Article pubs.acs.org/JPCC
Remarkable Fluorescence Enhancement versus Complex Formation of Cationic Porphyrins on the Surface of ZnO Nanoparticles Shawkat M. B. Aly,† Mohamed Eita, Jafar I. Khan, Erkki Alarousu, and Omar F. Mohammed* Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: Fluorescence enhancement of organic fluorophores shows tremendous potential to improve image contrast in fluorescence-based bioimaging. Here, we present an experimental study of the interaction of two cationic porphyrins, meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride (TMPyP) and meso-tetrakis(4-N,N,Ntrimethylanilinium)porphyrin chloride (TMAP), with cationic surfactant-stabilized zinc oxide nanoparticles (ZnO NPs) based on several steady-state and time-resolved techniques. We show the first experimental measurements demonstrating a clear transition from pronounced fluorescence enhancement to charge transfer (CT) complex formation by simply changing the nature and location of the positive charge of the meso substituent of the cationic porphyrins. For TMPyP, we observe a sixfold increase in the fluorescence intensity of TMPyP upon addition of ZnO NPs. Our experimental results indicate that the electrostatic binding of TMPyP with the surface of ZnO NPs increases the symmetry of the porphyrin macrocycle. This electronic communication hinders the rotational relaxation of the meso unit and/or decreases the intramolecular CT character between the cavity and the meso substituent of the porphyrin, resulting in the enhancement of the intensity of the fluorescence. For TMAP, on the other hand, the different type and nature of the positive charge resulting in the development of the CT band arise from the interaction with the surface of ZnO NPs. This observation is confirmed by the femtosecond transient absorption spectroscopy, which provides clear spectroscopic signatures of photoinduced electron transfer from TMAP to ZnO NPs.
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INTRODUCTION Cationic surfactant-stabilized zinc oxide (ZnO) nanoparticles (NPs) have recently inspired much scientific advancement due to their chemical stability, nontoxic nature, and inexpensive fabrication as well as their excellent thermal and structural features.1−4 Potential applications include, but are not limited to, biomedical imaging, photodynamic therapy, and antibacterial and optical detection of target bioconstituents.5−7 Heterostructures based on ZnO nanowires received special interest due to their versatile applications in many aspects including UV photodetectors, light-emitting diodes, dyesensitized or quantum dots-sensitized solar cells, and photoelectrochemical cells.8 On the other hand, water-soluble porphyrins have been also considered as an interesting class of compounds owing to their wide applications in medicine, biology, and sensors of various reactions.9 Photosensitization of CdTe quantum dots (QD) with porphyrin provide a new means of photosensitizer detection.10 In this system detection is based on fast quenching CdTe QD luminescence as a result of defects and oxidation of QD due to formation of singlet oxygen (1O2).10 Moreover, production of 1O2 by Zn-based QD sensitized with porphyrins is the basis of a new approach of photodynamic therapy for cancer treatment.11−13 Exposure of the sensitized QD to ionizing radiation will cause the nanoparticle to persist in luminescence which in turn activates the photosensitizers to produce cytotoxic reactive 1O2.11−13 © XXXX American Chemical Society
These potential applications motivated a great deal of research encountering interaction of zinc-based nanoparticles with porphyrins. Free-base porphyrin-functionalized ZnO NPs have been described as a universal photoelectrochemical platform.14 The strong absorption coefficient of porphyrin has been found to improve the photocurrent conversion efficiency of ZnO NPs, which is useful in photoelectrochemical biosensor applications.14 Hybrids composed of porphyrin− ZnO NPs have demonstrated cytotoxic activity and may be useful in photodynamic therapy in the treatment of cancer.15 The photoluminescent properties as well as energy transfer in some porphyrin−ZnO assemblies have also been examined.16,17 In these cases, the central hydrogen atoms of the free-base porphyrin were found to be replaced by zinc atoms after 2−4 weeks as indicated by Q-band features.11 Energy transfer was observed from the ZnO NPs to porphyrin in an analysis based on the Förster mechanism that estimated the average distance between porphyrin and the surface of the ZnO NPs to be 1.8 nm.12 Moreover, ZnO nanorods grafted with sulfonic acid functionalized free-base porphyrin were tested for potential applications in solid-state dye-sensitized solar cells. The Received: March 26, 2014 Revised: May 13, 2014
