AK_5) with nitrobenzene. The numbers of the curves correspond to the numbers of the solutions in Table 1.

—was strikingly close to the long-wavelength band and (\lambda_{\max}) of the corresponding dyes (3, Fig. 1 and 12, Fig. 2), formed by the addition of HCl and RX to the anhydrobase and of the corresponding complex of

Table 1

Comparison of the absorption spectra of components of the (AK) type with the spectra of molecular complexes of the ([AK \sim BK]) type and the corresponding dyes (B—K—A)

¹ (AK_1) (I) in 0.01 (N) HCl in EtOH. ² Ethyl perchlorate (AK_2). ³ 1,2-dichloroethane. ⁴ (AK_3) (III) in 0.001 (N) HCl in (CHCl_3). ⁵ Ethyl perchlorate (AK_4). ⁶ (AK_5) (V) in 0.01 (N) HCl in (CHCl_3).

of the above-mentioned (AK) with the acridine component (BK) (²–⁴). This indicates that (C_6H_5NO_2) appears in the complex as if in the form of an aprotonic acid. An explanation of the above-mentioned similarity of the curves and absorption regions of the complexes with (C_6H_5NO_2), with the acridine (BK)-component, and with the corresponding dye could be sought in ionization during formation of the complex (⁶). Another explanation is more probable: charge transfer upon excitation by light occurs not between the components forming the complex (⁷), but within one of the components, in the present case (AK), which acquires a partial charge ((\delta^+)) (⁸) as a result of complex conjugation (“complex mesomerism”) (⁹,¹⁰)*. Thus the systems (AK_1) and (AK_4) play, in the mentioned complexes, the role of the principal chromophoric component.

Fig. 3

Comparison of the spectra of solutions 1 and 3 with the spectra of solutions 4 and 6 (Table 1, Fig. 1) shows that annelation of the benzene ring in position 1,2 in 9-((p)-dimethylaminostyryl)-1,2-benzacridine ((AK_2), II) leads to a sharp hypsochromic shift of the curve compared with ((AK_1), I), both for the anhydro base (4) and for the dye salt (6) ((N)-ethyl perchlorate (AK_2) (II)). The cause of this could be (as a result of annelation) scattering of the conjugation effect (meso-effect) (¹³) in the ground state and upon transition to the excited state. However, the main cause should be considered the removal from coplanarity of the dimethylaminostyryl chromophoric group and the shaded parts of the molecule in Fig. 3, as may be assumed by analogy with benzo[c]phenanthrene (X-ray structural data (¹⁴)). These circumstances were reflected to an even sharper degree in the spectrum of the solution ((AK_2 + C_6H_5NO_2)): in the visible region of the spectrum there is only a small shift of the curve toward longer wavelengths (at (\lg \varepsilon = 1), (\Delta\lambda = +30) mμ) and a characteristic band in the region 600—700 mμ

* The concept of “complex mesomerism” (“complex resonance”) was advanced considerably earlier by Brackman (¹¹) and Mulliken (¹²).

which occurred in cases 2 and 11) is absent; there is only a weakly expressed bend (5, Fig. 1).

Replacement in (I) of the CH=CH group by the CH=N group in p-diethylaminoanilacridine-9-aldehyde ($AK_3$, III), with simultaneous replacement of the $\mathrm{NMe_2}$ group by $\mathrm{NEt_2}$,* caused, in the anhydro base (III), a bathochromic shift of the long-wavelength branch of the curve (at $\lg \varepsilon = 1$, $\Delta \lambda = +42$ m$\mu$).

For the solution of (III) in nitrobenzene, only a bathochromic shift of the curve with a bend at $\sim 590$ m$\mu$ was observed (8, Fig. 1). The shift was manifested to an even stronger degree in the dye (hydrochloride $AK_3$, Fig. 1, 9). Analogous phenomena were first observed by A. I. Kiprianov ($^{16}$). A similar observation was also made later ($^{17}$). Unfortunately, the isomer with the inverted position of the azomethine group $\mathrm{N=CH}$ is inaccessible, owing to the tautomeric structure of 9-aminoacridine.

In the case of 9-(p-dimethylaminostyryl)-3,4,5,6-dibenzacridine $AK_5$ (V), the presence of two condensed benzene nuclei leads to still greater dissipation of the conjugation effect. The spectra of the anhydro base and of the solution (V) in nitrobenzene are shifted strongly hypsochromically. For the molecular complex $[AK_5 \sim \mathrm{C_6H_5NO_2}]$, $\lambda_{\max} \sim 590$ m$\mu$ is observed, almost coinciding in bend with $\lambda_{\max}$ 600 m$\mu$ for the hydrochloride of $AK_5$ (Fig. 2, 14, 15).

The very small values of $\varepsilon$ (2, 5, 8, 11, 14, Table 1), of the order of 20–40, are considerably lower than the values of $\varepsilon$ that we observed for solutions of complexes of $\alpha$- and, respectively, $\beta$-naphthylamines with nitrobenzene ($\varepsilon = 1620$ and 208, respectively ($^{18}$)). We explain this by the presence in the anhydro bases (I–V) of the grouping

[

\overset{\delta+}{\mathrm{C}}=\overset{\delta-}{\mathrm{N}}-,
]

which leads to an increase in conjugation of $\mathrm{NMe_2}$ (in the dimethylaminostyryl grouping) and, as a result, to a decrease in basicity and in the capacity of the $AK$ system to interact with $\mathrm{C_6H_5NO_2}$. In the molecules of $\alpha$- and $\beta$-naphthylamines, the grouping $>\mathrm{C=N}-$ is absent. Investigation of the question of the possibility of replacing, in “donor-acceptor” complexes, the acridine or quinoline component by nitrobenzene gave new proof that the appearance of color upon formation of the complex is not connected with alcoholysis of the onium component and the formation of halochromic adducts with the $AK$ component (for other evidence, see ($^{2,3,4,7}$)).

Moscow Pedagogical Institute
named after V. P. Potemkin

Received
20 I 1960

CITED LITERATURE

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$^{18}$ N. A. Kitrosskii, V. A. Izmail’skii, Khim. nauka i prom., 1960 (in press).

* We were forced to do this in order to increase solubility ($^{15}$).