The Rise and Current Status of Polaritonic Photochemistry and Photophysics

Abstract

The interaction between molecular electronic transitions and electromagnetic fields can be enlarged to the point where distinct hybrid light-matter states, polaritons, emerge. The photonic contribution to these states results in increased complexity as well as an opening to modify the photophysics and photochemistry beyond what normally can be seen in organic molecules. It is today evident that polaritons offer opportunities for molecular photochemistry and photophysics, which has caused an ever-rising interest in the field. Focusing on the experimental landmarks, this review takes its reader from the advent of the field of polaritonic chemistry, over the split into polariton chemistry and photochemistry, to present day status within polaritonic photochemistry and photophysics. To introduce the field, the review starts with a general description of light-matter interactions, how to enhance these, and what characterizes the coupling strength. Then the photochemistry and photophysics of strongly coupled systems using Fabry-Perot and plasmonic cavities are described. This is followed by a description of room-temperature Bose-Einstein condensation/polariton lasing in polaritonic systems. The review ends with a discussion on the benefits, limitations, and future developments of strong exciton-photon coupling using organic molecules.

Figure 1

Sketch of the three different interactions between a photon and a two level system. (a) Absorption, (b) stimulated emission, and (c) spontaneous emission.

Figure 2

(a) Structure of a Fabry–Perot cavity with cavity modes at an integer number of λ/2. (b) Sketch of the two different wavevectors which are the origin of the dispersive behavior of a Fabry–Perot cavity. (c) Sketch of a plasmonic resonator treated as an open lossy system with complex-valued eigenmodes , where the imaginary part, describing the leakage rate for the nth cavity mode, contains all losses of the system (molecular, plasmonic, radiative).

Figure 3

(a) Sketch of a two level system representing a molecule (blue) inside an optical cavity with the λ cavity mode (red) on resonance to a molecular transition. The dissipation from the system is indicated by γ_c for the cavity photon and γ_m for the molecule. (b) Energy diagram showing that the molecular transition (blue) and the resonance cavity mode (red) couple, leading to the formation of hybrid states, polaritons (purple), separated in energy by the Rabi splitting ℏΩ_R. (c) Dispersion of the formed polaritons (purple) with indicated behavior of the cavity mode (red) and exciton (blue). (d) Corresponding Hopfield coefficients |α|² and |β|² to the polariton dispersion in ‘c’.

Figure 4

(a, b) Wave function overlap between a polariton and the exciton reservoir. The wave function of the polaritonic state is represented by a lambda wave, and the small spheres represent states in the exciton reservoir. Orange and green represent the initial and final excited states in an energy transfer reaction, respectively. The relaxation from a single exciton reservoir state to the polaritonic state is shown in ‘a’, and is a transition with a small wave function overlap. The relaxation from the polaritonic state to the exciton reservoir is shown in ‘b’ and is a transition where the small wave function overlap is compensated with the large number of available states in the exciton reservoir. (c) The effect of molecular transition broadening on the photonic contribution to the eigenstates. A system having a cavity mode coupled to 100 molecules with a narrow molecular transition (no disorder) and relatively broad molecular transition (disorder). In the narrow molecular transition coupling (red square), three distinct eigenstates are formed. The lower and upper polariton each having 50% photonic contribution along with 99 degenerate dark states having 0% photonic contribution and same energy as the molecular excited state. In the case of a realistically broad molecular transition (blue circles), the photonic contribution is no longer concentrated to two distinct states. Instead, the photonic contribution is with a varying degree distributed over many eigenstates. Reproduced from ref (117). Copyright 2021 John Wiley and Sons.

Figure 5

(a) Polariton relaxation pathways. From the upper polariton, the population can relax (black arrows) to the exciton reservoir (i), to the lower polariton (ii), or radiatively to the ground state (iii). From the exciton reservoir, the population can relax (green arrows) to the lower polariton or to the ground state. From the lower polariton, the population can relax radiativly (red arrow) to the ground state or transfer to the exciton reservoir. (b, c) Two-dimensional Fourier transform spectra of a TDBC J-aggregate in a red detuned cavity, monitored at (b) ϕ = 5° and (c) at ϕ = 20°. Here, broad-band visible light was used to probe the excitation dependence. Exothermic transitions can be followed by off-diagonal peaks in the lower right corner, and endothermic transitions can be followed by off-diagonal peaks in the top left corner. Reproduced from ref (132). Copyright 2020 Springer Nature. (d) Vibration assisted scattering. The shaded curve represents the total emission of the lower polariton (the spectrum was generated by summing all PL spectra at different detection angles) of a cavity containing TDBC. The total emission is plotted as a function of the energy separation, ΔE, from the exciton reservoir at 2.11 eV. The green spectrum represents Raman scattering. The peaks in the total emission from the lower polariton matches with the strongest Raman scattering peaks of the TDBC J-aggregate. Reproduced from ref (137). Copyright 2021 AIP Publishing. (e) The exciton reservoir can be viewed as being in a dynamic equilibrium with the lower polariton branch, which in turn can be approximated as a discrete set of states. This equilibrium heavily leans toward the exciton reservoir with the consequence being that the overall excited state lifetime often is dominated by the nonradiative rate of the exciton reservoir (green dotted arrow) rather than the polariton emission (red arrows).

