Electronic and Vibrational Properties of meso-Tetraphenylporphyrin

Article pubs.acs.org/JPCA

Electronic and Vibrational Properties of meso-Tetraphenylporphyrin on Silver Substrates Patrick Z. El-Khoury,† Karoliina Honkala,‡ and Wayne P. Hess*,† †

Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, FIN-40014 Jyväskylä, Finland

‡

ABSTRACT: The electronic and vibrational properties of mesotetraphenylporphyrin (mtpp) on silver substrates are investigated using UV−vis and surface-enhanced resonance Raman scattering (SERRS) spectroscopy. Whereas the vibrational signatures associated with the tetrapyrrole backbone exhibit minor variations throughout sequences of consecutively recorded SERRS spectra, the CC stretching vibrational modes localized on the meso-phenyl moieties of mtpp exhibit noticeable intensity fluctuations, masked in the average SERRS response. We attribute the observed vibrational-state-specific blinking events to conformational changes in mtpp, namely, torsional flexibility which mediates the coupling between the π-framework of the meso-phenyls and the underlying metal substrate.

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INTRODUCTION The dynamic coupling between radiation fields and molecular polarizabilities leads to Raman scattering. Of the challenges faced in the application of Raman spectroscopy to probe various properties of material systems, the poor efficiency of normal Raman scattering (cross sections on the order of 10−30 cm2 per molecule) is by far the most prohibitive. Early observations of surface-enhanced Raman scattering (SERS) revealed that this problem can be overcome,1−3 all while preserving the unique properties of Raman scattering. Ensuing works illustrated that the ultimate detection limit of a single molecule can be attained using SERS from molecules adsorbed on one or between plasmonic nanoparticles.4,5 This led to a variety of applications in the areas of ultrasensitive chemical detection and imaging, as highlighted in recent reviews.6−8 An intriguing feature in SERS is the observation of vibrational-state- and time-dependent intensity fluctuations, generally referred to as “spectral blinking”. Recent works associated such fluctuations with molecular reorientation at plasmonic nanojunctions.9−11 Namely, the scattering tensor that controls Raman activity can be written as Sn2α ∑ |εslαn′(Ω)εil|2

employed to account for spectral variations in nonresonant SERS9,10 and tip-enhanced Raman scattering11 (TERS), but the framework has been contested in a very recent report which probed tip-enhanced resonant Raman scattering (TERRS) from a single rhodamine 6G molecule.12 The authors reasoned that although eq 1 can describe intensity fluctuations in the absence of resonance, the TERRS tensor is governed by excited state properties (e.g., the excited state lifetime), which are independent of molecular orientation. Indeed, the orientation dependence in resonant Raman scattering (RRS) stems from the orientation of the transition dipole vector, which is vibrational-state-independent (at least in the Condon approximation). Herein, we investigate the origin of surface-enhanced resonant Raman scattering (SERRS) from meso-tetraphenylporphyrin (mtpp) on silver substrates. We find that whereas the vibrational signatures associated with the tetrapyrrole framework are unaltered throughout series of consecutively recorded SERRS spectra, the aromatic CC stretching modes localized on the meso-phenyl moieties of mtpp exhibit noticeable intensity fluctuations. Their origin is investigated. General Framework for the Interpretation. For more a detailed discussion of the operating principles in RRS and their analogues in SERRS, the reader is referred to prior works (e.g., the original publications of Bloembergen,13 Albrecht,14 Mukamel,15 and Lombardi/Birke).16 We only go over particulars which pertain to our present findings and ensuing discussion in the following sections.

