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-O2NC6H4NR2) (1a-f) and alkyl-4-nitrophenyl ether (p-O2NC6H4OR) (2a-c) solvatochromic pi* indicators have been characterized and compared with respect to: (a) solvatochromic bandshape, (b) sensitivity expressed as -s, (d (v) over bar max/d pi *), and (c) trends in -s with increasing length of alkyl chain(s) on the probe molecule. -Octyl 4-nitrophenyl ether (p-O2NC6H4OC8H17) (2b) and -decyl 4-nitrophenyl ether (p-O2N C6H4 OC10H21) (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,C- for example, N,N-didecyl-4-nitroaniline (p-O2NC6H4N (C10H21)(2)) and N,N-didodecyl-4-nitroaniline (p-O2NC6H4N (C12H25)(2)) (1d-f). A plot of -s versus C-n follows a 1st order decay fo.- 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 pi* 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 (c) 2007 John Wiley & Sons, Ltd.