Figure 6

(a) In a molecule having the possibility to form a twisted intramolecular carge transfer state (TICS) on the excited state surface, the lower polariton can facility the energy funneling toward the TICS. Reproduced from ref (164). Copyright 2021 AAAS. (b) Emission from DABNA-2 in two differently tuned cavities. A large change in the ratio of polariton versus excimer emission is seen. Reproduced from ref (165). Copyright 2022 the American Chemical Society. (c) The energy landscape of the photoisomer couple spiropyran (SPI) and merocyanine (MC). After excitation of MC (black arrow), relaxation can occur (orange arrows) to both MC and SPI. (d) When MC is placed in the strong coupling regime, the excited state photophysical pathways are perturbed (dashed orange arrows). For instance, strong coupling can introduce an energy barrier from the lower polariton branch of merocyanine to the energy minima on the excited state surface (I), which affects the photoisomerization. Reproduced from ref (166). Copyright 2012 John Wiley and Sons. (e) Absorption as a function of irradiation time of norbornadiene in the strong coupling regime, showing a contraction of the Rabi splitting as norbonadiene is photoswitched into tetracycline. Reproduced from ref (117). Copyright 2021 John Wiley and Sons.

Figure 7

(a) Energy sketch of the dispersive behavior of the polaritons (purple) for the case that the coupling strength between the first exciton (light blue) and the cavity photon (red) is larger than the interaction of the second exciton (dark blue) with the cavity photon. (b) Contribution of the photonic (red), first excitonic (light blue), and second excitonic (dark blue) part to the middle polariton corresponding to ‘a’. (c) Energy sketch of the reverse case where the interaction between second exciton with cavity photon exceeds the interaction between the first exciton and cavity photon. (d) Contribution to the middle polariton for the case corresponding to ‘c’. The figures were obtained from a coupled harmonic oscillator model using three oscillators representing the two excitons and one photon mode. (e) Bulk donor–acceptor film inside an optical microcavity. The film is the blend of two J-aggregates TDBC (yellow) and NK-2707 (red). Here, the TDBC J-aggregate is the donor and the NK-2707 J-aggregate is the acceptor for the energy transfer reaction. Reproduced from ref (179). Copyright 2014 Springer Nature. (f) Energy transfer from the TDBC J-aggregate donor to the BRK J-aggregate acceptor, with the two dyes separated with a 75 nm spacer. Reproduced from ref (180). Copyright 2017 John Wiley and Sons.

Figure 8

Pictorial illustration of (a) singlet fission and (b) triplet triplet annihilation. Here, TT represents the triplet pair, SF singlet fission, ET triplet energy transfer, and TTA triplet–triplet annihilation. The blue arrows represent the excitation event in a typical experiment, green arrows represent nonemissive excited state transfer events, and green dotted arrows represent intermolecular processes. Polaritonic states are shown in purple. The formation of polaritons affects the energy alignment with the triplet pair, resulting in modified kinetics of the SF and TTA processes.

Figure 9

(a) Arrhenius plot with calculated activation energies. Red represent neat film, dark purple represent Cavity 1 (E_T1 < E_LP) and light purple represent Cavity 2 (E_LP < E_T1) of DABNA-2. Reproduced from ref (217). Copyright 2021 Springer Nature. (b) Emission after triplet triplet annihilation in a tetracene crystal on top of an on resonance plasmonic array. Emission is enhanced when the system is on the array. Reproduced from ref (226). Copyright 2019 John Wiley and Sons. (c) The exciton reservoir consists of states of mostly singlet character. However, small contributions of high spin states can result in cavity-mediated triplet triplet annihilation enhancement. Reproduced from ref (227). Copyright 2020 the Royal Society of Chemistry. (d) The rate of sensitized TTA of DPP(PhCl)₂ in the strong couple regime (purple) and in film (red). Reproduced from ref (228). Copyright 2021 American Chemical Society.

