The changes of the polaritonic transition density are presented as a function of the coupling strength A 0 (varied along the horizontal axis of panel d) for the upper and lower polaritons, with a C-O bond length of 1.22 Å and at cavity energy ω c = 7.92 eV. Under this configuration, the cavity is nearly resonant with the molecular adiabatic transition from the ground state to the ψ A 1 1 state at the Franck-Condon points. Through the coupling-dependent mixing of the various electronic transition densities, the polaritonic transition density is modified for each coupling strength, and the results showcase how tuning the cavity parameters can modify the local electronic properties to facilitate chemical reactions or photophysical changes. Fig. shows the excitonic spectra of a (6,5) single-walled carbon nanotube (SWCNT) system inside an optical cavity for cavity frequencies ω c = (e) 2 E 11 , (f) , where E 11 is the lowest energy bright transition in the pristine SWCNT. This system has been the subject of recent exploration by the polaritonic community and has yielded many interesting results. It is well known that pristine SWCNTs are relatively dark to emission due to low-lying optical inactive electronic transitions. Here, the |ψ 0 , 2⟩ photon-dressed ground state is in resonance with the bright |E 11 ⟩ ≡ |ψ 6 , 0⟩ state. At resonance (panel g), the bright character of the E 11 state is split nearly symmetrically as a function of the light-matter coupling strength, A 0 . For negatively detuned cavity (panels e,f), the bright character is blueshifted. For a positively detuned cavity (panel h), the bright character is redshifted to below the manifold of low-lying dark states, effectively brightening the emission of the SWCNT system without the need for chemical functionalization or solvent doping. 56;57