A new 1,10-phenanthroline (Phen) derivative, 3,8-bis(5methylthiophen-2-yl)-1,10-phenanthroline (PHT1), has been synthesized and fully characterized including by single-crystal X-ray structure analysis. The electron-donating effects of the two 5-methylthiophene groups lead to distinctive fluorescence emission enhancement on complexation with Zn 2+ compared with non-emissive Phen. However, PHT1 shows stronger Zn 2+ -sensing ability than 3,8-bis(thiophen-2-yl)-1,10-phenanthroline (PHT0) or 3,8-bis(3-methylthiophen-2yl)-1,10-phenanthroline (PHT2) and longer-wavelength fluorescence emission (λ em = 461 nm), which is vital for studies of living systems, mainly because of the lower steric hindrance and the electron-donating effects of the 5-methyl groups on the thiophene rings. More interestingly, PHT1 dis-Eur. 3845 Figure 2. (a) UV/Vis and (b) fluorescence emission spectra of 10 μm PHT1 upon the addition of 0-1.2 equiv. Zn 2+ (CH 3 CH 2 OH/H 2 O, 9:1, v/v; λ ex = 325 nm).www.eurjic.org