RESUMO
The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2,6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2-dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the pyrene (Py) subunit leads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5-5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with ability to undergo CS.
RESUMO
A theoretical investigation on the electronic structure of 4-dimethylamino-4'-nitrostilbene (DANS), 4-(dicyanomethylene)-2-methyl-6-p-(dimethylamino) styryl-4H-pyran (DCM), and their protonated forms is presented in an effort to rationalize recent experimental results on the tuning of the emitted color of organic light-emitting diodes through photochemically induced protonation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out on the neutral and protonated forms of DANS and DCM, employing both the B3LYP and the CAM-B3LYP functionals. It was found that the CAM-B3LYP functional leads to better agreement than the B3LYP of the calculated with the experimental absorption lambda(max) for DANS, whereas B3LYP is more appropriate than CAM-B3LYP for DCM. The results of the calculations aid in a rationalization of the observed differences of the spectra of DANS and DCM upon protonation, and in particular those differences that make DANS a more attractive system for absorbance and emission tuning.
RESUMO
The odd-even effect is demonstrated, for the first time, in dilute polymeric solutions of polyethers, consisting of substituted luminescent quinquephenyl units which are connected by flexible aliphatic chains of 7-12 methylene groups. The effect, which is demonstrated by means of steady state and time resolved fluorescence anisotropy, has been attributed to the different mutual orientation of the luminescent dipoles, in the odd (7, 9, 11) and even (8, 10, 12) polymers. Namely, as the temperature of the solution is lowered the flexible aliphatic chains adopt the nearly all-staggered lowest energy conformation, which results in different mutual orientations of the fluorophores in the two types of polymers.
RESUMO
On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylamino-triphenylmethyl radical, 3(*) (lambda(max) at 343 and 403 nm), in very high quantum yield (Phi = 0.92). The intervention of the triplet state of 2 (lambda(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3(*) is further demonstrated using EPR spectroscopy. The detection of the S(1) state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (tau(f) = 1500 ps, Phi(f) = 0.085). The S(1) state is converted almost exclusively to the T(1) state (Phi(T) = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(*)(+), and (2) photodissociation of the C-Si bond, giving radical 3(*) as before in hexane. The formation of 2(*)(+) occurs through a two-photon process. Radical cation 2(*)(+) does not fragment further, as would be expected, to 3(*) via a nucleophile(MeCN)-assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2(*)(+) with the solvent (MeCN) and transfer to it of the electrofugal group Me(3)Si(+). The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.
