Ultrafast Relaxation Dynamics of 5,10,15,20-meso-Tetrakis Pentafluorophenyl Porphyrin Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy

Abstract

The ultrafast photophysical characterization of 5,10,15,20-meso-tetrakis pentafluorophenyl porphyrin (H2F20TPP) in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solution has been done in the femtosecond-picosecond time domain, by combining fluorescence up-conversion and femtosecond transient absorption spectroscopy. Fluorescence up-conversion studies on H2F20TPP were done demonstrating fluorescence dynamics over the whole spectral range from 440 to 650 nm when excited at 405 nm, 360.5 cm(-1) excess vibrational energy of Soret band (411 nm). Single-exponential decay with ∼160 ± 50 fs lifetime of Soret fluorescence (also called S2 fluorescence or B band fluorescence) at around 440 nm was observed. On going from 440 nm, S2 fluorescence to S1 fluorescence, (Q-band) around 640 nm (wavelength of 0-0 transition in the stationary spectrum), single-exponential fluorescence time profile turns into a multiexponential time profile and it could be resolved critically into five-exponential components. An ultrafast rise component with ∼160 ± 50 fs followed by two decay components: a very fast decay component with 200 ± 50 fs time constant and another relatively slower 1.8 ± 0.5 ps decay component. Next, a very prominent rise component with 105 ± 30 ps lifetime followed by long-lived 10 ns decay component. The initial rise of S1 (Q-band) fluorescence around 640 nm agreed with the decay time of S2 (Soret or B band) fluorescence indicates that internal conversion (IC) from relaxed S2 to vibrationally excited S1 occurs in the ∼160 fs time scale and subsequent very fast decay with 200 fs time constant, which is assigned to be intramolecular vibrational dephasing or redistribution. The 1.8 ps decay component of S1 fluorescence is attributed to be "hot" fluorescence from vibrationally excited S1 state, and it reveals the vibrational relaxation time induced by elastic or quasi-elastic collision with solvent molecules. The 105 ps rise component is the creation time of the thermally equilibrated S1 state population, and it could be attributed either to an excited state conformational relaxation/intramolecular charge transfer or a molecular cooling process by dissipation of excess energy within the solvent by inelastic collision. Finally, the decay of equilibrated S1(Qx state) occurs on 10 ns to S0 by fluorescence. Femtosecond resolved transient absorption studies on H2F20TPP in the spectral range 390-620 nm following both S2 (Soret band) and S1 (Qx) band excitation have been done and they complement the observations found in fluorescence up-conversion studies. The stimulated emission (SE) kinetics observed at 640 nm, S1 emission peak, in 2-10 ps time domain rebuilds a dynamic similar to that observed by fluorescence up-conversion study. The transient absorption kinetics upon S1 excitation were observed mainly to be biexponential with decay constants 105 ps and 10 ns, respectively. At a long time window (6 ns), a long-lived rise component could be predicted followed by two long-lived decay components for both the excitations in betw...

Investigation of Morphology‐Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin

“…Jtype or edge to edge arrangement and H-type or face to face arrangement. [16][17][18][19] J-type and H-type aggregated porphyrin exhibit red and blue absorption shift from their monomeric entity, respectively [20,21] and the J-type aggregation exhibits the faster energy transfer. [15,[22][23][24] The morphologies of porphyrinoid assembly are controlled by polarity, pH, concentration of porphyrin, porphyrin structure, aging time and counter ions of inorganic salt.…”

Fano Description of Single-Hydrocarbon Fluorescence Excited by a Scanning Tunneling Microscope

“…The apparent violation of the empirical Kasha rule, 51 which forbids molecular luminescence due to non-thermalized excitons may be rationalized in terms of a strongly reduced fluorescence lifetime. Conventional chromophores in solution or in the condensed phase exhibit a fluorescence relaxation time on the order of 1 ns, while the vibrational relaxation time in an electronic state can be an order of magnitude smaller 52,53. …”

Corrole–ferrocene and corrole–anthraquinone dyads: synthesis, spectroscopy and photochemistry

“…The fit curve is shown in Figure 8 and corresponding amplitudes are listed in Table 3. In analogy to the ultrafast dynamic of tetrapyrrolic systems reported elsewhere 45,50,51 the first component can be assigned to the vibrational cooling processes. However, FU signal profile of the Cor-AQ dyad shows very fast decay and it is nicely fit by bi-exponential components with 2 ps lifetime, a major component, along with a very weak and long 3 ns lifetime which was kept as constrained.…”