Charge-Transfer Complexation and Excited-State Interactions in

ARTICLE pubs.acs.org/JPCC

Charge-Transfer Complexation and Excited-State Interactions in Porphyrin-Silver Nanoparticle Hybrid Structures Sean Murphy,†,‡ Libai Huang,*,† and Prashant V. Kamat*,†,‡,§ †

Radiation Laboratory, ‡Department of Chemistry and Biochemistry, and §Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States

bS Supporting Information ABSTRACT: Highly photoactive porphyrin is shown to form charge-transfer complex with silver nanoparticles. Complexation of tetra(4-aminophenyl) porphyrin (TAPP) with Ag nanoparticles is confirmed by ground-state absorption and Raman spectroscopy. Strong Raman enhancement indicates both electromagnetic and chemical enhancement. Evidence of chemical enhancement includes a selective enhancement of porphyrin Raman bands. Fast charge separation in the complex is indicated by ultrafast transient absorption and fluorescence upconversion measurements. The charge-separated state is shown to have a lifetime of 116 ( 6 ps. Porphyrin substituents are shown to play a role in the formation of charge-transfer complex.

1. INTRODUCTION Interaction of photoactive molecules with metallic nanostructures has garnered much recent interest.1,2 When incident light excites coherent oscillation of the free electrons in metallic nanoparticles, localized surface plasmon resonance (LSPR) is created.3 This effect produces a strong optical near-field (or evanescent field) that decays exponentially away from the metal nanoparticle surface with length scales on the order of 10
200 nm.3 The strong optical near-field leads to enhancement of Raman signals of nearby molecules.4
14 In addition to the strong field-enhancement effects, surface plasmons can couple strongly to the molecular electronic transitions to form hybrid molecular-plasmon states.15
19 The LSPR can also interact with the excited states of the molecules, which results in phenomena such as shortening of molecular radiative lifetime,20
23 energy transfer,24
26 charge transfer,16,27 and mediation of molecular energy redistribution.15,28 Charge transfer between photoactive molecules and nearby metallic nanostructures has been extensively studied.16,27,29
32 Such interactions produce modulated optical and electrochemical properties that are desirable for applications including electrocatalysis,33 photoenergy conversion, 29
31,34,35 and molecular sensing.36
38 Hybrid molecular-plasmonic nanostructures may be assembled by various means.1,28,30,31,33,39,40 An understanding of the interactions between metallic nanoparticles and photoactive molecules is therefore crucial for the realization of applications. When photoactive molecules possess absorption near the frequency of the LSPR coupling is expected to be strong.1,20,39,41,42 Porphyrins possess strong absorption in the visible range, εmax > 105 M
1 cm
1. This absorption can overlap significantly with the r 2011 American Chemical Society

plasmon band of Ag nanoparticles. There is great interest in porphyrins for application in artificial photosynthesis.34,43,44 Porphyrins coupled to metallic nanostructures have been shown to possess desirable properties.19,24,28,29,33
35,45 These properties include charge transfer,29,34,35 plasmon-enhanced electrical conduction,45 and electrocatalyic activity.33 However, ultrafast time-resolved studies of interactions between porphyrins and silver nanostructures has been limited.19 This study focuses on interaction between tetra(4-aminophenyl) porphyrin (TAPP) with terminal amine functional groups and solution synthesized Ag nanoparticles. Optical properties of the hybrid system are characterized by UV
visible absorption, Raman, ultrafast transient absorption, and femtosecond fluorescence upconversion spectroscopies. Charge-transfer complexation is evidenced by ground-state absorption. Surface-enhanced resonance Raman spectroscopy (SERRS) results exhibit selective enhancement and frequency shifts of certain vibrational modes indicating charge transfer enhancement. Excited-state dynamics of the hybrid system are studied by femtosecond transient absorption spectroscopy and fluorescence upconversion spectroscopy. These timeresolved studies indicate fast charge separation and subsequent recombination. To determine the role of porphyrin substituents in facilitating interaction with the surface of Ag nanoparticles, three different porphyrins were selected based on electronic character of their terminal functional groups. This allows for examination of the role of charge-transfer in Raman enhancement. Received: June 17, 2011 Revised: October 7, 2011 Published: October 13, 2011 22761

