Ultrafast nonlinear optical (NLO) properties of four
novel Corroles
in the visible spectral range (660–800 nm) were evaluated using
picosecond Z-scan technique. Ultrafast excited state dynamics have
also been appraised with picosecond (ps) and femtosecond (fs) degenerate
pump–probe techniques using excitation wavelengths of 800 and
600 nm, respectively. The excitation by 800 nm photons resulted in
two-photon absorption at adequately high peak intensities, thereby
facilitating the access to higher excited states (S
n
). The nonradiative relaxation mechanisms from these states,
reflected in the pump–probe data, consisted of double-exponential
decay with a slow component in the range of 54–277 ps and faster
component in the range of 2.0 to 2.5 ps. When excited with 600 nm
photons (unfocused), photoinduced absorption was observed with the
first excited state S1 being populated, and as a consequence
single decay was observed in the data of all molecules studied. These
retrieved lifetimes were analogous to those obtained with picosecond
pump–probe data. The long lifetime is attributed to nonradiative
decay from the S1 state with possible contribution from
triplet states, whereas the shorter lifetime is attributed to the
internal conversion (S2 to S1*), followed by
vibrational relaxation (S1* to S1) processes.
Time-resolved fluorescence lifetime measurements revealed the magnitude
of radiative lifetimes to be in the nanosecond regime. NLO coefficients
were evaluated from the Z-scan data at wavelengths of 660, 680, 700,
740, and 800 nm. Large two-photon absorption coefficients (β)/cross-sections
(σ2) at 740 nm/680 nm were recorded for these molecules,
making them apposite for applications such as two-photon induced photodynamic
therapy and lithography. Figure of merit, T, was
<1 for all molecules at 740 and 800 nm, suggesting that these molecules
find use in photonic device applications.
Ultrafast excited state dynamics of dinaphthoporphycenes were investigated using femtosecond and picosecond degenerate pump-probe techniques at 600 nm and 800 nm, respectively. Femtosecond pump-probe data indicated photo-induced absorption at 600 nm resulting from two-photon/single photon excitation, whereas picosecond pump-probe data demonstrated photo-bleaching which was a consequence of three-photon absorption. The fastest lifetimes (100-120 fs) observed are attributed to the intramolecular vibrational relaxation, the slower ones (1-3 ps) to internal conversion, and the slowest components (7-10 ps) to non-radiative decay back to ground state. Z-scan studies in the 560-600 nm range were also carried out. V
Sterically hindered molecules like the title compounds, combined with their strong NLO coefficients make them potential candidates for photonic applications.
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