(1)

n

εli,s

where are the enhanced incident and scattered local radiation fields, αn′ are the molecular polarizability derivative tensors of the nth vibrational eigenstate, and Ω = {α, β, γ} are the Euler angles which dictate molecular orientation relative to the local fields. Accordingly, the optical response of a single scatterer (or a few) recovers the full tensor nature of Raman scattering, and the orientation of a molecule with respect to the incident and scattered fields dictates the intensity of the nth vibrational state in the spectrum. This framework has been © 2014 American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: December 17, 2013 Revised: February 14, 2014 Published: February 20, 2014 8115

dx.doi.org/10.1021/jp412360b | J. Phys. Chem. A 2014, 118, 8115−8123

The Journal of Physical Chemistry A

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⎡ ⎤ |⟨F |μs |K ⟩ K |μi |I ⟩|2 ⎥ SB2α − Im⎢ l l l l ⎢⎣ (ωKI − ωi + iγKI )(ωKF − ωs − iγKF )(ωFI − ωi + ωs + iγFI ) ⎥⎦

In the absence of specific resonances between the incident photon and excited electronic states of the material system, both (i) the damping parameters associated with excited electronic states and (ii) the time ordering between the bra and ket states can be neglected. In this case, Bloembergen’s 48 third-order electrical susceptibility terms13 are reduced to 24, only 4 of which satisfy the resonance condition ωFI − ωli + ωls + iγFI (see Figure 1); where ωFI is the frequency difference

(3)

between the initial and final vibrational states on the ground electronic state, ωli,s are the frequencies of the incident and scattered radiation fields, and iγFI corresponds to the dephasing time constant between the initial and final vibronic states. To stay on-topic, we only illustrate a diagrammatic representation of one term which contributes to nonresonant Raman scattering, shown in Figure 1A. It can be shown that in the absence of resonance, the total scattering signal is given by SA2 α

∑ K

ωKI − ωil

+

∑ k

⟨F |μi |K ⟩⟨K |μs |I ⟩ ωKI + ωsl

(4)

⎡ ⎤ |⟨F |μs |K ⟩⟨K |μi |I ⟩|2 ⎥ SD2 α − Im⎢ l l ⎢⎣ (ωKI − ωi + iγKI )(+ iγK )(ωKF − ωs − iγKF ) ⎥⎦

(5)

where iγσρ are the dephasing constants between states σ and ρ for σ ≠ ρ, and γK is the lifetime of the excited vibronic state K, see Figure 1B−D. By summing eqs 3−5 and assuming that these processes are taking place on Born−Oppenheimer surfaces, it can be shown that the total RRS signal is governed by the modulus square of the product between the transition moments for absorption and scattering; the line shape function only acts as a weighting factor for the Franck−Condon absorption/secondary radiation events. Note that although resonant polarization is achieved in Figure 1B, this diagram is similar to the off-resonance Raman diagram depicted in Figure 1A in the sense that the excited state is not populated. This is not the case for the processes shown in Figures 1C,D, where the excited vibronic state K is populated prior to emission, and molecular evolution on the excited state potential energy surface contributes to the total resonant scattering signal. Overall, the initial, intermediate, and final states can be of molecular origin to describe normal (off-resonance) and resonant Raman scattering, or of mixed molecular-metallic (e.g., molecule-to-metal or metal-to-molecule charge-transfer) origin to describe surface and tip-enhanced Raman scattering and their resonant analogues, as recently demonstrated by Lombardi and Birke.16 Whether or not the levels of theory typically employed to tackle the SERRS/TERRS problem can correctly describe excited states of mixed origin is an entirely different problem in itself. Practical Considerations. The spectral density I(ω) of a dynamical variable x(t) can be related to the Fourier transform of its autocorrelation function C(t) (or x(0)x(t)) by17

Figure 1. Time circuit diagrams for nonresonant (A) and resonant (B−D) Raman scattering. Time increases from left to right. The states and frequencies of the incident/scattered fields are labeled in Panel A.

