Charge transfer emissive singlet excited states and photoinduced electron transfer properties in the diarylideneacetone compounds (RCHCH)₂CO; R = phenyl, 1- and 2-naphthyl, 3-(N-ethylcarbazoyl), and 4-(C₅H₅)Fe(C₅H₄C₆H₄CHCH(CO)CHCHC₆H₅)

Abstract

. 68,2278 (1990). Four diarylideneacetone compounds ((RCHCH)2C0, where the aryl groups are phenyl (dba), 1-naphthyl (I-dNapha), 2-naphthyl (2-dNapha), and 3-(N-ethylcarbazoyl) (dNECa)), and 4-(C5H5)Fe(C5H4C6H,CHCH(CO)CHCH(C6H,) (dba-Fc) have been prepared and characterized. The compounds are found to be fluorescent and photochemically and reversibly electrochemically active. The lowest-energy absorption bands for the diarylideneacetones are assigned to a charge transfer (CT) electronic transition, except for dba-Fc, in which a ferrocenyl ligand field transition assignment is made. The 77 K C T absorption and fluorescence bands are vibrationally structured (vibrational spacings = 1260-1360 cm-I). While the fluorescence decay at 293 K is monoexponential, the excited state fluorescence lifetimes (TF) for the 77 K samples exhibit double exponential decays, the short component being 0.38-0.64 ns and the long one 3.5-10.9 ns. The photophysical results are interpreted in terms of excited state deactivation processes dominated by radiationless pathways that are associated with the presence of fluorescent species with different geometries. Only the dNECa compound is found to be fluorescent in solution at 298 K (TF = 1.65 ns; @F = 0.39 * 0.02). Cyclic voltammetry and coulometry measurements suggest that a reversible one-electron reduction process and an irreversible higher potential one-electron reduction process take place in the -I to -2 V vs. SSCE range. In addition, dba-Fc also exhibits an electrochemically reversible one-electron oxidation wave at 0.52 V vs. SSCE centered at the ferrocenyl group. These results together with the spectroscopic electronic data have permitted evaluation of the reduction potentials of the lowest singlet (CTexcited states (El-'*); they range from 1.4 to 2.2 V vs. SSCE, with dba being the strongest photooxidizing agent and dNECa the weakest. :Photoinduced intermolecular electron transfer reactions have been investigated by steady state fluorescence techniques and picosecond flash photolysis spectroscopy for dNECa and dba, respectively. The bimolecular deactivation rate constants, k , for the reductive photoinduced electron transfer reactions of dNECa with diphenylamine (DPA) (k = (2.65 L 0.25) X loq M-' s-') and N, N, N', Nf-tetramethylphenylenediamine (TMPM) (k, = (1.38 k 0.03) X 10' M-' s-' ) have been obtained in THF solutions at 293 K. No fluorescence quenching is observed when oxidative and energy transfer quenchers are used with dNECa. For the non-emissive dba compound at room temperature, picosecond flash photolysis experiments show that quenching of the broad dba transient band (-500 nm) does indeed occur between 5 and 10 ns.

Nature and characterization of the singlet and triplet excited states of the dibenzylideneacetone and di-3-(N-ethylcarbazoylidene)acetone bridging ligands

“…respectively (9). The lower-frequency mode could be associated to ring deformation coupled with v(C=C).…”

“…Dba (14) and dNECa (9) were prepared according to standard '~u t h o r to whom correspondence may be addressed.…”

“…The CT assignment for the lowest energy absorption band has been established (9). The variation of the dipole moment should be polarized along the main ( z ) axis of dNECa in the C,, point group (s,cis-s,cis conformation).…”

“…In the free state the ligand adopts primarily the s,cis-s,cis conformation (7) and appears to be particularly photoreactive (8). Electron transfer and cis,trans isomerization reactions have been observed in the lowestenergy singlet (9) and triplet excited states, respectively (8). The luminescing singlet excited state has been assigned to a charge transfer (CT) state from the aryl groups to the ketone (9).…”

“…Electron transfer and cis,trans isomerization reactions have been observed in the lowestenergy singlet (9) and triplet excited states, respectively (8). The luminescing singlet excited state has been assigned to a charge transfer (CT) state from the aryl groups to the ketone (9). There has been no spectroscopic evidence for an tz -+ r * transition associated with the presence of the carbonyl group although the molecule is' closely related to benzophenone for which the lowest energy n r * singlet and triplet excited states are well known (10).…”