The Photophysics of Three Naphthylmethylene Malononitriles

Abstract

The solvent dependence of the photophysical properties of three naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedinitrile (1-MN), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2-(3,4-dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-MN), was studied in order to determine their potential utility as fluidity probes and to make comparisons to the better studied benzylidene malononitriles. Density functional calculations were used to understand the possible conformational states related to rotation about the vinyl-aromatic bond ("τ"). Absorption and emission frequencies, extinction coefficients, fluorescence quantum yields, and fluorescence lifetimes were measured in 11 representative solvents. Both the computational and experimental results indicate that the S0 → S1 transitions of these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result from relaxed S1 states which do not differ qualitatively from the Franck-Condon states reached by absorption. In 2-MN, time-resolved emission reveals the presence of two ground-state conformers ("a" and "b" differing by ∼180° rotation about τ) coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer. Fluorescence lifetimes vary from ∼1 ps in 1-MN to ∼200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a), the lifetimes vary systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence decay in naphthylmethylene malononitriles is likely to be the same as that of the benzylidene malononitriles-twisting about the double bond in S1, which leads to rapid internal conversion via a conical intersection with S0.

Synthesis and Enhanced Linear and Nonlinear Optical Properties of Chromophore–Au Metal Cluster Oligomers

“…Therefore, the shortening of the excited state lifetime is due to the increase in the transition dipole moment of the chromophore−Au 25 nanocluster dimers compared to the single Au 25 nanoclusters. A similar correlation between transition dipole and fluorescence lifetime has been observed in naphthylmethylene malononitriles and certain fluorophores 85,87,88 Another example is the shortening of emission lifetime of attached fluorophores due to energy transfer to a plasmonic metal nanoparticle with large dipole moment, generating a nonfluorescent state, the "dark states". 89,90 The increase of transition dipole moment combined with the competitive nonradiative relaxation from the core to the surface states contributed to the shortening of the nanocluster's core excited state lifetime.…”

Solute Rotation in Ionic Liquids: Size, Shape, and Electrostatic Effects

“…Time-resolved fluorescence decays of DMAn, CMAn, and DMAc + in [Im 41 ][BF 4 ] (0.1−0.2 OD) were collected using a previously described time-correlated single photon counting (TCSPC) instrument. 58 In short, fundamental pulses from a cavity-dumped Ti:sapphire oscillator (Mira 900 and PulseSwitch pumped by a Coherent Verdi V5 laser) were frequency doubled using a BBO crystal to achieve excitation wavelengths of 400 nm (DMAn), 410 nm (CMAn), and 450 nm (DMAc + ). Vertical excitation was employed and polarized emission decays, I VV (t), I VH (t), and magic angle were collected at 90°t o the excitation beam.…”

Electron Transfer Kinetics between an Electron-Accepting Ionic Liquid and Coumarin Dyes

“…Time-Resolved Fluorescence Measurements. A home-build time-correlated single photon counting (TCSPC) instrument 49 was used to measure lifetimes in the range 25 ps−15 ns. For the experiments described in this paper, excitation pulses were provided by the frequency-doubled output of a cavity-dumped Ti:Sapphire oscillator (Coherent Mira 900 + PulseSwitch) tuned to ∼800 nm.…”