⟨F |μs |K ⟩⟨K |μi |I ⟩

⎡ ⎤ |⟨F |μs |K ⟩⟨K |μi |I ⟩|2 ⎥ SC2α − Im⎢ l l ⎢⎣ (ωKI − ωi − iγKI )(− iγK )(ωKF − ωs − iγKF ) ⎥⎦

I(ω) =

2

1 2π

∞

∫−∞ C(t )e−iωtdt

(6)

A recent report from our group18 demonstrated that there are distinct advantages to deriving surface-enhanced spectroscopic properties from ab initio molecular dynamics simulations, through the example of a prototypical finite silver cluster-organic complex, namely, Ag7-benzenethiol. This approach overcomes the implicit approximations made in eq 1, principally, that reorienting the molecule with respect to the incident/scattered radiation fields (or vice versa) does not perturb its electronic structure. We found that propagating the density matrix of Ag7-benzenethiol beyond its minimum energy configuration (where spectroscopic properties are typically evaluated) on the ground electronic state significantly modulates its electronic density, dipoles, and polarizabilities. This is a result of considerable conformational flexibility to the seemingly rigid model system, chiefly, torsional motion, which mediates the coupling between the π-framework of the chemisorbed organic molecule and the metal. We find that a similar underlying mechanism accounts for the observables in this report. Note that the large size of our molecular system (mtpp with a finite silver slab to simulate the metal) is restrictive from a computational standpoint. Moreover,

L(ωFI − ωil + ωsl)

(2)

The states are defined in Figure 1A: μi,s are the dipole moment operators in the incident (i) and scattered (s) polarization directions, and ωσρ (σ ≠ ρ) corresponds to the frequency difference between states σ and ρ. The first and second sums are the resonant and nonresonant terms of the familiar Kramers−Heisenberg expressions, whereas the third corresponds to a Lorentzian line shape broadened by the dephasing time constant, γFI. When the incident photon energy is resonant with an excited electronic state of the material system, the Kramers− Heisenberg relations are no longer appropriate. In RRS, one needs to globally account for the various properties of the excited states encountered in the scenarios diagrammatically illustrated in Figure 1B−D. Here, Bloembergen’s 48 terms13 are reduced to 3 which retain three resonant factors in their respective denominators. The expressions which describe the resonant scattering signals can be described by 8116



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Figure 2. Left panel: UV−vis absorption spectrum of mtpp in toluene. An extinction coefficient of 17 735 M−1 cm−1 is calculated at the excitation wavelength used throughout this work (514 nm). Right panel: linear dependence of absorption on solute concentration in the 0−5 mM range. The deviation from linearity at 10 mM is an indication of solute aggregation in toluene. The red arrow points to the solute concentration used to prepare the mtpp films (2.5 mM), falling within the linear Lambert−Beer regime.

to a spectrometer (Holespec f/1.8i, Kaiser Optical System). The irradiation intensity was attenuated to 1−5 μW/μm2, as higher laser powers were found to induce (i) the appearance of additional peaks in the SERRS spectra that possibly arise from photochemical products of mtpp and (ii) a dramatic loss in signal intensity over the time scale over our measurements. The effective instrument resolution in the micro-Raman experiments is on the order of 8 cm−1. Computational. The isolated free base porphyrin (mtpp) was fully optimized at the B3LYP/6-31g* level of theory, using the methodologies implemented in Gaussian 09.21 Unconstrained geometry optimization was followed by normal-mode analysis and static Raman scattering activity calculations. The computed orientationally averaged vibrational spectra were scaled by a single factor of 0.966, and they are represented as sums of Lorentzian functions individually broadened by 8 cm−1 to match their experimental counterparts. Vertical transition energies were computed from the optimized minima at the TD B3LYP/6-31g* level of theory. Similar exploratory calculations (not shown) were performed to explore the electronic and vibrational properties of dimers of mtpp, its doubly deprotonated dianion, and various oxidation states of mtpp. The Ag-bound system (Ag-mtpp) was modeled using the gpaw software package.22 The metal substrate was simulated using three layers of Ag in the (111) orientation. The dimensions of the unit cell used for the simulation of a single mtpp molecule on Ag (111) are 3.004 × 1.737 nm, resulting in a 225-atom model system. The positions of the Ag atoms constituting the silver layer furthest away from the molecular system were fixed throughout the geometry optimization procedure, whereas all other atomic positions were energyminimized. We employed both the PBE23 and the vdW-DF24 exchange-correlation functionals, the latter used to account for the effects of van der Waals forces on the computed adsorption energies. The predicted binding energies were +1.17 and −0.23 eV, obtained using the PBE and vdW-DF functionals, respectively.