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2. MATERIALS AND METHODS 2.1. Silver Nanoparticle and Hybrid Nanostructure Synthesis. Silver nitrate (MP Biomedicals ACS reagent grade), sodium

borohyrdride (Alfa Aesar 98%), sodium citrate dihydrate (J.T. Baker 99.02%), citric acid anhydrous (Fluka g99.5%), 5,10,15, 20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP) (TCI), tetrakis(4-carboxyphenyl) porphyrin (TCCP) (TCI), and 5,10, 15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TmPyP) (Aldrich) were used as received. A modified version of the procedure described by Lee and Meisel4 was used to prepare Ag nanoparticles. In brief, to 125 mL of 1 mM silver nitrate in deionized (DI) water, 5 mL of 47 mM sodium citrate along with 5 mL of 52 mM citric acid in DI water was added with vigorous stirring. After 5
15 min of stirring, a solution of sodium borohyrdride (130 mM) in DI water was added dropwise with vigorous stirring. Growth of the plasmon peak was monitored by UV
visible absorption spectrophotometry. Sodium borohyrdride was added until the plasmon peak stopped growing. Approximately 0.9 mL of 130 mM sodium borohyrdride was necessary for complete reduction. TAPP-Ag nanoparticle hybrid nanostructures were prepared by mixing an ethanolic solution of TAPP with Ag nanoparticles in water. TAPP powder was dissolved in ethanol, sonicated for 5
15 min, and stirred for several hours to prepare an ethanolic solution. Hybrid samples were prepared just prior to optical measurements. Samples for time-resolved measurements were prepared by deaerating the porphyrin and Ag nanoparticle solutions separately by purging with N2 and then mixing in a sealed cuvette under an inert atmosphere. 2.2. Methods. A Varian Cary 50-Bio UV
visible spectrophotometer was used for ground-state absorption measurements. A Horiba Jobin Yvon Fluorolog-3 spectrofluorometer was used to acquire steady-state fluorescence spectra (Supporting Information, Figure S1). The emission lifetime of neat TAPP (Supporting Information, Figure S2) was determined using a Horiba Jobin Yvon DataStation HUB in time-correlated single photon counting mode (TCSPC) with a 660 nm long pass filter placed before the emission monochromator and a 638 nm NanoLED diode at a 500 kHz repetition rate providing excitation. Raman experiments were carried out using a Renishaw Raman microscope (RM1000) equipped with a 514 nm Argon ion laser excitation source. Integrated intensities for Raman bands were determined by fitting individual bands to a Lorentzian function using Renishaw WiRE software. A 1 kHz amplified Ti:sapphire laser system (Clark MXR CPA2010) was used for femtosecond transient absorption measurements. The pump and probe beams came from a splitting of the 150 fs, 775 nm, ∼1 mJ/pulse fundamental output of the Ti: sapphire amplifier. Ninety-five percent of the 775 nm fundamental was doubled to create 387 nm pump pulses, whereas the other 5% was used for white light continuum probe pulses. (See the supporting information of ref 34 for more information regarding the transient absorption system.) Femtosecond fluorescence spectroscopy was carried out using a fluorescence upconversion spectrometer (Ultrafast Systems Halcyone) fed by a Ti:Sapphire oscillator (80 MHz repetition rate, Coherent Chameleon). A pulse picker was used to reduce the repetition rate of the laser to 5 MHz to allow photoexcited porphyrin molecules sufficient time to relax between excitation events.

Figure 1. Ground-state absorption spectra acquired using a 2 mm path length optical cell: (a) TAPP and Ag nanoparticles, (b) TAPP, and (c) Ag nanoparticles in a 50:50 ethanol
water mixture.

Figure 2. Raman spectra acquired using 514 nm excitation: (a) neat TAPP powder; (b
e) 10 μM TAPP and Ag nanoparticles: (b) 0.05 mM Ag0, (c) 0.10 mM Ag0, (d) 0.13 mM Ag0, and (e) 0.21 mM Ag0; (f) 10 μM TAPP; and (g) Ag nanoparticles (0.21 mM Ag0). Spectra (b
g) are solution samples in a 50:50 ethanol
water mixture acquired using a 2 mm path length optical cell. Spectra in the top panel are offset for clarity. See Table 1 for Raman band assignments.

