Reply to "Comment on: 'Isolating Vibrational Polariton 2D-IR Transmission Spectra'".

In a Comment on our recent Letter, the authors take issue with our method of refining 2D-IR transmission spectra to remove a background contribution that arises from nonpolaritonic molecules in the cavity. In our response to their Comment, we describe how our approach was motivated by the previous work of the authors, and we present a spatially dependent molecule-cavity Tavis-Cummings model that can account for the significant response from localized molecules with nonzero oscillator strengths. The telltale signature of the localized molecule response is the spectral diffusion dynamics of the bare W(CO)6 molecules in the polar butyl acetate solvent. Inhomogeneous broadening is absent from polaritonic states due to the extreme degree of exchange narrowing in coupling very large numbers of molecules to a cavity mode.

Isolating Polaritonic 2D-IR Transmission Spectra.

Strong coupling between vibrational transitions in molecules within a resonant optical microcavity leads to the formation of collective, delocalized vibrational polaritons. There are many potential applications of "polaritonic chemistry", ranging from modified chemical reactivity to quantum information processing. One challenge in obtaining the polaritonic response is removing a background contribution due to the uncoupled molecules that generate an ordinary 2D-IR spectrum whose amplitude is filtered by the polariton transmission spectrum. We show that most features in 2D-IR spectra of vibrational polaritons can be explained by a linear superposition of this background signal and the true polariton response. Through a straightforward correction procedure, in which the filtered bare-molecule 2D-IR spectrum is subtracted from the measured cavity response, we recover the polaritonic spectrum.

Probing the Formation and Conformational Relaxation of Previtamin D3 and Analogues in Solution and in Lipid Bilayers.

The photosynthesis of vitamin D3 in mammalian skin results from UV-B irradiation of provitamin D3 (7-dehydrocholesterol, DHC) at ca. 290 nm. Upon return to the ground state, the hexatriene product, previtamin D3, undergoes a conformational equilibration between helical gZg and more planar tZg and tZt forms. The helical gZg forms provide a pathway for the formation of vitamin D3 via a [1,7]-sigmatropic hydrogen shift. Steady state photolysis and UV transient absorption spectroscopy are combined to explore the conformational relaxation of previtamin D3 formed from DHC in isotropic solution and confined to lipid bilayers chosen to model the biological cell membrane. The results are compared with measurements for two analogues: previtamin D2 formed from ergosterol (provitamin D2) and previtamin D3 acetate formed from DHC acetate. The resulting spectral dynamics are interpreted in the context of simulations of optical excitation energy and oscillator strength as a function of conformation. In solution, the relaxation dynamics and steady state product distributions of the three compounds are nearly identical, favoring tZg forms. When confined to lipid bilayers, the heterogeneity and packing forces alter the conformational distributions and enhance the population of a gZg conformer capable of vitamin D formation.

Direct comparison of amplitude and geometric measures of spectral inhomogeneity using phase-cycled 2D-IR spectroscopy.

Two-dimensional infrared (2D-IR) spectroscopy provides access to equilibrium dynamics with the extraction of the frequency-fluctuation correlation function (FFCF) from the measured spectra. Several different methods of obtaining the FFCF from experimental spectra, such as the center line slope (CLS), ellipticity, phase slope, and nodal line slope, all depend on the geometrical nature of the 2D line shape and necessarily require spectral extent in order to achieve a measure of the FFCF. Amplitude measures, on the other hand, such as the inhomogeneity index, rely only on signal amplitudes and can, in principle, be computed using just a single point in a 2D spectrum. With a pulse shaper-based 2D-IR spectrometer, in conjunction with phase cycling, we separate the rephasing and nonrephasing signals used to determine the inhomogeneity index. The same measured data provide the absorptive spectrum, needed for the CLS. Both methods are applied to two model molecular systems: tungsten hexacarbonyl (WCO6) and methylcyclopentadienyl manganese tricarbonyl [Cp'Mn(CO)3, MCMT]. The three degenerate IR modes of W(CO)6 lack coherent modulation or noticeable intramolecular vibrational redistribution (IVR) and are used to establish a baseline comparison. The two bands of the MCMT tripod complex include intraband coherences and IVR as well as likely internal torsional motion on a few-picosecond time scale. We find essentially identical spectral diffusion, but faster, non-equilibrium dynamics lead to differences in the FFCFs extracted with the two methods. The inhomogeneity index offers an advantage in cases where spectra are complex and energy transfer can mimic line shape changes due to frequency fluctuations.