simulating SERRS spectra from ab initio molecular dynamics simulations would require propagating the density matrix on (at the very least qualitatively correct) excited electronic states of the total system, which is not feasible at this time. As such, we adopt a more qualitative approach to interpreting our current results with the aforementioned observations in mind, guided by tools of density functional theory.

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METHODS Sample Preparation. The solution used to prepare the samples consists of a 2.5 mM solution of meso-tetraphenylporphyrin (mtpp, Frontier Scientific, 95%) in anhydrous toluene (Aldrich, 99.8%). The first sample was prepared by spin-casting the mtpp/toluene solution onto a 0.1 mm glass support, followed by vacuum-evaporation to rid the sample of residual solvent molecules. The second sample consists of mtpp coated atop a 15 nm Ag film grown on the glass support by arcdischarge physical vapor deposition. The sputtering target was purchased from Ted Pella, Inc. (99.99% purity), and the film thickness was monitored in situ using a quartz crystal microbalance. The third sample comprises mtpp sandwiched between two Ag layers, prepared by sputtering an additional 15 nm of Ag on top of an mtpp-coated glass-supported 15 nm Ag layer. Freshly prepared samples were used to minimize the effects of Ag oxidation on the observables. Optical Characterization. The steady state UV−vis absorption spectra of the liquid and solid samples were collected using the Cary 50 UV−vis spectrophotometer. Our Raman microspectroscopy setup has been previously described elsewhere.19,20 Briefly, measurements were conducted under ambient laboratory conditions using an inverted optical microscope (Axiovert 200, Zeiss). The incident 514 nm continuous wave monochromatic light (Innova 300, Coherent) is attenuated using a variable neutral density filter wheel, reflected off a dichroic beamsplitter, and tightly focused onto the sample surface using an oil-immersion objective (1.3 NA, 100×). The backscattered radiation is collected through the same objective, transmitted through a beamsplitter, and filtered using a long pass filter. The resulting radiation is detected by a liquid nitrogen cooled charge coupled device coupled (CCD) 8117



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Figure 3. Left panel: UV−vis absorption spectra of (i) a 15 nm thick glass-supported Ag surface, (ii) a porphyrin film coated onto a thin glass support, (iii) a porphyrin film coated onto a 15 nm glass-supported Ag surface, (iv) a porphyrin film sandwiched between two 15 nm Ag layers, and (v) an 0.028 mM mtpp/toluene solution. The green arrow highlights that our choice of excitation wavelength populates the high-energy wing of the Qy(1,0) state in the free base. Right panel: HOMO − 1 → LUMO + 1 (160 → 163) and HOMO → LUMO (161 → 162) transitions, which contribute to the ground → Qy transition by ∼43 and ∼56%, respectively.

Figure 4. Left panel: stable surface-enhanced resonance Raman (SERR) trajectory (mtpp on a 15 nm Ag film) at an incident laser power of 5 μW/ μm2. Right panel: signatures of the photoinduced damage of mtpp at early times followed by an almost complete disappearance of the molecular response toward the end of the trajectory at incident laser powers as low as ∼15 μW/μm2. Each of the sequentially acquired spectra is individually integrated over a time period of 10 s.