3. RESULTS AND DISCUSSION 3.1. Ground-State Optical Properties. Ground-state absorption spectra for TAPP in the presence and absence of Ag nanoparticles are compared in Figure 1. Upon mixing of TAPP with Ag nanoparticles in solution, a new absorption band from 500 to 700 nm is apparent. Red-shifted broad absorption attributed to charge-transfer bands has been observed for donor (porphyrin)
acceptor systems.46,47 Charge transfer absorption has also been related to SERS enhancement for molecular-silver nanoparticle systems.48 The surface plasmon band of Ag nanoparticles appears damped in the presence of TAPP. Surface plasmon band dampening upon chemisorption of various species has been previously observed.49,50 Chemisorption often leads to the formation of complexes capable of charge-transfer transitions.6,51 The amino functional groups of TAPP allow for strong adsorption on the surface of Ag nanoparticles. We therefore 22762

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Table 1. Assignment of SERRS Active Modes for TAPPa mode

a

frequency (cm
1) observed by

frequency observed by

514 nm excitation

Wang et al. (cm
1)53

difference (cm
1)

description53 b

ν13

1538

FT-Raman 1545


7

ν (CβCβ + CmCα)

ν16

1484

FT-Raman 1492


8

ν (CβCβ)

ν23

1272

computed 1279


7

ν (CαCβ)s + δ(phC-H)

ν24

1238

FT-Raman 1238

0

ν (Cm-phC)

ν93

1017

computed 1014

+3

δ (phC-H + pyrN-H)

ν53

354

computed 357


3

δ (phenyl wag)

or ν54

354

FT-Raman 333

+21

δ (CαCmCα) asymmetric

Modes denoted by bold-face type are shown to experience selective enhancement (Figure 3). b ν denotes a stretch vibration while δ denotes a bend.

assign the red-shifted absorption (500
700 nm) to a chargetransfer band arising from the interaction between Ag nanoparticles and TAPP.46,47,52 Complexation between TAPP and Ag nanoparticles are explained in terms of the following equation TAPP þ Ag nanoparticle f TAPP 3 3 3 Ag nanoparticle

ð1Þ

Equation 1 depicts complexation between TAPP and Ag nanoparticles. Additional evidence of charge-transfer complexation is provided by SERRS results. (See Sections 3.2 and 3.4.) 3.2. Surface-Enhanced Resonance Raman Spectroscopy. Surface-enhanced resonance Raman spectroscopy was used to investigate interaction between TAPP and the surface of Ag nanoparticles. Figure 2 shows SERRS spectra of TAPP-Ag nanoparticle complex in solution acquired using 514 nm excitation. A resonance Raman (RR) spectrum of TAPP powder is included for comparison. Significant enhancement of the TAPP Raman signal is observed for the complex. For the same concentration of TAPP, we are able to observe an increase in the SERRS signal with increasing Ag nanoparticle concentration (traces b
e). Assignment of TAPP vibrational modes was made using the work of Wang et al.53 The modes observed are summarized in Table 1 and Table S2 of the Supporting Information. Because 514 nm excitation overlaps with both the surface plasmon band of Ag nanoparticles and charge-transfer band of the complex, both electromagnetic and chemical enhancement should play a role in our measurements. Charge-transfer chemical enhancement has been shown for molecules that undergo chemical binding to metal surfaces.6,54 Chemical contributions can add a factor of ∼102 to the overall enhancement. The origin of chemical enhancement can be explained in terms of differences in the Raman polarizability for metaladsorbed molecule systems, as opposed to free molecules.5,32 To investigate further chemical enhancement, we studied Raman band intensity as a function of Ag nanoparticle/TAPP ratio. Please note that porphyrin is always in excess. The Ag nanoparticle/TAPP ratio is estimated by FAg 4 NA ð2Þ n ¼ πrp3 3 MAg 1 ½Ag0  ½Ag nanoparticle  ¼ n ½TAPP ½TAPP