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3. RESULTS AND DISCUSSION Steady-State Absorption and Fluorescence Spectra. The absorption and emission spectra of TMPyP as a function of the ZnO NPs dispersed in deionized water are given in Figure 1. The recorded absorption spectra of the porphyrins,
conversion efficiency of the solar cells was found to be improved in the presence of a low concentration of the dye.18 In general, bright fluorescence with photostable and biocompatible near-infrared fluorophores has been difficult to achieve with the exception of metallic nanoparticles due to the plasmon resonant properties. Here, we describe a marked enhancement in fluorescence and the formation of a charge transfer complex in aqueous solutions of meso-tetrakis(1methyl-pyridinium-4-yl)porphyrin chloride (TMPyP) and meso-tetrakis(4-N,N,N-trimethylanilinium)porphyrin chloride (TMAP) upon interaction with ZnO NPs, respectively. These two porphyrins were carefully selected for comparison based on the differences in the type and nature of the positive charge of the meso substituent. Whereas on TMPyP the positive charge on the pyridinium group is delocalized along the aromatic system of the pyridine ring, the positive charge on TMAP is localized on the nitrogen atom. Moreover, the steric hindrance of the three methyl groups connected to the nitrogen carrying the positive charge was used to address the impact of the steric hindrance on the fluorescence quantum yield. Our experimental measurements demonstrate for the first time a clear relationship between the dramatic fluorescence enhancement (sixfold higher relative to free TMPyP) and the formation of the charge transfer complex by changing the nature and the positive charge of the meso unit of the cationic porphyrin. This finding indicates that the nature and the location of the positive charge of the meso unit play key roles in the observed fluorescence enhancement.
2. EXPERIMENTAL SECTION
Figure 1. (A) Absorption (inset shows the Q-band absorption) and (B) emission (measured at λex = 580 nm) of TMPyP (3.3 μM) and the successive addition of ZnO NPs over the range from 0 to 0.14 μL/mL. Absorption of ZnO NPs (2.5 μL/mL) and its photoluminescence (multiplied by a factor of 10 for clarity) measured using λex = 580 nm are given as the dotted curve.
Materials. Zinc oxide nanoparticles were purchased from Sigma-Aldrich as a 50 wt % dispersion in water with particle sizes less than 100 nm (measured by dynamic light scattering technique). Meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride (TMPyP) and meso-tetrakis(4-N,N,Ntrimethylanilinium)porphyrin chloride (TMAP) were supplied by Frontier Scientific. Stationary Spectroscopy. Absorption spectra were measured on a Cary 5000 UV−vis−NIR spectrophotometer (Varian Inc.), while the steady-state photoluminescence spectra were measured using a Jobin−Yvon−Horiba Fluoromax-4 spectrofluorometer. Raman Spectroscopy. Raman spectra were measured on a LabRam Aramis Raman specrometer from Horiba Jobin Yvon, using a laser wavelength of 473 nm and an integration time of 60 s. Time-Resolved Spectroscopy. Time-resolved absorption decays were measured with a pump−probe setup in which a white light continuum probe pulse was generated in a 2 mm thick sapphire plate contained in an Ultrafast System LLC spectrometer by pulse energy of a few microjoules. The fundamental output came from a Ti:sapphire femtosecond regenerative amplifier operating at 800 nm with 35 fs pulses and a repetition rate of 1 kHz. Spectrally tunable (240−2600 nm) femtosecond pulses generated by an Optical Parametric Amplifier (Light Conversion LTD) and a white light continuum were used, respectively, as the pump (excitation) and probe beams in the pump−probe experimental setup (Helios).
