Solvatochromic properties of long alkyl chain π* indicators: comparison of N , N ‐dialkyl‐4‐nitroanilines and alkyl 4‐nitrophenyl ethers

Hydrophobic forms of the N , N ‐dialkyl‐4‐nitroaniline (DNAP) ( p ‐O 2 NC 6 H 4 NR 2 ) ( 1a–f ) and alkyl‐4‐nitrophenyl ether ( p ‐O 2 NC 6 H 4 OR) ( 2a–c ) solvatochromic π* indicators have been characterized and compared with respect to: (a) solvatochromic bandshape, (b) sensitivity expressed as − s , ( ${\rm d}\bar v_{\max } $ / d π * ), and (c) trends in − s with increasing length of alkyl chain(s) on the probe molecule. Octyl 4‐nitrophenyl ether ( p ‐O 2 NC 6 H 4 OC 8 H 17 ) ( 2b ) and decyl 4‐nitrophenyl ether ( p ‐O 2 N C 6 H 4 OC 10 H 21 ) ( 2c ) were synthesized and their solvatochromic UV/Vis absorption bands were found to maintain a Gausso‐Lorentzian bandshape for the indicators in non‐polar and alkyl substituted aromatic solvents, for example, hexane(s) and mesitylene. Corresponding absorption bands for 1a–f display increasing deviation from a Gausso‐Lorentzian shape in the same solvents as the alkyl chains on the indicator are increased in length all the way to C 10 and C 12 , for example, N , N ‐didecyl‐4‐nitroaniline ( p ‐O 2 NC 6 H 4 N (C 10 H 21 ) 2 ) and N , N ‐didodecyl‐4‐nitroaniline ( p ‐O 2 NC 6 H 4 N (C 12 H 25 ) 2 ) ( 1d–f ). A plot of − s versus C n follows a 1st order decay for the DNAP indicators but is linear for the alkyl 4‐nitrophenyl ethers. A discussion of how the long alkyl chains on the two types of indicators affect the orientation and overlap of n and π * orbitals, and resulting solvatochromic bands is presented. For DNAP, overextending the alkyl chains to obtain greater hydrophobic character may cause the alkane component to dominate solute‐solvation processes at the expense of the probe's fundamental solvatochromic character. Copyright © 2007 John Wiley & Sons, Ltd.