cis/trans-[Pt((CN)-N-& BOTTOM;)(C VCR)(CNBu t )] Isomers: Synthesis, Photophysical, DFT Studies, and Chemosensory Behavior

The synthesis, structures, and isomerizationof a seriesof trans- and cis-alkynyl/isocyanidecycloplatinated compounds [Pt((CN)-N-& BOTTOM;)(C VCR)(CNBu (t) )] are presented, including a detailed comparativestudy of their photophysical properties, supported by theoreticalstudies. Additionally, a study showing the sensitivity and selectivityin the photophysical response of two pairs of cis/trans isomers toward the highly toxic Hg2+ is included. cis/trans Isomerismcan be acrucial factor for photophysical properties. Here, we report the synthesisand optical properties of a series of trans- and cis-alkynyl/isocyanide cycloplatinated compounds [Pt((CN)-N-& BOTTOM;)(C VCR)(CNBu t )][R = C6H4-4-OMe 1, 3-C4H3S 2; (CN)-N-& BOTTOM; = 2-(2,4-difluorophenyl)pyridine(dfppy) (a), 4-(2-pyridyl)benzaldehyde (ppy-CHO) (b)]. The trans-forms do not isomerize thermallyin MeCN solution to the cis forms, but upon photochemicalirradiation in this medium at 298 K, a variable isomerization to the cis forms was observed. This behavior is in good agreementwith the theoretically calculated energy values. The trans/cis configuration, the identity of the cyclometalated,and the alkynyl ligand influence on the absorption and emission propertiesof the complexes in solution, polystyrene (PS) films, and solid stateare reported. All complexes are efficient triplet emitters in allmedia (except for trans -1a and trans -2a in CH2Cl2 solution at 298 K), with emission wavelengthsdepending mainly on the cyclometalated ligand in the region 473-490nm (dfppy), 510-550 (ppy-CHO), and quantum yields (& phi;)ranging from 18.5 to 40.7% in PS films. The combined photophysicaldata and time-dependent density functional theory calculations (TD-DFT)at the excited-state T-1 geometry reveal triplet excitedstates of L-3 & PRIME;LCT (C VCR & RARR; (CN)-N-& BOTTOM;)/(IL)-I-3 ((CN)-N-& BOTTOM;) character with minor (MLCT)-M-3 contribution. The dfppy (a) complexes showa greater tendency to aggregate in rigid media than the ppy-CHO (b) and the cis with respect to the trans, showing red-shifted structureless bands of (MMLCT)-M-3 and/or excimer-like nature. Interestingly, trans -1a,2a and cis -1a,2a undergo significant changes in the ultraviolet(UV) and emission spectra with Hg2+ ions enabling theiruse for sensing of Hg2+ ions in solution. This is clearlyshown by the hypsochromic shift and substantial decrease of the low-energyabsorption band and an increase of the intensity of the emission inthe MeCN solution upon the addition of a solution of Hg(ClO4)(2) (1:5 molar ratio). Job's plot analysis estimateda 1:1 stoichiometry in the complexation mode of Hg2+ by trans -2a. The binding constant(log K) calculated for this system from absorptiontitration data resulted to be 2.56, and the limit of the detection(LOD) was 6.54 x 10(-7) M.