Figure 1A, in the absence of ZnO NPs were typical of free-base porphyrins with an intense Soret band located at 422 nm and four Q-bands over the range of 500−630 nm. TMPyP exhibits a broad fluorescence spectrum covering Q(0,0) and Q(0,1) over 600−800 nm as shown in Figure 1B. The absorption and emission spectra of ZnO NPs are given in Figure 1 (dotted line). Successive addition of ZnO NPs to a fixed porphyrin concentration resulted in obvious spectral changes as displayed in the inset of Figure 1A. The Soret band initially displayed a decrease in the optical density. As the concentration of the ZnO NPs increased, an observable red-shift occurred. In the Qregion, the initially detected four characteristic bands changed upon addition of ZnO NPs. The Q-band at 518 nm diminished as the concentration of ZnO NPs increased, while another band at 560 nm started to develop in parallel. The absorption spectra of porphyrins have been the subject of extensive studies as a key issue in identifying symmetry in porphyrin macrocycles.19−22 The spectra of free-base porphyrins, due to the proton axis, exhibit an intense Soret band corresponding to S0 → S2 electronic transition in the near-UV along with four Q-bands in the visible region for the Qy (1,0), Qy (0,0), Qx (1,0), and Qx (0,0) transitions.14 In metalated porphyrins with square symmetry, the Qx and Qy transitions become degenerate and B
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the four Q-bands overlap and form only two peaks.20 Hence, the key issue to understand the spectral changes in porphyrin− ZnO NP assemblies is to consider the interaction that significantly alters the symmetry of the porphyrin macrocycle and the spectral features of the absorption spectrum as a consequence of this interaction. The absorption spectrum of the TMPyP−ZnO NPs hybrid was collected after 24 h and is shown in Figure 2A where a clear
mentioning that our excitation is at 580 nm where the optical density of the TMPyP with and without ZnO NPs is almost identical. A comparison between the TMPyP emissions alone and in the presence of ZnO NPs is shown in Figure 2B where the vibronic structure of the emission becomes more resolved and there is a clear blue-shift from 655 nm for TMPyP to 627 nm in the presence of ZnO NPs. These observed changes, in line with the absorption behavior, indicate the strong interaction of the porphyrin with the surface of the nanoparticles. Again, the blue-shift recorded in the emission reinforces the suggested adopted SAT configuration where the energy difference between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) is increased relative to free-base porphyrin.23,25,26 Surprisingly, the emission spectra were further enhanced as the concentration of the ZnO NPs increased. The maximum enhancement observed after 24 h was found to be by a factor of about six as measured at the first vibronic of the emission band. It is well established that insertion of a metal into a porphyrin cavity will diminish emissions due to enhanced intersystem crossing.27 We also examined the addition of zinc acetate into a porphyrin solution (Figure 2C and D). The absorption spectrum given in Figure 2C exhibited behavior typical of the metal insertion into porphyrin in terms of absorption as indicated by the red-shift in the Soret band and the vibronics of the Q-bands. Moreover, fluorescence quenching displayed in Figure 2D is in line with the increased intersystem crossing associated with metal insertion into porphyrin macrocycle. This behavior with zinc acetate suggests that the interaction we observed with the ZnO NPs was not due to the insertion of a metal into the porphyrin cavity. On the contrary, we attribute the observed emission enhancement to an increase in the rigidity of the TMPyP as a result of its binding on the surface of ZnO NPs. To confirm this, we measured the emission of TMPyP in methanol (MeOH) and ethylene glycol (EG) with equal optical densities at the excitation wavelength as shown in Figure 3. The viscosity values of these solvents are 0.59 and 13.5 mPa/s for MeOH and EG, respectively.28 We found that the emission intensity increased by a factor of about two when going from MeOH to EG, supporting the idea that the fluorescence enhancement is correlated to increase in rigidity. Furthermore, several reported examples indicate that hindered
Figure 2. (A) Absorption (inset shows Q-bands region) and (B) emission (measured at λex = 560 nm) of TMPyP (2.5 μM) (black) and TMPyP (2.5 μM)−ZnO NPs (0.25 μL/mL) assembly (red); (C) absorption (inset shows Q-region absorption) and (D) emission (measured at λex = 560 nm) of TMPyP (0.2 μM) with successive addition of Zn(CH3COO)2 from 0 to 11 μM.
