Ultrafast Studies of Excited-State Dynamics of Phthalocyanine and Zinc Phthalocyanine Tetrasulfonate in Solution

Abstract

Femtosecond measurements of transient absorption, bleach, and stimulated emission are used to study the excited-state dynamics of phthalocyanine tetrasulfonate (PcS4) and zinc phthalocyanine tetrasulfonate (ZnPcS4) in solution. In water the excited-state decay process is fast and dominated by energy relaxation due to intermolecular aggregation. In dimethyl sulfoxide (DMSO) both PcS4 and ZnPcS4 exist predominantly in the monomeric form and exhibit very different dynamics from that of the aggregates. The decays are much slower and the observed processes are strongly dependent on the probe wavelength. For PcS4 in DMSO, when probed at 790 nm, the dynamics are dominated by stimulated emission which is observed for the first time in solution. At other wavelengths either transient absorption or bleach dominates the signal. All the observed dynamics can be well fit using a double-exponential function with a fast and slow component. The fast decay has a time constant of 10 ± 4 ps for both phthalocyanines while the slow decay has a time constant of 370 ps for PcS4 and 460 ps for ZnPcS4, respectively. The overall excited-state decay dynamics correlate well with the recovery of the ground electronic state, indicating that the recovery is the predominant process on this time scale. On the basis of a simple three-state kinetic model, the fast decay (10 ps) is attributed primarily to a conversion from the second to the first excited singlet state, possibly involving vibrational relaxation in S1. There might also be a small contribution from aggregates. The first excited-state S1 subsequently decays with a time constant of 130 ps for PcS4 and 160 ps for ZnPcS4, respectively. This decay is due to a combination of radiative and nonradiative relaxation from S1 to S0 and intersystem crossing from S1 to the triplet state.

Facile Design of a Plasmonic Nanolaser

“…The PL emission peaking at 708 nm is assigned to the longitudinal plasmonic emission of the AuNRs. Pcts dissolved in water shows two strong broad absorption bands: the Soret band appears in the near-UV (332 nm) and the Q-band in the red (630 nm) [11]. The Q-band arises from in-plane electronic (π → π*) transitions.…”

“…The resulting time constants with t 1 = 22 ps and t 2 = 65 ps reflect ultrashort decay or rise dynamics. In case of a laser dye such as Pcts picosecond-resolved dynamics are characteristic for stimulated emission, providing the deactivation of the S 1 state and the repopulation of the S 0 ground state [11].…”

Mechanism of the Zn(II)Phthalocyanines’ Photochemical Reactions Depending on the Number of Substituents and Geometry

“…The fluorescence rate (k f " 1{τ f ) was determine by time-resolved measurements, already described. The relaxation time of the second excited state to the first excited state is known to be around 300 fs for phthalocyanines and porphyrins [30,31]. Solving numerically the rate equations and using only as an adjustable parameter the excited state absorption cross-section, σ 1n pλq, we were able to model (solid lines) the single-pulse Z-scan signatures at 532 nm (symbols), as shown in Figure 4.…”

Metallo-Phthalocyanine Near-IR Fluorophores: Oligonucleotide Conjugates and Their Applications in PCR Assays

“…With very few exceptions, neither MPc dimers nor higher order cofacial aggregates fluoresce (47)(48)(49). Electronic adsorption spectra of MPc's can provide insight into groundstate aggregation processes (50); a split and blue-shifted Q-band typically indicates cofacial ground-state aggregation of Pc rings (19,44,46,51,52).…”