Excited-State Absorption and Ultrafast Relaxation Dynamics of Porphyrin, Diprotonated Porphyrin, and Tetraoxaporphyrin Dication

Abstract

The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B --> Qy internal conversion is followed by the Qy --> Qx process, vibrational relaxation, and thermalization in the Qx state with time constant approximately 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B --> Q internal conversion and thermal equilibration in the Q state. The B --> Q time constant has been determined to be 25 ps. The large difference with respect to the B --> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm-1 in TOxP2+ vs 6100 cm-1 in H2P. The relaxation dynamics of H4P2+ has a first ultrafast component of approximately 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT --> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).

Effect of protonation on the singlet–singlet excited-state absorption of meso-tetrakis(p-sulphonatophenyl) porphyrin

“…3a. This fact, together with the small fluorescence Stokes shift observed previously [4,17], indicates that the TPPS 4 vibronic structure is preserved upon excitation [18,19]. In other words, the molecular structure is the same for all singlet states participating in the absorption process and the D 2h molecular symmetry, characteristic for the ground state other hand, the ratio r 1n (k)/r 01 (k) % 2, observed in the entire Q-band region shows that the S 1 ?…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Going to the unsubstituted molecule, the non radiative processes follow a decay pattern similar to that of tetraphenylporphyrin [306]. The femtosecond transient absorption results on porphyrin are presented in Figure 51 while Figure 52 shows the decay kinetics of the 412 nad 465 nm ESA bands.…”

“…The results from three recent calculations relative to porphyrin and presented in Table 13 differ from experiment [299,304] by no more than 0.2 -0.3 eV not only for the Q x , Q y and B x , B y pairs but also for the weak transitions, usually denoted N x , N y and L y , and observed at higher energies with respect to the Soret band. Exciting into the Soret absorption band of tetraoxaporphyrin as indicated in Figure 48 radiative and non radiative decay processes are promoted [305,306]. The fluorescence emissions from the first and second excited state, Q → S 0 and B → S 0 , are easily observed with quantum yield 0.11 and 0.035, respectively [305].…”

“…The B lifetime could only be estimated to be ≤ 100 ps, due to the instrumental resolution of the time-resolved fluorescence apparatus, and indirectly evaluated from quantum yield and radiative time constant to be ≈ 40 ps. Transient absorption pump-probe experiments have been set up in the femtosecond time domain [306] following experimental procedures described in detail [160,307,308] in order to characterize the non-radiative relaxation channels from the B state. In general, transient absorbances ∆A arise from four different contributions [307], excited state absorption (ESA), stimulated emission (SE), stimulated Raman gain (SRG) and bleaching (B).…”

Enhanced energy transport in genetically engineered excitonic networks

“…The sample was contained in a 2 mm path-length quartz cell and was stirred continuously with a small magnetic bar to refresh the solution and avoid photo degradation. More detailed set-up information can be found in previous works [31][32][33] .…”