red-shift of 14 nm is observed in the Soret band, and the Qbands are reduced to two bands. A smaller red-shift of 8 nm in the Soret band was previously reported for grafted porphyrin on ZnO nanorods18 also with the disappearance of two Qbands. This was attributed to a strong interaction causing the porhpyrin to lie horizontally with its cavity in close contact with the surface of the nanorod. Here, a larger red-shift in the Soret band with a similar reduction to two Q-bands takes place. Usually, large red-shift of the Soret band could be indicative of a considerably distorted structure caused by out-of-plane displacement of the metal center.23,24 Complexations outside the cavity adopt the configuration of sitting-atop (SAT) complexes where the S0−S2 energy spacing decreases compared to free-base porphyrin, and consequently the Soret band encounters red-shift.23,25,26 We therefore suggest that a strong interaction takes place between the porphyrin cavity and the surface of the nanoparticle adopting the SAT configuration, which in turn results in structural deformation in the porphyrin cavity and the consequential increase in symmetry of the cavity as reflected in the Q-band behavior. TMPyP in the absence of ZnONPs exhibited broad fluorescence over the 650−720 nm range. Successive addition of ZnO NPs resulted in a blue-shift along with a dramatic increase of the emission intensity (see Figure 1B). It is worth
Figure 3. Absorption and emission spectra of TMPyP in MeOH and EG measured after excitation at 590 nm. C
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cycle onto the methylpyridyl substituents.9,37 The interaction between the π-system of the porphyrin macrocycle and the pyridinium group requires a rotation toward a coplanar conformation.37 Thus, any interaction that hinders the pyridinium group from adopting the coplanar configuration will consequently disturb the intramolecular CT. This agrees with our experimentally measured emission from a viscous solvent in which the molecular rotation is hindered38 and as a consequence the emission intensity is increased. A similar situation is possible in the interaction with ZnO NPs in which the rotation of the pyridinium group is hindered and the coupling between the CT and unperturbed S1 is thus hindered, leading to an increase in the intensity of the fluorescence. To understand the effects of the charge density of the porphyrin cavity, the bulky group, and the type of positive charge of the meso substituent on the interaction with the ZnO NPs surface, we extended the study to include the interaction of mesotetrakis(4-N,N,N-trimethylanilinium)porphyrin chloride (TMAP) with ZnO NPs. The positive charge of TMAP is localized on the nitrogen atom (see Figure 5B, inset) to which
or restricted molecular rotation rigidifies the molecule and in turn reduces nonradiative excited-state deactivation and improves photoluminescence intensity.29−31 Raman Spectroscopy of TMPyP-ZnO NPs. The recorded Raman shifts provide additional evidence for the strong interaction between the porphyrin macrocycle and the ZnO NPs as indicated by the distinctive differences between the Raman spectra of TMPyP and the TMPyP−ZnO NPs assembly. Figure 4 shows the Raman shifts recorded for
Figure 4. Raman spectra of TMPyP (black) and its assembly with ZnO NPs (red) using a laser wavelength of 473 nm.
TMPyP and the TMPyP−ZnO NPs assembly. Several features of recorded shifts enforce the proposed interaction as follows. The band at 332 cm−1 for TMPyP, which can be assigned to inplane bending of the porphyrin core, disappears when ZnO NPs are added.32−34 The in-plane bending of pyridine ring band that appears in free porphyrin at 970 cm−1 also disappears when ZnO NPs are added, indicating that the bending of the pyridine ring after binding onto the ZnO surface is forbidden and providing clear evidence for a strong interaction between the porphyrin macrocycle and surface of the ZnO NPs.35 Moreover, the disappearance of the pyridine bending band of TMPyP at 1298 cm−1 also indicates an interaction with the cavity when ZnO NPs are added.32 Furthermore, the band at 1364 cm−1, assigned to the stretching of the (Cα−N) mode, shifts to 1353 cm−1 when ZnO NPs are added, indicating the less degree of freedom for this vibration upon binding.32−34,36 The Raman spectra of TMPyP−ZnO NPs provide solid evidence of the binding to ZnO NPs in a configuration similar to the metalation. Effect of the Molecular Structure. It has been reported that the pyridinium meso subsitiuents in TMPyP are nearly perpendicular to the plane of the porphyrin due to steric hindrance.37 This cationic porphyrin experiences an intramolecular charge transfer (CT) from the tetrapyrrole macro-
Figure 5. (A) Absorption, (B) emission (measured at λex = 550 nm) of TMAP (4 μM) with the successive addition of ZnO NPs over the range from 0 up to 1.2 μL/mL, and (C) emission of TMAP in the presence of 0.8 μM Zn(CH3COO)2 (measured at λex = 550 nm).