Figure 10

Single-emitter Rabi splitting at room temperature. (a) Schematic of a nanometer-thick self-assembled monolayer (SAM) of molecules in the gap of a plasmonic nanocavity with nanoparticle-on-mirror geometry (top left). Scheme of a picocavity with the optical field attracting an adatom to the molecule tip (top right). Simulated energy of picocavities versus reaction coordinate of adatom when molecule tip-adatom separation z decreases by light (solid) and when there is no molecule (dashed), showing a reduced energy barrier for adatom extraction (bottom left). Time-series of surface-enhanced Raman scattering spectra showing the lifetime of a picocavity by new intense vibrational modes appearing (bottom right). Reproduced from ref (333). Copyright 2022 AAAS. (b) Schematic of scanning tunneling microscopy setup with the atomistic protrusion at the silver tip, giving the enhancement of photoluminescence from a molecule under the tip (left). Simultaneously recorded tip-enhanced photoluminescence photon image and scanning tunneling microscope photon image (inset) of a single zinc phthalocyanine molecule on three-monolayer-thick NaCl/Ag(100). Reproduced from ref (334). Copyright 2020 Springer Nature. (c) Ab initio calculations of photoabsorption spectra of Al₁₄₇ dimers with a 10 Å gap coupled to a various number of benzene molecules showing the ultrastrong coupling even for the single molecule (left). The same for Mg₂₀₁ dimers with a single benzene molecule for various dimer gaps (right). Reproduced from ref (337). Copyright 2022 American Chemical Society. (d) Schematic of the interfacial capillary force-assisted method for positioning quantum dots in the gap of bowtie structures (left). Scattering spectra of silver bowtie antennas with one, two, and three CdSe/ZnS quantum dots in the gap shows record values of Rabi splitting (right). Reproduced from ref (339). Copyright 2016 Springer Nature. (e) Schematics of a scanning probe microscopy setup to study quantum dots in the strong coupling regime (left) and slit-like scanning probe combining the near-field spectroscopy technique with the ideas of gap nanoantennas (right). Reproduced from refs (344) and (345). Copyright 2018 AAAS and 2019 AAAS. (f) Schematic (top left) of the self-positioning of quantum dots at the sharper corners near the ends of gold nanorods, where the electric field (bottom left) and Rabi splitting (right) are maximized. Reproduced from ref (346). Copyright 2022 American Chemical Society. (g) Schematic of a single quantum dot in the gap between a silver nanocube and a gold film (right). The location of the quantum dot near the corner of the nanocube, where the field enhancements are the largest, gives a 1900-fold increase of the room-temperature emission (right). Reproduced from ref (348). Copyright 2016 American Chemical Society.

Figure 11

Plexcitonic photochemistry and photophysics. (a) Schematic of the photobleaching reaction of J-aggregates strongly coupled to a silver nanoprism. (b) Photobleaching mechanism of a strongly coupled system and transitions between various states: polaritons provide a fast relaxation from the excited to the ground state, bypassing the long-lived triplet. (c) Schematic of the photobleaching process as a function of plasmon-exciton detuning. The transparency of arrows shows high/low probability of the corresponding transition, demonstrating that red-detuned particles are more stable than blue-detuned ones. (d) The relative change in the concentration of active organic molecules for various plasmon-exciton detuning, compared to the uncoupled bare J-aggregates. (a)-(d) are reproduced from ref (354). Copyright 2018 AAAS. (e) Inverted population of the triplet state showing the optimal balance between the detuning and coupling energy for which the photobleaching suppression is maximized (the stability line). Reproduced from ref (366). Copyright 2020 American Chemical Society. (f) The photodegradation rate of dye molecules vs molecular concentration. The red dots are experimental data obtained from the different samples. The blue and green are full and simplified theoretical curves, respectively. Reproduced from ref (368). Copyright 2020 the American Chemical Society. (g) Coherent ultrafast manipulation of the coupling energy in strongly coupled J-aggregate/metal hybrid nanostructures by controlling the exciton density on a 10 fs time scale. Reproduced from ref (127). Copyright 2013 Springer Nature. (h) The left panel shows a schematic of molecules coupled to a plasmonic nanocavity and sketch of photoisomerization reaction between a trans- and cis-isomer. The right block of the six panels shows a calculation of the photoisomerization suppression under strong coupling for single molecules. The top row in it shows how the formation of polaritons provides new minima in the potential energy surfaces and even a barrier at larger coupling strength. The bottom row shows the corresponding trapping of the nuclear wavepacket, which means that the photochemical reaction is slowed down. Reproduced from ref (170). Copyright 2016 Springer Nature. (i) Coupled-cluster calculation of plexcitonic strong coupling reveals the modification of the ground-state (GS) electronic density caused by the molecule-plasmon interaction. The direction of charge transfer (CT) is along the arrow in the right panel. Reproduced from ref (388). Copyright 2021 the American Chemical Society.