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from (v) to obtain the respective “background free” UV−vis spectra shown. Although the solution spectrum exhibits the typical absorption bands of porphyrins, that is an intense and sharp Soret (B) band and four weak Q bands,25 the solid samples (mtpp on both glass and silver) exhibit splitting and broadening in the Soret region. For instance, the B-band centered at 419 nm in the liquid phase spectrum broadens and splits into two broad bands centered at ∼400 and ∼430 nm in the glass-supported mtpp film. The observation of similar peaks in the UV−vis spectra of structurally related systems was previously implicated with dimerization/aggregation in porphyrin films.26,27 Of relevance to our present investigation is the observation of a ∼10 nm red-shift of the Qy(1,0) band, such that the 514 nm excitation wavelength used in the resonance Raman experimentswhich coincides with the band maximum in solutionexcites the high-energy wing of the corresponding dimers/aggregates state in the solid samples. The change in

RESULTS UV−vis Spectroscopy. Figure 2 shows the solvent subtracted absorption spectrum of meso-tetraphenylporphyrin (mtpp) in toluene. An extinction of 17 735 M−1 cm−1 is computed at 514 nm, the excitation wavelength used in the SERRS experiments discussed in the following sections. A linear dependence of absorption on solute concentration in the 0−5 mM range ensures the absence of aggregation in solution prior to coating mtpp onto the silver/glass substrates. Figure 3 shows the UV−vis spectra of (i) mtpp in solution, (ii) a glasssupported 15 nm thick Ag layer (Ag15 from hereon), (iii) mtpp coated onto a thin glass support, (iv) mtpp on Ag15, and (v) mtpp sandwiched between Ag15 and an additional 15 nm of Ag. We subtract the contribution of the glass support to the total absorption of (ii) and (iii), the total absorption of (ii) from (iv), as well as the sum of absorption of (ii) + (ii) (before and after subtracting the contribution from the glass support) 8118



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Figure 5. Left paneltop to bottom. SERRS/RRS spectra of (i) an mtpp film sandwiched between two 15 nm Ag layers, (ii) a porphyrin film coated onto a 15 nm glass-supported Ag surface, (iii) a porphyrin film coated onto a thin microscope slide, and (iv) mtpp powder. The lowest trace corresponds to the computed B3LYP/6-31g* Raman spectrum. Right panel: mixed molecular−metallic and pure molecular scattering signatures derived from the spectra shown in the left panel. The black trace was obtained by subtracting (iii) from (ii), whereas the red trace was obtained by subtracting (i) from (ii). Notice the resemblance between the former and the magenta trace in the left panel, as well as the correspondence between the latter and the blue trace in the left panel.

electron density associated with ground to Qy excitation is also schematically illustrated in Figure 3, revealing that the electronic density on the occupied/virtual Gouterman-type orbitals28 are predominantly localized on the pyrroles and mesocarbons. We note that the same holds for ground to Qy excitation in the dimer structure (not shown). SERRS Spectroscopy. Because the excitation wavelength used is resonant both with an excited electronic state in mtpp and in its dimers/aggregates, sample degradation is a major concern. Our resonant molecular reporters are particularly susceptible to photoinduced damage at asperities and defects sites sustained throughout the inhomogeneous metal terrain where local fields are maximally enhanced and the optical response is predominant. In Figure 4, we show that laser intensities as low as 15 μW/μm2 induce sample degradation within the integration time (10 s per spectrum) employed in this study. This is followed by the emergence of vibrational signatures which likely arise from photoproducts of mtpp and an almost complete loss of signal within minutes. The aforementioned is contrasted with a stable and reproducible signal over the typical time scales of our measurement (a few minutes) at incident intensities in the 1−5 μW/μm2 range.