three methyl groups are attached, providing a steric hindrance. On the other hand, the positive charge on TMPyP is more efficiently delocalized with the porphyrin π-system.39 Thus, the major difference between these two porphyrins lies in the type, distance, and delocalization of the positive charge with the porphyrin π-system. In addition, two different oxidation potentials are reported for TMAP and TMPyP that are 1.15 and >1.30 eV, respectively.39 D
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The steady-state absorption and emission spectra of TMAP in the presence of ZnO NPs are shown in Figure 5. The absorption spectra in Figure 5A showed a new broad structured absorption band from 450 to 700 nm together with a red-shift in the Soret band of about 8 nm with the addition of ZnO NPs. This broad red-shifted band has been reported previously in the literature as a charge-transfer band for donor (porphyrin)− acceptor systems.40−42 Based on the resemblance of our measured spectra with those reported previously, we attribute this band to a charge-transfer band arising from the interaction between the TMAP and ZnO NPs. As can be seen in Figure 5B, with the successive addition of ZnO NPs, fluorescence of TMAP experiences significant changes where a new band at 600 nm begins to develop while the band at 700 nm suffers from quenching together with red-shift in the 742 nm band. The band formed at 600 nm is due to symmetry change brought by adopting the SAT complex configuration. This is confirmed by studying the zinc acetate effect on TMAP which is given in Figure 5C. The observed quenching of the 700 nm band indicates the formation of weakly luminescent complex. This is in agreement with the ground-state CT complex as indicated by the ground-state absorption (see Figure 5A).This behavior differs from that observed for TMPyP. In fact, this behavior is expected when we consider the difference in the structure between the two porphyrins. Unlike TMPyP, the positive charge on the bulky trimethylammonium groups of TMAP cannot be delocalized to the π-conjugation system.43 Consequently, the electron density will be concentrated on the porphyrin macrocycle, which is obvious in the oxidation potential value, and facilitate its interactions with electron acceptor.43 A red-shift is detected in the 640 nm band of TMAP fluorescence upon ZnO NPs addition that becomes clear at higher concentrations added from ZnO NPs as can be seen in Figure 5B. In order to examine the origin of this shift, we experimentally examined the emission of TMAP as compared with that of TMAP−Zn acetate mixture; the recorded spectra are given in Figure 5C. It is clear from the spectra that metal insertion into the cavity produces a red-shift for the emission band at 645−652 nm. This indicates that symmetry change of the porphyrin macrocycle by interaction with zinc acetate induces the observed red-shift associated with LUMO−HOMO energy gap change.23,25,26 Hence, we can assume the porphyrin cavity in TMAP−ZnO NPs adopts the same symmetry of the metalated porphyrin. A similar behavior has been reported for porphyrin-grafted ZnO nanorods.18 Raman Spectroscopy of TMAP−ZnO NPs. The Raman spectroscopy measurements support the absorption and fluorescence measurements indicating different behaviors between TMAP and TMPyP in their interactions with ZnO NPs as displayed in Figure 6. The recorded Raman spectra reveal that the key peaks for free-base porphyrin appear fairly similar for TMAP and its ZnO NPs assembly. These bands can be summarized as the stretching of the porphyrin macrocycle44 at 325 cm−1 and the expansion of the pyrroline groups along the N···N axis located at 965 cm−1 and that along the NH···NH direction assigned to 1002 cm−1.45 However, careful comparison of the spectra measured for TMAP with those for TMAP− ZnO NPs reveals some differences. The band at 325 cm−1 for TMAP appeared as a weaker band in the TMAP−ZnO NPs assembly. There is also a shift to a lower frequency of about 10 cm−1 observed in the δ(phC−H + pyrN−H)42 band at 1018 cm−1. Another shift observed in the band at 1361 cm−1 is assigned to νs(Cα−N) + δ(Cβ−H).44 These shifts can be
Figure 6. Raman spectra of TMAP and its assembly with ZnO NPs using a laser wavelength of 473 nm.