ð3Þ

where n is the number of silver atoms per nanoparticle, rp is the approximate radius of Ag nanoparticles, FAg is the density of silver, MAg is the molecular weight of silver, [Ag0] is the concentration of silver atoms (assuming complete reduction), and NA is Avogadro’s number. The approximate diameter, ∼10 nm,

of the Ag nanoparticles is estimated by the position of the surface plasmon peak, ∼390 nm, in accord with previous reports from the literature.1,11,12 This estimate is in agreement with TEM measurements (Supporting Information, Figure S9A
E) that indicate a distribution of Ag nanoparticle sizes ∼7
20 nm in diameter. In addition to estimates based on concentration of Ag0 and TAPP, an estimate of the number of TAPP molecules that may be accommodated per nanoparticle has been made based on surface area. This estimate contains the assumption that TAPP only lays flat on the nanoparticle surface, which we later suggest is not the case. For monolayer coverage and an effective porphyrin diameter11 of ∼2 nm, an estimate of only 80 TAPP/Ag nanoparticle is made. However, this estimate does not allow for the possibility of adsorption through interaction between the terminal amine functional groups of TAPP and the surface of Ag nanoparticles. Such interaction would allow for a more efficient stacking of porphyrins: rather than lying flat, TAPP may take on a perpendicular orientation with respect to the nanoparticle surface. Furthermore, spectroscopic evidence shows that TAPP interacts more strongly with Ag nanoparticles than two other free-base porphyrins with different terminal functional groups (Figure 7B and Supporting Information Figure S4 to be discussed later). Therefore, it is suggested that more than 80 TAPP molecules may be accommodated per Ag nanoparticle. Figure 3 shows the relative intensity for a collection of SERRS active bands as a function of [Ag0]/[TAPP]. For high enough [Ag0]/[TAPP] ratios, the relative intensity for select Raman bands increases markedly. A reference band at 1238 cm
1 (ν24) was chosen for comparison. This band (ν24) was chosen as a reference because of its prominence in the RR spectra reported by Wang et al.53 and the RR spectra of neat TAPP powder (Figure 2 and Supporting Information Figure S3). For smaller Ag nanoparticle/TAPP ratios, the relative intensities of the studied Raman bands do not undergo significant change. Once the Ag nanoparticle/TAPP ratio becomes greater than ∼1/1000, a dramatic selective enhancement of the ν13 (1538 cm
1) and ν16 (1484 cm
1) modes was observed. The ν23 (1272 cm
1) and ν93 (1017 cm
1) modes also experience selective enhancement but to a lesser degree. The relative intensity of ν13 increases by a factor of 3.3, whereas ν16 increases by a factor of 2.7 in going from Ag nanoparticle/TAPP ratios of 1/1500 f 1/500. It is suggested that when the ratio of Ag nanoparticles to TAPP becomes >1/1000 a measurable proportion of TAPP in solution is able to chemisorb on the nanoparticle surface. As more Ag nanoparticle surface area/porphyrin molecule is made available, a higher percentage of porphyrin molecules in solution is able to interact directly with the surface and participate in chemical binding. 22763

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Chart 1. TAPP Moleculea

Figure 3. Relative integrated intensities for SERRS active bands of TAPP. The ratio of Ag0 to TAPP was varied while the relative intensity for selected bands compared with the reference band centered at 1238 cm
1 was determined (See Table 1 for description of the SERRS active modes): (a) 354 cm
1/1238 cm
1 (ν54/ν24), (b) 1272 cm
1/1238 cm
1 (ν23/ν24), (c) 1017 cm
1/1238 cm
1 (ν93/ν24), (d) 1484 cm
1/ 1238 cm
1 (ν16/ν24), and (e) 1538 cm
1/1238 cm
1 (ν13/ν24).