Figure 12

Self-hybridized polaritons in open cavities. (a) Schematic of an hBN slab with phononic self-polaritons (left) and reflectance spectra of an optically thick hBN on a polystyrene substrate. The traces for different thicknesses are offset for the sake of clarity (right). Reproduced from refs,. Copyright 2021 AIP Publishing and 2022 American Chemical Society. (b) Edge view and schematic of a face-centered-cubic crystal consisting of gold nanoparticles (left) and the plasmon-polariton dispersion of it with a large Rabi frequency (Ω_R = 1.4 ω_pl) corresponding to the deep strong coupling regime (right). Reproduced from ref (414). Copyright 2020 Springer Nature. (c) Schematic of a water droplet with vibrational Mie self-polaritons (left) and the splitting of Mie TM-modes for a water sphere depending on its radius (right). Reproduced from ref (61). Copyright 2021 AIP Publishing. (d) Schematic and electron-microscopy images (scale bar 100 nm) of self-assembled nanotubular J-aggregates supporting self-hybridized exciton-polaritons (left) and femtosecond pump spot spatiotemporal dynamics showing the typical scale of enhanced exciton transport (right); here the dotted line shows the radial Gaussian standard deviation (σ). Reproduced from ref (60). Copyright 2021 Springer Nature. (e) Schematic of DBR structure coated with TDBC molecules supporting Bloch surface waves polaritons (left) and gradual expansion of the polariton cloud obtained with pump–probe reflection microscopy. Reproduced from ref (253). Copyright 2023 Springer Nature. (f) Schematic of Bloch surface wave exciton-polaritons in WSe₂ slab on DBR substrate (left) and the polariton propagation length as a function of their wavelength (right). Reproduced from ref (422). Copyright 2022 the American Chemical Society. (g) Schematic of the experimental setup (left) and calculated dispersion color map with blue squares indicating experimental points (right) from the observation of self-hybridized exciton-polaritons in MoSe₂ slabs beyond the light line. Reproduced from ref (424). Copyright 2017 Springer Nature. (h) Schematic of a perovskite CsPbBr₃ nanowire with excitonic self-polaritons (left), optical images in the lasing mode operation of CH₃NH₃PbX₃ [X = I (red), Br (green), Cl (blue)] perovskite nanowires (center), and emission spectra for a subwavelength CsPbBr₃ nanocube (right). Reproduced from refs (61), (408), (425), and (426). Copyright 2021 AIP Publishing, 2018 the American Chemical Society, 2015 Springer Nature, and 2023 Song Jin.

Figure 13

Schematic representation of the energy levels and relaxation processes in (a) a photon laser and (b) a polariton laser.

Figure 14

(a) Threshold curves of polariton lasers for different concentrations of organic molecules. (b) Emission quantum efficiency of the same molecules as a function of the concentration in the polymer matrix (PMMA). Reproduced from ref (18). Copyright 2017 Optica Publishing. (c) Scattering of an organic exciton-polariton fluid by a defect. (d) Superfluidic organic BEC moves friction-less along the defect. Reproduced from ref (447). Copyright 2017 Springer Nature.

Figure 15

Polariton laser threshold in units of fluence (μJ cm^–2) for several organic materials. The data are sorted by the type of organic material and also by increasing fluence. Note that no distinction is made between incident and absorbed fluence, as these two quantities are used in the literature. Single crystal: (1)-, (2)-, (15)-, (19)-; Polymer: (5)-, (13)-; Oligofluorenes: (6)-, (7)-, (8)-, (9)-, (10)-, and (11)-, (14)-, (16)-, (23)-, (24)-; Dyes: (3)-, (4)-, (12)-, (17)-, (18)-, (20)-, (21)-; Proteins: (22)-, (25)-. The red dashed line indicates the value of the threshold below which electric pumping would be able to overcome the losses associated with carrier injection. The black squares around some threshold values group the same organic material with the intention to distinguish it from other compounds in situations where they are very close together in threshold value.