中央研究院 原子與分子科學研究所

To explore non-adiabatic effects caused by electromagnetic (EM) vacuum fluctuations in molecules, we develop a general theory of internal conversion (IC) in the framework of quantum electrodynamics and propose a new mechanism, “quantum electrodynamic internal conversion” (QED-IC). The theory allows us to compute the rates of the conventional IC and QED-IC processes at the first-principles level. Our simulations manifest that, under experimentally feasible weak light–matter coupling conditions, EM vacuum fluctuations can significantly affect IC rates by an order of magnitude. Moreover, our theory elucidates three key factors in the QED-IC mechanism: the effective mode volume, coupling-weighted normal mode alignment, and molecular rigidity. The theory successfully captures the nucleus–photon interaction in the factor “coupling-weighted normal mode alignment”. In addition, we find that molecular rigidity plays a totally different role in conventional IC versus QED-IC rates. Our study provides applicable design principles for exploiting QED effects on IC processes.

The Huang–Rhys (HR) factor, a dimensionless factor that characterizes electron–phonon (vibronic) coupling, has been extensively employed to investigate a variety of material properties. In the same spirit, we propose a quantity called the polaritonic HR factor to quantitatively describe the effects of (i) light–matter coupling induced by permanent dipoles and (ii) dipole self-energy. The former leads to polaritonic displacements, while the latter is associated with the electronic coupling shift named reorganization dipole self-coupling. In the framework of macroscopic quantum electrodynamics, our theory can evaluate the polaritonic HR factor, reorganization dipole self-coupling, and modified light–matter coupling strength in an arbitrary dielectric environment without free parameters, whose magnitudes are in good agreement with the previous experimental results. We believe that this study provides a useful perspective on understanding and quantifying light–matter interactions in media.

In this study, we develop a theory of multichromophoric excitation energy transfer (MC-EET) in the framework of macroscopic quantum electrodynamics. The theory we present is general for studying the interplay between energy transfer and fluorescence in the presence of arbitrary inhomogeneous, dispersive, and absorbing media. The dynamical equations of MC-EET, including energy-transfer kernels and fluorescence kernels, allow us to describe the combined effects of molecular vibrations and photonic environments on excitation energy transfer. To demonstrate the universality of the MC-EET theory, we show that under specific conditions, the MC-EET theory can be converted to three representative theories. First, under the Markov approximation, we derive an explicit Förster-type expression for plasmon-coupled resonance energy transfer [Hsu et al., J. Phys. Chem. Lett. 8, 2357 (2017)] from the MC-EET theory. In addition, the MC-EET theory also provides a parameter-free formula to estimate transition dipole–dipole interactions mediated by photonic environments. Second, we generalize the theory of multichromophoric Förster resonance energy transfer [Jang et al., Phys. Rev. Lett. 92, 218301 (2004)] to include the effects of retardation and dielectric environments. Third, for molecules weakly coupled with photonic modes, the MC-EET theory recovers the previous main result in Chance–Prock–Silbey classical fluorescence theory [Chance et al., J. Chem. Phys. 60, 2744 (1974)]. This study opens a promising direction for exploring light–matter interactions in multichromophoric systems with possible applications in the exciton migration in metal–organic framework materials and organic photovoltaic devices.

Richard Feynman stated that “The theory behind chemistry is quantum electrodynamics”. However, harnessing quantum-electrodynamic (QED) effects to modify chemical reactions is a grand challenge and currently has only been reported in experiments using cavities due to the limitation of strong light–matter coupling. In this article, we demonstrate that QED effects can significantly enhance the rate of electron transfer (ET) by several orders of magnitude in the absence of cavities, which is implicitly supported by experimental reports. To understand how cavity-free QED effects are involved in ET reactions, we incorporate the effect of infinite one-photon states into Marcus theory, derive an explicit expression for the rate of radiative ET, and develop the concept of “electron transfer overlap”. Moreover, QED effects may lead to a barrier-free ET reaction whose rate is dependent on the energy-gap power law. This study thus provides new insights into fundamental chemical principles, with promising prospects for QED-based chemical reactions.

Our previous study [S. Wang et al., J. Chem. Phys. 153, 184102 (2020)] has shown that in a complex dielectric environment, molecular emission power spectra can be expressed as the product of the lineshape function and the electromagnetic environment factor (EEF). In this work, we focus on EEFs in a vacuum–NaCl–silver system and investigate molecular emission power spectra in the strong exciton–polariton coupling regime. A numerical method based on computational electrodynamics is presented to calculate the EEFs of single-molecule emitters in a dispersive and lossy dielectric environment with arbitrary shapes. The EEFs in the far-field region depend on the detector position, emission frequency, and molecular orientation. We quantitatively analyze the asymptotic behavior of the EFFs in the far-field region and qualitatively provide a physical picture. The concept of EEF should be transferable to other types of spectra in a complex dielectric environment. Finally, our study indicates that molecular emission power spectra cannot be simply interpreted by the lineshape function (quantum dynamics of a molecular emitter), and the effect of the EEFs (photon propagation in a dielectric environment) has to be carefully considered.

We study the emission power spectrum of a molecular emitter with multiple vibrational modes in the framework of macroscopic quantum electrodynamics. The theory we present is general for a molecular spontaneous emission spectrum in the presence of arbitrary inhomogeneous, dispersive, and absorbing media. Moreover, the theory shows that the molecular emission power spectra can be decomposed into the electromagnetic environment factor and lineshape function. In order to demonstrate the validity of the theory, we investigate the lineshape function in two limits. In the incoherent limit (single molecules in a vacuum), the lineshape function exactly corresponds to the Franck–Condon principle. In the coherent limit (single molecules strongly coupled with single polaritons or photons) together with the condition of high vibrational frequency, the lineshape function exhibits a Rabi splitting, the spacing of which is exactly the same as the magnitude of exciton–photon coupling estimated by our previous theory [S. Wang et al., J. Chem. Phys. 151, 014105 (2019)]. Finally, we explore the influence of exciton–photon and electron–phonon interactions on the lineshape function of a single molecule in a cavity. The theory shows that the vibronic structure of the lineshape function does not always disappear as the exciton–photon coupling increases, and it is related to the loss of a dielectric environment.