This chemical binding leads to a selective enhancement of Raman bands. The reason for selective enhancement lies in the selection rules derived from the charge transfer mechanism.32 We now offer an explanation for selective enhancement of the aforementioned vibrational modes: The ν13 (1538 cm
1) and ν16 (1484 cm
1) modes are in the high-frequency region of our SERRS spectra. High-frequency modes have been shown to experience a greater degree of charge transfer enhancement. This is due to larger Franck
Condon coupling between the vibrational states of the ground electronic state and those of the charge-transfer excited state for high-frequency modes.32 These modes are described as (CβCβ + CmCα) stretches and (CβCβ) stretch, respectively (see Chart 1). The ν23 (1272 cm
1) and ν93 (1017 cm
1) modes are described as (CαCβ) stretching(phC-H) bending and (phC-H + pyrN-H) bending, respectively. These modes, ν23 and ν93, constitute the two other most prominent bands observed in our SERRS spectrum for TAPP. Wang et al.53 did not observe these modes using 514 nm RR, and thus our observance of these modes is most likely the result of charge-transfer interaction between TAPP and Ag nanoparticles. Chemical binding of analytes to metal surfaces has been shown to contribute to spectral changes associated with SERS spectra. These spectral changes include shifts in the frequency of Raman active vibrations along with changes in the relative intensity for these bands.54 Comparison of our data to that reported by Wang et al.53 reveals shifts in the frequency for the vibrational modes experiencing selective enhancement (Table 1 and Figure 3). Therefore, we attribute selective enhancement and shifts in vibrational frequency for SERRS active modes to charge-transfer chemical enhancement. 3.3. Femtosecond Transient Absorption and Fluorescence Upconversion Spectroscopy. Femtosecond transient absorption was employed to study the excited-state interaction between TAPP and Ag nanoparticles. Figure 4A displays transient absorption spectra of neat TAPP recorded following 387 nm laser pulse excitation. As confirmed from emission lifetime measurements (Supporting Information, Figure S2), the singlet excited state of TAPP is fairly long-lived, τobs ≈ 8 ns. Therefore, the

a

Label system adopted from the work of Wang et al.53

TAPP transient absorption spectrum does not exhibit changes over a 40 ps time window. The transient absorption kinetic trace recorded at 566 nm for neat TAPP shows a small bleach that does not exhibit any recovery within 500 ps (Figure 5A, trace a). In contrast, transient absorption spectra recorded following excitation of TAPP-Ag nanoparticle complex exhibit significant changes over the time window we monitor (Figures 4B and 5A,B). We observe the formation of a new photoinduced bleach band unique to the complex centered near 566 nm. This bleach mirrors the absorption of the charge-transfer band observed in ground-state measurements (Figure 1), indicating the depletion of the ground state of the charge-transfer complex. The difference absorbance monitored at 566 nm initially shows a positive absorption that quickly decays to produce a bleached absorption with a rise time of 13 ps (Figures 4B and 5A,B). We explain this unusual transient absorption profile as the overlap of two transient signatures with different lifetimes: namely, a short-lived excited-state absorption and a longer-live photoinduced bleach. Photoexcitation at 387 nm can excite both TAPP and Ag nanoparticle and therefore bleaches the ground-state absorption of the complex around 560 nm. The excited-state absorption corresponds to (TAPP 3 3 3 Ag nanoparticle)*, which decays by charge transfer from TAPP to Ag nanoparticle to form a charge-separated state: TAPP(+) 3 3 3 Ag nanoparticle(
). As the excited state decays, the bleaching becomes dominant and reflects the formation of long-lived charge transfer state. The charge-separation process is summarized in terms of the following equations (the time constant for charge separation is discussed along with fluorescence quenching data) hv

TAPP 3 3 3 Ag nanoparticle s fðTAPP 3 3 3 Ag nanoparticleÞ ð4Þ fast ðTAPP 3 3 3 Ag nanoparticleÞ s f TAPPð þ Þ 3 3 3 Ag nanoparticleð
Þ

ð5Þ 22764

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Figure 4. Femtosecond transient absorption spectra for (A) neat TAPP and (B) TAPP with Ag nanoparticles in 50:50 ethanol
water mixture recorded at (a) 1.2, (b) 2.2, (c) 5, (d) 10, (e) 20, and (f) 40 ps probe delays in a 2 mm optical cell. The samples were degassed using N2 prior to measurements.