nonradiative dissipation pathways, leading to ultrashort excited state lifetimes. As previously eluded to, this is also marked by the broadening of the UV−vis absorption bands. This in turn allows for Raman scattering to predominate over fluorescence (the dephased radiative process) which typically takes place on nanosecond time scales. Energy transfer to the metal in the hybrid organic−metallic substrates is also expected to lead to strong fluorescence quenching. Nonetheless, the recorded SERRS spectra are further complicated by static and dynamic molecule−metal interactions, and in the present resonance Raman study, possible evolution on excited electronic states of molecular and/or mixed molecular−metallic origin. There are subtle yet characteristic differences between the resonance Raman spectra collected from the glass- (blue line) and silver-supported (olive line) mtpp films, see Figure 5. Those include (i) variations in the underlying broad background scattering signal, (ii) an apparent broadening of the molecular response in the 1300−1400 cm−1 region, and (iii) the appearance of a weak Raman band at ∼1600 cm−1 all observed in the average response of the Ag-featuring substrate and not observed in the spectra recorded from the glasssupported mtpp film (see the blue highlighted region in Figure 5). By elimination, the observed differences can be naı̈vely attributed to molecule−metal interactions, the details of which deserve further scrutiny. We begin by inspecting the background scattering signal in the Ag-featuring substrates. Here, we have specifically designed a substrate in which the observables are selectively biased toward scattering of mixed molecularmetallic origin. In a recent report from our group,19 we employed a similar strategy to distinguish between the background signal and the molecular response in Raman scattering at plasmonic nanojunctions. Herein, we probe the inelastic scattering response from an mtpp film sandwiched between two 15 nm Ag layers, where strong molecule−metal interactions are forced by design. We find that in this case, the molecular response is dwarfed by a broad scattering signal (see Figure 5), consistent with prior observations and assignments.9,19 We take the concept a step forward in the plots shown in the right panel of Figure 5 to further illustrate the principle. We find that the molecular signal (line spectrum, red

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DISCUSSION Figure 5 shows the average resonance Raman and SERRS spectra collected from the various samples probed in this work. Each of the experimental spectra shown represents a global/ time-averaged response, obtained by averaging thousands of individual spectra collected from tens of different spots from several different samples prepared on different days. As such, both sample integrity and reproducibility are ensured. We begin by inspecting the resonant molecular response from mtpp powder (red line) and the glass-supported mtpp film (blue line). The two spectra are similar, except for a broad background response toward higher energies in the powder, assigned to the onset of fluorescence in mtpp. The absence of the fluorescent background in the mtpp film is a sign of aggregation, distinct from the intermolecular interactions present in the crystalline form. It appears that relatively strong intermolecular interactions in the film open way for ultrafast 8119



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Figure 6. Selected structural parameters of the optimized isolated and silver-bound mtpp models. See the Computational section in Methods for more details.

general, and the sequences shown in Figure 7 are illustrative of the general temporal evolution of the recorded SERRS spectra. Possible Interpretation. We now turn the reader’s attention to a recent report from our group,18 where we correlated time-dependent conformational changes in a model SERS system with changes in its vibrational density of states and spectroscopic properties. We found that molecular evolution along a torsional degree of freedom in Ag7benzenethiol controls the coupling between the π-framework of the aromatic molecule and the metal cluster. We propose that a very similar mechanism accounts for our current observations, and molecular evolution on the excited electronic state does not need to be invoked to interpret our results. In effect, mtpp which consists of a rigid tetrapyrrole framework linked to more flexible meso-phenyl moieties (for rotation about the single bond connecting the meso-carbon to the mesophenyls) is an ideal system to test our previous proposal.18 Following the absorption of a 514 nm photon, the electron density is localized on the tetrapyrrole framework of mtpp. That the resonant signal arises from the process shown in Figure 1B (no excited state evolution) would explain the diminished Raman scattering activity of the ∼1600 cm−1 aromatic CC stretching vibration localized on the mesophenyls in the crystalline form as well as in the glass supported mtpp film. The signal in the absence of the metal seems to be governed by Franck−Condon factors. In the Ag-featuring samples, thermally (or photothermally) induced torsional motion about the single bond connecting the meso-carbon to the meso-phenyls (on the ground state) mediates the coupling between the phenyls and the underlying metal. When the phenyls are parallel or perpendicular to the underlying metal, the coupling between these moieties and the underlying metal is switched either on or off. In other words, two different

