Effects of pH on the ultrafast transient absorption of iron (III) meso-tetrakis(4-N-methyl-pyridiniumyl) porphyrin (Fe³⁺TMPyP) molecular complexes

Highlights

• Excited-state dynamics absorption of Fe³⁺TMPyP complexes was performed.

• Femtosecond Z-scan and ultrafast transient absorption techniques were employed.

• Fast relaxations, with lifetimes around 2.0 picoseconds, were observed.

• The fast relaxation was attributed to internal conversion relaxation processes.

• μ-oxo-dimers slightly affect the relaxation of Fe³⁺TMPyP.

• Fe³⁺TMPyP porphyrin can be useful for biomedical photothermal applications.

Abstract

The meso-tetrakis (methylpyridiniumyl) porphyrin complexed with Fe³⁺ (Fe³⁺TMPyP) is an irregular porphyrin, possessing unfilled d external shells with d⁶ electronic configuration. In aqueous solution, Fe³⁺TMPyP exists in a complex equilibrium of different forms. Changing the solution pH, these complexes can bind to other species (as OH- and H₂O) or form physical aggregates (μ-oxo-dimers), that alter their excited states properties and affect their potential applications. In the present work, we report excited state absorption dynamics of Fe³⁺TMPyP molecular complexes in different pH employing femtosecond Z-Scan and ultrafast transient absorption techniques. Complementary data were obtained by UV–vis absorption and fluorescence emission spectroscopies. For 532 nm excitation, the Z-Scan technique shows a decrease in the absorption coefficient for Fe³⁺TMPyP at pH 4.0 and an increase at pHs 7.0 and 9.0. Ultrafast transient absorption reveals the same behavior in the 525−570 nm region for samples at pHs 7.0 and 9.0. Fast relaxations, with lifetimes around 2.0 picoseconds, were obtained for all samples studied, and were attributed to internal conversion relaxation processes. Although the molecular aggregation accelerates the relaxation process of regular porphyrins, μ-oxo-dimers slightly affect the relaxation of Fe³⁺TMPyP. The prompt conversion of absorbed light energy in heat observed to Fe³⁺TMPyP porphyrins can be useful for biomedical photothermal applications, such as Photoacoustic Imaging (PAI) and Photothermal Therapy (PTT).

Introduction

Porphyrins are versatile molecules because their physical and chemical properties can be adjusted only by changing some aspects of their molecular structure. Furthermore, their broad-band UV–vis absorption spectra associated with their excited state properties, provides the opportunity for several kinds of applications [[1], [2], [3], [4]].

Paramagnetic metalloporphyrins have been proposed as contrast agents in nuclear magnetic resonance (NMR) tomography for cancer diagnostics [5]. More recently, it was demonstrated that porphyrin complexation with metal ions increases the photoacoustic response [6]. Its photothermal property suggests its potential use in biomedical applications like contrast agents for photoacoustic imaging [7]. Other possible applications for such molecules are their use in optical switching and limiting that can be achieved due to their nonlinear absorption response assigned to resonant excited state absorption [[8], [9], [10]]. For all these applications, the understanding of how structural modifications or environment changes act on the porphyrin excited state features, such as absorption cross-sections, lifetimes, and quantum yields are still a key research field.

Differently from regular porphyrins, such as free-base and Zn²⁺ complexes ones, Fe³⁺ porphyrins are known to be irregular porphyrins, because they possess unfilled d external shells with d⁶ electronic configuration [1]. Furthermore, the metal orbitals can be positioned between the π filled and π* empty orbitals of the porphyrin ring. Since these metal orbitals are considered as electron acceptors, the irregular porphyrins usually present the charge transfer (CT) of an unpaired metal electron to the π conjugated ring, producing a strong fluorescence quenching and reducing the porphyrin’s π* state deactivation lifetime [1]. In this way, the absorbed energy is promptly converted into heat via non-radiative processes (internal conversion). Compounds that present highly efficient photothermal effect can be employed in Photoacoustic Imaging (PAI) [7] as well as in Photo-Thermal Therapy (PTT) [11], in which the efficiency is dependent on the structure and the excited state photophysical properties of the compound.

It is well known that changes in the environment pH of free-base porphyrins can protonate its central ring, affecting their photophysical features such as the ground and excited state absorptions, and relaxation processes [12,13]. The present work reports on the excited state photophysical parameters of meso-tetrakis (methylpyridiniumyl) porphyrin with Fe³⁺ ions (Fe³⁺TMPyP) inserted in the center of macrocycle ring at different pHs. Such a procedure was adopted since iron complexes can bind to other species as OH- and H₂O in the solution and also form physical aggregates (μ-oxo-dimers) depending on the pH [14,15]. The bonding to other species and the formation of dimers can change the excited state properties of the iron porphyrins, affecting their potential applications.

To investigate the fast relaxation time of Fe³⁺TMPyP’s excited state decay at different pHs, ultrafast transient absorption technique probed with a broad-band light source was used to track the excited state deactivation time and the spectral features of the excited state absorption bands. Once this technique measures the difference between the excited and ground state transmittances, the excited state absorption cross-section for the spectral window of the transient absorption experiment was obtained using a known excited state absorption cross-section at a given wavelength [16]. We obtained the particular excited state absorption cross-section employing the single wavelength ultrafast Z-Scan technique [13,17]. Complementary data were obtained with UV–vis absorption spectroscopy.