Fourier Transform Fluorescence-Encoded Infrared Spectroscopy.

While time-resolved infrared (IR) vibrational spectroscopy provides insight on structural dynamics of solution-phase systems, current techniques are limited to high concentrations. Fluorescence-encoded infrared spectroscopy (FEIR) can be used to encode IR-driven vibrational excitations into excited electronic states that fluoresce, which can be detected at lower concentrations than a coherently detected IR signal. Here, we report on the development of Fourier transform FEIR as an alternate approach for high-sensitivity IR spectroscopy. Upon driving vibrational excitation with a pair of IR fields with a variable time delay, an interferometric component was observed in the encoded fluorescence. This signal can be Fourier transformed to obtain a vibrational spectrum. By additionally varying the time delay of the encoding pulse following the second IR pulse, we observed frequency-difference oscillations, allowing us to construct a 2D correlation spectrum of coupled vibrations. Response functions for this experiment have been modeled, which reproduce the observed spectral features and relate them to excitation pathways using diagrammatic perturbation theory. The pathways observed in a 2D FEIR spectrum arise from the excitation of vibrational populations and coherences between coupled vibrations.

Two-Photon-Excited Fluorescence-Encoded Infrared Spectroscopy.

We report on a method for performing ultrafast infrared (IR) vibrational spectroscopy using fluorescence detection. Vibrational dynamics on the ground electronic state driven by femtosecond mid-infrared pulses are detected by changes in fluorescence amplitude resulting from modulation of a two-photon visible transition by nuclear motion. We examine a series of coumarin dyes and study the signals as a function of solvent and excitation pulse parameters. The measured signal characterizes the relaxation of vibrational populations and coherences but yields different information than conventional IR transient absorption measurements. These differences result from the manner in which the ground-state dynamics are projected by the two-photon detection step. Extensions of this method can be adapted for a variety of increased-sensitivity IR measurements.

Aqueous colloidal acene nanoparticles: a new platform for studying singlet fission.

Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films.

Efficient singlet fission discovered in a disordered acene film.

Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPT's pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet fission yield, with 1.22 triplets being created per excited singlet. This triplet production occurs over two principal time scales, with ~50% of the triplets appearing within 1 ps after photoexcitation followed by a slower phase of triplet growth over a few hundred picoseconds. To fit these kinetics, we have developed a model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission. Singlet excitons directly excited at these sites can undergo fission rapidly, while singlet excitons created elsewhere in the film must diffuse to these sites to fission.

Observation of Triplet Exciton Formation in a Platinum-Sensitized Organic Photovoltaic Device.

Organic photovoltaics (OPVs) constitute a promising new technology due to their low production costs. However, OPV efficiencies remain low because excitons typically diffuse only ∼5-20 nm during their lifetime, limiting the effective thickness of the light-absorbing layer. One strategy to improve OPVs is to increase exciton lifetimes by converting them into triplet states, which typically persist 10(3)-10(5) times longer than singlet excitons. We present femtosecond transient absorption and steady-state photovoltaic measurements of a model OPV system consisting of diphenyltetracene (DPT) films doped with platinum tetraphenylbenzoporphyrin (Pt(TPBP)). Photoexcitation of Pt(TPBP) creates a singlet excitation that rapidly intersystem crosses to a triplet state before transferring to the DPT host matrix. This transfer is rapid and efficient, occurring in 35 ps with an 85% conversion ratio of porphyrin singlets to DPT triplets. These triplet excitons lead to enhanced photocurrent response that increases with device thickness.