Fig. shows the excitonic spectra of the system inside the cavity (see Theoretical Details) when the cavity energy is at half-resonance with the bright E 11 transition (i.e., when |ϕ 0 , 2⟩ is in resonance with |E 11 , 0⟩) and so shows weak splitting of the effective upper and lower polaritons which form through coupling of the |ϕ 0 , 2⟩ and |E 11 = ϕ 6 , 0⟩ states. At very large coupling strengths, the lowest-energy excited polaritonic state |Φ 1 ⟩ becomes partially bright due to the mixing with the E 11 state over the large energy difference of nearly 1.0 eV. Fig. -f show the same information but with varied cavity energy: (d) 1.5 eV, (e) 2.0 eV, (f) 2.5 eV. Interestingly, the resonant case (panel e) shows nearly perfect Rabi splitting (denoted as Ω R ) as a function of coupling strength. The weakly negative (panel d) and positive (panel f) detunings showcase a very important result. Here, one can demonstrate that the bright character of the E 11 state can be manipulated by tuning the cavity energy and coupling strength by either (d) blueshifting or (f) red-shifting the bright state. In the ladder, the polaritonic system should be extremely emissive in the infrared telecommunications wavelengths. With low numbers of effective photons, this system could then operate as a source of single photons without the need for covalent functionalization. Until now, chemical functionalization was the only method to brighten the emission of these materials. In this letter, using time-dependent density functional theory (TD-DFT), we have studied ab initio, molecular exciton-polaritons, exploring various ways to analyze excited polaritonic states drawing from well-known excited state analysis methods from the electronic structure community. The community relies on simple methods to compute properties of excited states for high-throughput quantum chemistry, and with these tools, one is able to explore the physics and/or chemistry of any polaritonic system from first principles by focusing first on the complicated and computationally expensive molecular system, followed by the inclusion of the photonic degrees of freedom which can then be exactly diagonalized accounting for the exact electron-photon correlation up to the truncation of the basis size. This is in stark contrast to many recent works focusing on the development of electronic-photonic structure methods. There is strong merit to formulating such an approach as it allows a variational solution to the mixed electron-photon problem -meaning that the electronic and photonic bases are optimized to give the correct eigenstates of the QED Hamiltonian given only a few original adiabtic-Fock states as a starting point. However, there are two main issues at the present time: (I) These electronicphotonic structure methods are not widely available/tested in commercially available software. (II) These methods require treating the electronic and photonic DOFs on the same footing even though the photonic DOFs are much simpler than their electronic counterparts.