trace) can be confidently differentiated from the mixed molecular-metallic scattering signal (broadened spectrum, black trace). Namely, subtracting the molecular response nascent from the glass-supported mtpp film from the total SERRS response of the Ag-supported mtpp film yields a heavily broadened “background” signal, bearing strong resemblance to the inelastic scattering response recorded from the Agsandwiched mtpp film. Conversely, the pure molecular line spectrum can be recovered by subtracting the optical response of the Ag-sandwiched mtpp film from the spectrum of the Agsupported mtpp. The computed Raman spectrum of mtpp is shown in Figure 5. We attribute the differences between the experimental and computed spectra to the limitations of the level of theory and model systems used to simulate the Raman spectra. Nonetheless, we can confidently assign the ∼1600 cm−1 SERRS signatures to the aromatic CC stretching vibrations localized on the meso-phenyls. Interestingly, theory predicts significant scattering activity at ∼1600 cm−1 for the isolated gas-phase molecule. Experimentally, this signature was only observed in the Ag-supported films (see the blue highlighted region in Figure 5), but this was not observed in the Raman spectra recorded from the powder and glass-supported mtpp films. This is neither a consequence of chemical changes (e.g., deprotonation) nor a result of a dramatic change in the molecular/electronic structure (e.g., flat adsorption geometry) in mtpp upon adsorption onto to silver substrates (see Figure 6). It is a dynamic effect. Sequentially recorded SERRS spectra are informative in this regard (see Figure 7). Whereas the vibrational signatures of the porphyrin macrocycle display little variations throughout the recorded sequences, time-dependent changes in the relative intensities of the ∼1600 cm−1 Raman signatures are evident. We stress that these observations are 8120



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Figure 7. SERRS trajectories of mtpp on Ag. The left panel illustrates sequentially acquired Raman spectra, each of which was individually integrated for 10 s. Selected individual spectra from the sequences are shown in the right panels, along with the averaged SERRS response (see Figure 5) and the simulated B3LYP/6-31g* spectrum. Note how unlike the first two sequences, the last trajectory illustrates an irreversible trajectory.

Franck−Condon regions of the excited electronic state of the total system are accessed from two different starting ground vibronic states, a dark and a bright state. Our suggested

mechanism is overall consistent with a prior suggestion by Haran29 of a two state geometry for blinking in crystal violet. We stress that an excited state analogue of the proposed 8121



