Article pubs.acs.org/JPCA
Ultrafast Relaxation Dynamics of 5,10,15,20-meso-Tetrakis Pentafluorophenyl Porphyrin Studied by Fluorescence UpConversion and Transient Absorption Spectroscopy P. Hemant Kumar,† Yeduru Venkatesh,†,‡ Doddi Siva,† B. Ramakrishna,† and Prakriti Ranjan Bangal*,†,‡ †
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, 500007, India ‡ Academy of Scientific and Innovative Research, 2-Rafi Marg, New Delhi, 110001, India S Supporting Information *
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 between 450 and 500 nm probe wavelengths. The lifetimes of these components were longer-lived than were possible to exactly measure using our existing femtosecond transient absorption system. However, this apparent rise component is assigned to be a Tn ← T1 transition, and the longest decay component is attributed to the lifetime of the T1 state.
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spectroscopy.12−15 In this context, 5,10,15,20- meso-tetrakis pentafluorophenyl porphyrin (H2F20TPP) is an unique derivative of tetraphenyl porphyrin (H2TPP), a well-studied model system in porphyrin chemistry, where 20 peripheral H atoms of H2TPP are replaced by as many F atoms. Although its
INTRODUCTION
The low energy ground-state and excited-state properties of porphyrin derivatives are influenced by conformational distortions of the porphyrin macrocycle associated with substituent at the periphery of the macrocycle are of great interest.1−11 The electron transition in the lowest excited state of porphyrin is generally observed in the red and near IR spectral regions. The properties of this excited state and the pathways of its deactivation were studied using kinetic © XXXX American Chemical Society
Received: August 22, 2014 Revised: January 22, 2015
A
DOI: 10.1021/jp512137a J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
In this Article, we intend to present a detailed picture of relaxation dynamics in the S2 and S1 states of free base H2F20TPP using the two above-mentioned techniques in analogy to H2TPP and its metal counterpart, which have been studied thoroughly.12−15
steady-state spectroscopic features grossly parallel that of the parent H2TPP molecule, the uniqueness of this molecule lies in its electron-accepting capability in singlet excited state in sharp contrast to its parent H2TPP molecule, which is known as a πelectron donor. For instance, in our earlier report, we have observed that H2F20TPP takes part in photoinduced reductive electron transfer reaction in association with different aliphatic and aromatic amines.16 To a further extent, we also have provided evidence for the participation of this molecule in a proton coupled electron transfer (PCET) event in the trimolecular transition state, where its enhanced reductive fluorescence quenching behavior by different phenol derivatives in the presence of different pyridine bases has been invoked as electron transfer from a hydrogen bonded phenol−pyridine base pair to the excited singlet state of H2F20TPP with concerted motion of bound proton to associated pyridine.17 Hence, this molecule is found to be a potentially important mimic system to better understand activity in biology, in particular in photosynthesis,18 where tyrosine H-bonded to histidine participates in the PCET process in photosystem-II, which is a porphyrin-based complex.19−26 So, at this juncture, detailed understanding of the ultrafast molecular relaxation process of this pigment could help one better comprehend the long-range electron transfer reaction chain in the photosynthesis reaction center in green plants. Although numerous studies on the ultrafast dynamics of different model tetrapyrrole compounds were carried out to explore the molecular relaxation mechanisms in view of the operation of these pigments in electron transfer events in the reaction center of photosynthesis,3,10,11 studies on this aspect for this pigment are sparse.16,17,27 Relaxation dynamic pertaining to higher singlet state S2 can be rationalized from the kinetic profile in a few picosecond time window by multiexponential fitting, whereas the singlet-totriplet transition dynamic can be observed from the kinetic profile in a picosecond-to-nanosecond time window. The relaxation process from the upper excited singlet states of different free base tetrakis porphyrin derivatives and their metal counterparts have been studied over the years, but the characterization of the ultrafast relaxation process of the H2F20TPP molecule of interest here has yet to be explored. Hence, for the time being, femtosecond spectroscopic studies of H2F20TPP by fluorescence up-conversion and transient absorption techniques are not only indispensible but also the quintessential choice of investigations. Among the different experimental methods exploiting the investigation of ultrafast molecular processes, combined use of fluorescence up-conversion and transient absorption spectroscopy has the leading advantage, as both methods collect the same information on the molecular relaxation process, such as internal conversion, intramolecular vibrational relaxation, and molecular cooling within the solvent, but in two different ways, giving opportunity to cross check the observations. In general, fluorescence up-conversion techniques measure the temporal evolution of “transient fluorescent species” and provide the relaxation dynamic of the system from a higher vibrational level to a lower vibration level of the same or different electronic states, whereas the latter one measures the temporal evolution of “transient species” from a lower vibrational level to a higher vibrational level of the same or different electronic states. In a sense, these two techniques complement each other, and the same strategy of data analysis is applicable for both techniques.
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EXPERIMENTAL SECTION Sample Preparation and Steady State Measurement. 5,10,15,20-meso-Tetrakis pentafluorophenyl porphyrin (H2F20TPP) was purchased from Porphyrin System, Germany, and used as received. Spectroscopic grade dichloromethane (DCM) and tetrahydrofuran (THF) were purchased from Aldrich Chemicals, USA. A 4:1 mixture of DCM and THF was used for all experiments. All UV/vis spectra were recorded using a Hitachi U-2910 spectrophotometer before and after laser exposure to the sample to check sample degradation if any. All steady-state fluorescence spectra were recorded at room temperature by a Fluorolog-3 spectrofluorimeter (Horiba Jobin Yvon, USA). Femtosecond Laser Apparatus. A mode-locked automated broadband tuning from 690 to 1040 nm Ti:sapphire laser (Mai Tai HP, Spectra Physics, USA) pumped by 14 W frequency doubled Nd:YVO4 (532 nm) was used as the master oscillator. It produces laser pulses of 4 mJ pulses centered at 800 nm, having a full width at half-maximum (fwhm) of