attributed to the CT complex formed between the TMAP and the surface of ZnO NPs.42,46,47 Time-Resolved Spectroscopy. Femtosecond transient absorption information was collected using an Ultrafast Systems Helios UV−NIR spectrometer integrated with a Ti:sapphire femtosecond regenerative amplifier operating at 800 nm with 35 fs pulses and a repetition rate of 1 kHz. A short description of the setup is given in the Experimental section, whereas more details can be found elsewhere.48 Figure 7 displays the transient absorption of free TMAP (see Figure 7A) and with TMAP−ZnO NPs assembly (Figure 7B) recorded with 550 nm laser pulse excitation. The excited-state kinetic traces of TMPyP, TMAP, and their ZnO NPs assemblies are displayed in Figure 8 (A and B). Without ZnO NPs, the transient absorption of TMAP shows that the excited-state absorption extends over 450−810 nm with no observable changes in the spectral features monitored over a 1 ns time window. This observation is expected because the reported singlet excited state for free-base porphyrin is fairly long-lived (8.5 ns).42,49−51 The transient absorption spectra for the TMAP−ZnO NPs assembly, on the other hand, exhibit a faster decrease over the same time window, suggesting a fast excited-state deactivation compared with TMAP alone. Moreover, a new spectral band is observed over the 500−540 nm range, which can be attributed to the TMAP radical cation.52 Transient absorption spectra indicate that the formation of the cation radical band is faster than our time resolution of 120 fs. In fact, the kinetic trace for ZnO NPs extracted at 536 nm (see Figure 8A) exhibits two different lifetime components, one short-lived with a characteristic time constant of ≈37 ps and the other long-lived. It is possible to attribute the short-lived component to the charge recombination associated with TMAP•+. The long-lived one might be coming from the E
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Figure 8B, excluding the CT event under our experimental conditions.
4. CONCLUSION We present a unique model system that dramatically increases the fluorescence of cationic porphyrin in the presence of ZnO NPs. Our experimental results suggest that the hindrance of the rotational relaxation of TMPyP caused by electrostatic binding with the surface of ZnO NPs plays a key role in the observed fluorescence increase. More specifically, the TMPyP−ZnO NPs assembly showed increased rigidity and/or the mixing between the low-lying CT state and the singlet excited state was reduced, hence enhancing the fluorescence properties of the porphyrin. This approach is potentially valuable not only for improving the sensitivity of low-quantum yield fluorophores but also for improving fluorescence-based bioimaging. On the other hand, TMAP manifested itself in a new ground-state CT complex on the surface of ZnO NPs. This observation was confirmed experimentally by femtosecond broadband transient absorption spectroscopy with a temporal resolution of 120 fs, which provides a clear signature for porphyrin−cation radical formation, resulting from photoinduced electron transfer from TMAP’s excited singlet state to ZnO NPs.
Figure 7. Transient spectra at indicated delays after 550 nm pulse excitation of TMAP (A) and the TMAP−ZnO NPs assembly (B).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
On leave from the Chemistry Department, Faculty of Science, Assiut University, Egypt. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
S.M.B.A. is grateful for the postdoctoral fellowship provided by Saudi Basic Industries Corporation (SABIC). The research reported in this publication was supported by the King Abdullah University of Science and Technology.
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Figure 8. (A) Comparison of the kinetics trace at 536 nm of TMAP (red) and the TMAP−ZnO NPs assembly (green). The solid line is the calculated fit. (B) Kinetic traces at 487 nm of TMPyP (orange) and the TMPyP−ZnO NPs assembly (green) after 425 nm pulse excitation.
lifetime of the unreacted TMAP which is reported in literature to be ∼8−9 ns, which is longer than the detection limit of our experimental setup.39 This agrees with our suggested scenario of a photoinduced charge transfer, and it is in line with the observed ground-state CT complex formed as indicated by the steady-state absorption. It is worth mentioning that TA measurements of TMPyP and its assembly with ZnO NPs show no change in the excited-state absorption as indicated in F
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