Photophysical Processes Occurring in a Zn-phthalocyanine in Ethanol Solution and on TiO₂ Nanostructures

Abstract

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)­phenyl] phthalocyaninatozinc­(II) (ZnPc) in solution and adsorbed on TiO2 and ZrO2 nanoparticle films is characterized by stationary and time-resolved spectroscopies in the subpicosecond to nanosecond time interval. The comparison between the solution and the solid substrate data allows us to identify different pathways of the energy and electron relaxation. On the solid substrate, the presence of H-aggregates adds a further nonradiative deactivation channel competing with the charge injection into the TiO2-conducting band, thus providing an explanation of the reduced efficiency of the charge transfer processes. The comparison between the kinetics recorded after excitation of the S0–S2 transition and those recorded after excitation of the S0–S1 transition provides an estimate of the internal conversion between S2 and S1 which occurs very efficiently and on an ultrafast (<50 fs) time scale. The ground-state recovery characterized by the decay of the bleaching band in the transient spectra slows down in TiO2 samples and is taken as evidence of the charge injection in this kind of sample.

Tuning the Structure and Photophysics of a Fluorous Phthalocyanine Platform

“…Breaking the symmetry of the degenerate LUMO orbitals, for instance, could cause the Q-band transitions to become nondegenerate. 55 An extreme case of this is seen in free-base Pcs, in which the D 4 h symmetry has been completely broken to D 2 h , resulting in the Q-band splitting into two distinct peaks. An intermediate case might produce some of the broadening observed, as well as the asymmetry seen in some of the absorbance bands.…”

Understanding the influence of disorder on the exciton dynamics and energy transfer in Zn-phthalocyanine H-aggregates

“…Since the analyzed time-window is much shorter than the excited state lifetime of the dye, the long time-component is fixed to the value of 4 ns, which we previously found on the same system by transient absorption measurements performed on a more extended timescale. 6 The transient signal shows a limited lineshape evolution, as evidenced by normalizing the EADS (see the inset of Fig. 2b).…”

“…We previously investigated the ultrafast behavior of the isolated chromophore in ethanol and adsorbed on titania and zirconia nanostructured films by means of transient absorption spectroscopy, obtaining evidence of H-aggregation of the dye deposited on the oxide films. 6,38 The absorption spectrum of the dye in ethanol (Fig. 1c) shows a main spectral feature composed of two peaks, centered at 14 860 cm À1 and 14 580 cm À1 , attributed to the S 0 -S 1 transition, with a vibronic progression visible at higher energies (15 700 cm À1 and 16 300 cm À1 ).…”

“…† The negative spectral features (blue color) are due ground state bleaching (GSB) and stimulated emission (SE). 6 The small positive signal between 15 000 cm À1 and 16 000 cm À1 is the low-frequency tail of the excited state absorption (ESA) that is very broad and weak, 6 inducing a small perturbation on the spectral window under investigation.…”

“…1,2 Such molecules represent an alternative to porphyrins and perylene, displaying higher chemical and thermal stability while maintaining analogous electronic properties. 1,[3][4][5] Phthalocyanine dyes indeed possess elevated fluorescence quantum yield (QY), intense absorption cross-section in a wide spectral region and anisotropic electrical and optical properties, features that make them suitable for acting as light-harvesting material in Dye Sensitized Solar Cells (DSSCs), [6][7][8][9] inorganic solar-cell junctions, as well as for the fabrication of electroluminescent devices. 10 Due to their extended p aromatic structures, these molecules have large third-order susceptibility and can be used for the development of non-linear optic devices.…”