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(3) Moskovits, M. Surface Roughness and the Enhanced Intensity of Raman Scattering by Molecules Adsorbed on Metals. J. Chem. Phys. 1978, 69, 4159−4161. (4) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (5) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670. (6) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105−1125. (7) Le Ru, E. C.; Etchegoin, P. G. Single-Molecule Surface-Enhanced Raman Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 65−87. (8) Pettinger, B.; Schambach, P.; Villagomez, C. J.; Scott, N. TipEnhanced Raman Spectroscopy: Near-Fields Acting on a Few Molecules. Annu. Rev. Phys. Chem. 2012, 63, 379−399. (9) Banik, M.; El-Khoury, P. Z.; Nag, A.; Rodriguez-Perez, A.; Guarrottxena, N.; Bazan, G. C.; Apkarian, V. A. Surface-Enhanced Raman Trajectories on a Nano-Dumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse. ACS Nano 2012, 6, 10343−10354. (10) El-Khoury, P. Z.; Hess, W. P. Raman Scattering from 1,3Propanedithiol at a Hot Spot: Theory Meets Experiment. Chem. Phys. Lett. 2013, 581, 57−63. (11) El-Khoury, P. Z.; Hu, D.; Hess, W. P. Junction Plasmon-Induced Molecular Reorientation. J. Phys. Chem. Lett. 2013, 4, 3435−3439. (12) Sonntag, M. D.; Chulhai, D.; Seideman, T.; Jensen, L.; Van Duyne, R. P. The Origin of Relative Intensity Fluctuations in SingleMolecule Tip-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2013, 135, 17187−17192. (13) Bloembergen, N. Encounters in Nonlinear Optics: Selected Papers of Nicolaas Bloembergen, with Commentary; World Scientific Publishing Co. Pte. Ltd.: River Edge, NJ, 1996. (14) Albrecht, A. C. On the Theory of Raman Intensities. J. Chem. Phys. 1961, 34, 1476−1484. (15) Mukamel, S. The Principles of Nonlinear Optical Spectroscopy; Oxford University Press: New York, 1995. (16) Lombardi, J. R.; Birke, R. L. A Unified Approach to SurfaceEnhanced Raman Spectroscopy. J. Phys. Chem. C 2008, 112, 5605− 5617. (17) Noid, D. W.; Koszykowski, M. L.; Marcus, R. A. A Spectral Analysis Method of Obtaining Molecular Spectra from Classical Trajectories. J. Chem. Phys. 1977, 67, 404−408. (18) El-Khoury, P. Z.; Bylaska, E. J.; Hess, W. P. Time Domain Simulations of Chemical Bonding Effects in Surface-Enhanced Spectroscopy. J. Chem. Phys. 2013, 139, 174303. (19) El-Khoury, P. Z.; Peppernick, S. J.; Hu, D.; Joly, A. G.; Hess, W. P. The Origin of Surface-Enhanced Raman Scattering of 4,4′Biphenyldicarboxylate on Silver Substrates. J. Phys. Chem. C 2013, 117, 7260−7268. (20) El-Khoury, P. Z.; Hu, D.; Apkarian, V. A.; Hess, W. P. Raman Scattering at Plasmonic Junctions Shorted by Conductive Molecular Bridges. Nano Lett. 2013, 13, 1858−1861. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (22) Mortensen, J. J.; Hansen, L. B.; Jacobsen, K. W. Real-Space Grid Implementation of the Projector Augmented Wave Method. Phys. Rev. B 2005, 71, 035109. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. (25) Stone, A.; Fleischer, E. The Molecular and Crystal Structure of Porphyrin Diacids. J. Am. Chem. Soc. 1968, 90, 2736−2748.

mechanism following excited state population (see Figures 1C,D) cannot be ruled out, as all three diagrams contribute to the signal when the incident photon is resonant with an excited electronic state of the system. That said, the excited state diagrams do not necessarily have to be invoked, and a similar mechanism is potentially operative in normal SERS spectroscopy of flexible systems. As a general statement, it appears that time-dependent conformational variations which perturb the electronic structure of molecular systems chemisorbed/ physisorbed onto metals comprise another factor which needs to be considered in an effort to faithfully account for spectral blinking in both nonresonant and resonant surface-enhanced Raman spectroscopy.

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CONCLUSIONS We analyze the electronic and vibrational spectra of mesotetraphenylporphyrin (mtpp) films on various substrates to understand the origin of spectral blinking in Ag-featuring mtpp substrates. We attribute the observed vibrational-state-specific intensity variations to time-dependent conformational changes which affect the coupling between mtpp and the underlying metal. In particular, we propose that torsional flexibility controls the coupling between the π-framework of the mesophenyls of mtpp and the metal and accounts for the observed time-dependent spectral variations. We concede that the arguments made to support our premise are qualitative for the most part. That said, our conclusions are consistent with recent reports from our group and others. We hope that this investigation fuels future theoretical and experimental works aimed at attaining a detailed understanding of specific molecule−metal interactions which govern surface-enhanced spectroscopic processes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS W.P.H. acknowledges support from the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. P.Z.E. acknowledges support from the Laboratory Directed Research and Development Program through a Linus Pauling Fellowship at Pacific Northwest National Laboratory (PNNL) and an allocation of computing time from the National Science Foundation (TG-CHE130003). K.H. acknowledges computing time provided by CSC the Finnish IT Center for Science. This work was performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated for DOE by Battelle.

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REFERENCES

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Electronic and Vibrational Properties of meso-Tetraphenylporphyrin

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