Figure 5. (A) Transient absorption kinetic traces with ΔA monitored at 566 nm for (a) 10 μM TAPP, (b) 10 μM TAPP and 0.08 mM Ag0 (Ag nanoparticles), (c) 10 μM TAPP and 0.16 mM Ag0 (Ag nanoparticles), (d) 10 μM TAPP and 0.24 mM Ag0 (Ag nanoparticles), and (e) 0.32 mM Ag0 (Ag nanoparticles) in 50:50 ethanol
water mixture in a 2 mm optical cell. (B) Kinetic trace and fit for the growth of the bleach at 566 nm for 10 μM TAPP and 0.16 mM Ag0 (Ag nanoparticles). The data were fit with a double exponential function: one exponent for the rise of the positive signal (τ < 1 ps) and one exponent for the growth or formation of the bleach signal (τgrowth ≈ 13 ( 0.4 ps). Inset represents a single exponential fit to a component of the bleach recovery (τrecovery ≈ 116 ( 6 ps).

As we increase the concentration of Ag nanoparticles, the amplitude of the bleach signal increases (Figure 5A). Because TAPP is in excess, it is expected that increasing the concentration of Ag nanoparticles leads to higher degree of charge-transfer complexation in the ground state. At the Ag0 concentration of 0.24 mM, we see a quick recovery of the bleach with a lifetime of 116 ( 6 ps. This initial recovery corresponds to the recombination of the charge-separated pair to yield back ground-state complex (eq 6). 116ps

TAPPð þ Þ 3 3 3 Ag nanoparticleð
Þ s f TAPP 3 3 3 Ag nanoparticle

ð6Þ The small residual bleaching as evident from the tail absorption is attributed to the charged products, which become stabilized by the solvent medium. Silver nanoparticles have a characteristic electron
phonon coupling decay lifetime of ∼3 ps.16 For Ag nanoparticles alone, the kinetic trace at 566 nm (trace e in Figure 5A) shows positive signal at early times. This signal arises due to broadening of the

SPR band upon absorption of photons from the femtosecond pump laser pulse.55 To evaluate contribution of Ag nanoparticles, if any, to the transient absorption spectral differences in the present experiments, transient absorption spectra for colloidal Ag nanoparticles alone were recorded under similar experimental conditions (Supporting Information Figure S6 and kinetics in Figure 5A). To ensure that the transient signatures observed for TAPP-Ag nanoparticle complex are not simply due to interactions between TAPP and Ag+ ions, residual reducing agent, or capping agent, transient spectra were recorded for a series of blank solutions. These blanks included (1) TAPP with an excess of sodium borohydride, sodium citrate, and citric acid and (2) TAPP with an excess of silver nitrate (Supporting Information, Figure S7A,B). The transient absorption spectral characteristics for these blanks are different from those for the TAPP-Ag nanoparticle hybrid. This observation confirmed that the transient signatures seen for the hybrid are indeed due to interaction between TAPP and Ag nanoparticles. To better understand the excited-state interaction between TAPP and Ag nanoparticles, we employed ultrafast fluorescence 22765

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Figure 6. Fluorescence lifetime spectra acquired using a 2 mm path length optical cell: (a) TAPP and Ag nanoparticles and (b) TAPP in 50:50 ethanol
water mixture. The solid line is the best doubleexponential fit for (a) using the emission lifetime for neat TAPP observed by TCSPC (Supporting Information, Figure S2) as the slow component (∼8 ns) and fitting the fast component with a lifetime of 6 ps. The samples were degassed using N 2 prior to measurements. Emission was monitored at λ max for TAPP: 672 nm (Supporting Information, Figure S1).

