Solid‐state Structures of N‐Substituted Michler’s Ketones and Their Relation to Solvatochromism

The solvatochromism (νmax) of N-substituted Michler’s ketones − including 4′-[bis(2-acetoxyethyl)amino]-4-(dimethylamino)benzophenone [MK(OAc)2, 1a], 4,4′-bis(diethylamino)benzophenone [MK(NEt2)2, 1e], 4,4′-bis(4-ethoxycarbonylpiperazino)benzophenone [MK(pipOEt)2, 2a], 4,4′-bis(piperidino)benzophenone [MK(pip)2, 2b], 4,4′-bis(morpholino)benzophenone [MK(mor)2, 2c], 4,4′-bis(piperazino)benzophenone [MK(pipaz)2, 2d], 4,4′-bis[4-(2-hydroxyethyl)piperazino]benzophenone [MK(pipazOH)2, 2e] and 1,4-bis(4-benzoylphenyl)piperazine (BBP, 3) − has been studied at 298 K in various solvents with wide ranging dipolarities and hydrogen-bonding abilities. The solvatochromic properties (νmax)s = (νmax)0 + sπ* + aα + bβ have been analysed in terms of the Kamlet−Taft linear solvation energy (LSE) relationship. The effect of the solvent on the UV/Vis spectral characteristics (νmax) has been determined quantitatively for all compounds in terms of the Kamlet−Taft solvent-independent correlation coefficients s, a, and b. It was found to be mainly the dipolarity/polarizability (π*) and hydrogen bond acidity (α) of the solvent that contribute to the bathochromic UV/Vis band shift. As strongly basic sites are present as substituents at the peripheries of the molecules, protonation takes place at the secondary nitrogen atom of, for example, MK(pipaz)2 (2d) in strongly hydrogen bond-donating (HBD) solvents. This specific type of substitution therefore gives rise to an ambiguous solvatochromic property, which makes the basicity (β term) of the solvent important. The solid-state structures of 2b (C23H28N2O), 2c (C21H24N2O3), and 3 (C30H26N2O2) have been determined by single-crystal X-ray structure analysis. Compound 2b crystallizes in the trigonal space group P3121 with a = 948.11(11), b = 948.11(11), c = 1818.0(3) pm, α = β = 90, γ = 120° and Z = 3, whereas 2c crystallizes in the orthorhombic space group Pna21 with a = 1259.9(2), b = 910.16(17), c = 1586.2(3) pm, α = β = γ = 90° and Z = 4. Compound 3 crystallizes in the triclinic space group P with a = 1034.0(2), b = 1079.9(2), c = 1127.2(2) pm, α = 72.062(4), β = 73.361(4), γ = 74.549(4)° and Z = 2. The UV/Vis band shifts are comparatively discussed with regard to the LSE relationships and the results of the solid-state structure determinations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2002)