upconversion spectroscopy. (For more information regarding fluorescence upconversion, see ref 56.) Figure 6 shows fluorescence lifetime profile for (a) TAPP with Ag nanoparticles and (b) TAPP alone using 400 nm excitation. We observe a shorter lifetime for TAPP fluorescence in the presence of Ag nanoparticles (τQ ≈ 6 ( 0.4 ps). These results show that the excited charge transfer complex formed following the laser pulse excitation (eq 4) decays with a lifetime of 6 ps to form a charge-separated state (eq 5). Fluorescence quenching by metal nanostructures has generally been attributed to one of the two mechanisms: electron transfer and energy transfer.24,29,31 Previous studies have shown that the lifetimes associated with energy transfer are generally longer: ∼72
200 ps.21,24,57 Fast electron transfer can be expected for strongly interacting systems where there is a mixing between molecular and metallic orbitals.51 Given our ground-state absorption (Figures 1A and 7A and Figure S5 in the Supporting Information) and SERRS results (Figures 2, 3, and 7B and Figure S4 of the Supporting Informaiton), we characterize TAPP-Ag nanoparticles as a strongly interacting system. Therefore, we postulate that electron transfer from TAPP to Ag nanoparticles is the dominant fluorescence quenching mechanism. The lifetime of ∼6 ps observed by upconversion is similar to the rise-time, ∼13 ps of the transient absorption feature attributed to the bleaching of the complex (Figure 5B). Therefore, we attribute the 6 ps lifetime to the decay of (TAPP 3 3 3 Ag nanoparticle)* to form the charge-separated state TAPP(+) 3 3 3 Ag nanoparticle(
). The charge-separated state relaxes with a lifetime of ∼116 ps (eq 6 and Figure 5A,B). A similar lifetime for a charge-separated state was reported by Hranisavljevic et al. in a study of J-aggregate-Ag nanoparticle hybrids.27 The charge-separated state with a 300 ps lifetime was observed using ultrafast transient absorption.27 These results confirm that the charge-transfer event follows the laser pulse excitation of TAPP-Ag nanoparticle hybrids. The absorption of porphyrin radical cation species in the red region of the visible spectrum has been previously reported.58,59

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We made an effort to observe the TAPP+• radical cation over pertinent time scales using femtosecond transient absorption spectra. However, the dominance of bleaching of the chargetransfer band in the visible made it difficult to probe the TAPP+• radical cation selectively. Characterization of the TAPP+• transient radical cation is also complicated by the molecules’ tendency to take on hyperporphyrin resonance forms as observed during chemical and electrochemical oxidation.60 The 800
1140 nm transient absorption spectrum of TAPP reveals a broad excited-state absorption that decays quickly in the presence of Ag nanoparticles (Supporting Information, Figure S11) similar to the decay measured by fluorescence upconversion. 3.4. Role of Porphyrin Substituents. The role of TAPP’s terminal amino substituents in interaction with Ag nanoparticles was investigated. Two other free-base porphyrins were chosen for comparison based on electronic character of their terminal functional groups: 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TmPyP) and tetrakis(4carboxyphenyl)porphyrin (TCCP). It was found that TCCP did not significantly modify the extinction properties of Ag nanoparticles (Figure 7A). However, TmPyP did affect the extinction of Ag nanoparticles. This suggests that the terminal substituents of TmPyP play a role in surface interaction with Ag nanoparticles, whereas TCCP does not have strong interaction with the nanoparticle surface. Absorption spectra of 10 μM TAPP, TmPyP, and TCCP all with the same concentration of Ag nanoparticles are included in Figure S12 of the Supporting Information.

SERRS results (Figure 7B) indicate that among the three porphyrins TAPP has the strongest interaction with Ag nanoparticles. TAPP displays a significantly more intense SERRS spectrum (Figure 7B (a)) than the two other porphyrins. This suggests the amine substituents of TAPP facilitate direct interaction with the nanoparticle surface. TmPyP displays some SERS enhancement in the presence of Ag nanoparticles (trace b). However, TCCP shows no appreciable enhancement (trace c). Electronic character of the terminal functional groups is shown to play a role in interaction with Ag nanoparticles. For example, electron-donating groups such as
NH2 enable stronger interaction. However, carboxyl groups (TCCP) that are slightly electron-withdrawing in nature do not lead to strong interaction. TmPyP may interact with Ag nanoparticles through attraction between positively charged 1-methyl-4-pyridinio groups and residual reducing or capping agent anions near the nanoparticle surface. Neat TAPP, TmPyP, TCCP, and Ag nanoparticles only display ethanol solvent peaks in their Raman spectra (Supporting Information, Figure S4). To rule out the possibility of differences in pH leading to differences in the SERRS enhancement observed 22766

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Figure 7. (A) Ground-state absorption spectra acquired using a 2 mm path length optical cell: (a) TmPyP and Ag nanoparticles, (b) TCCP and Ag nanoparticles, (c) Ag nanoparticles, (d) TmPyP, and (e) TCCP in 50:50 ethanol
water mixture. (B) Surface-enhanced resonance Raman (SERRS) spectra acquired using a 2 mm path length optical cell: (a) TAPP and Ag nanoparticles, (b) TmPyP and Ag nanoparticles, and (c) TCCP and Ag nanoparticles in 50:50 ethanol
water mixture. (Inset) Absorption spectra acquired using a 2 mm path length optical cell for (a) 9 μM TAPP, (b) 9 μM TmPyP, and (c) 9 μM TCCP where the green arrow represents 514 nm excitation.

for the three porphyrins, we measured the pH of porphyrin-Ag nanoparticle solutions. It was found that the pH of the different porphyrin-Ag nanoparticle solutions was very similar (5.0 ( 0.5). To confirm further that porphyrin substituents play a role in the interaction with the surface of Ag nanoparticles, transient absorption spectra were acquired for TCCP and TmPyP in Ag nanoparticle suspension (Supporting Information, Figure S8). Transient spectra for TmPyP in Ag nanoparticle suspension indicate porphyrin
nanoparticle surface interaction. Spectra for TCCP in Ag nanoparticle suspension appear as a convolution of the transient spectra for free TCCP and free Ag nanoparticles. Therefore, it is shown that porphyrins with substituents that lead to strong ground-state interaction also lead to excited-state interaction (TAPP and TmPyP), whereas porphyrin that has little interaction in the ground state does not display excited-state interaction (TCCP).

4. CONCLUSIONS A porphyrin derivative, TAPP, and Ag nanoparticles undergo charge-transfer complexation in the ground state, as evidenced by SERRS and red-shifted (500
700 nm) ground-state absorption. By overlapping the charge-transfer band with the Raman excitation wavelength, we have succeeded in introducing chemical enhancement of the SERRS signal. The charge-transfer state as monitored from the ultrafast spectroscopy measurements exhibits a charge-separation time constant of ∼6 ps and subsequent recombination time constant of ∼116 ps. By comparing the SERRS and transient absorption spectroscopy results of two other free-base porphyrins, we conclude that the amino substituents of TAPP play a significant role in its complexation with Ag nanoparticles. The strongly interacting TAPP-Ag nanoparticle system highlights the importance of functional substituents in facilitating charge-transfer enhancement in SERS and excitedstate charge separation. ’ ASSOCIATED CONTENT

bS

Steady-state emission spectrum along with an emission lifetime trace for neat TAPP; molar Supporting Information.

extinction coefficients for the four main absorption bands of TAPP observed by UV
visible absorption spectroscopy; a resonance Raman spectrum for TAPP powder; 514 nm Raman spectra for neat TAPP, neat TmPyP, neat TCCP, and Ag nanoparticles in solution; UV
visible absorption spectra for TAPP-Ag nanoparticle hybrids with increasing Ag nanoparticle concentration; transient absorption spectra for Ag nanoparticles; transient absorption spectra for TAPP with sodium citrate, citric acid, and sodium borohyrdride; transient absorption spectra for TAPP with silver nitrate; transient absorption spectra for TmPyP and TCCP in the presence of Ag nanoparticles; TEM images of Ag nanoparticles in the presence and absence of TAPP; transient absorption spectra for TAPP and kinetic traces for TAPP and TAPP in the presence of Ag nanoparticles acquired by probing from 800 to 1150 nm; and UV
visible absorption spectra for the same concentration of TAPP, TmPyP, and TCCP all in the presence of Ag nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected].

’ ACKNOWLEDGMENT The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. We thank Prof. Greg Hartland and the Strategic Research Investment Program of the University of Notre Dame for the use of fluorescence upconversion spectrometer. We also thank Ian Lightcap for TEM images, Kevin Tvrdy, and Clifton Harris for helpful discussions. This is contribution no. NDRL 4889 from the Notre Dame Radiation Laboratory. ’ REFERENCES (1) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Preparation and optical absorption spectra of dye-coated Au, Ag, and 22767

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