From Constraint to Control: Molecular Symmetry and Excited-State Decay from Primordial Molecules to Protein Chromophores

Molecular fluorescence—the absorption of a photon followed by re-emission from an excited electronic state—is the outcome of a long physico-chemical evolution, ranging from atomic transitions in the early universe to complex aromatic chromophores. In this Perspective we frame molecular fluorescence as a dissipative process and trace its genealogy: atomic emission during reionization, with the hydrogen Lyman-α line; the formation of the first molecules following recombination; and the synthesis in the circumstellar envelopes of evolved stars of the carbonaceous aromatic systems from which polycyclic aromatic hydrocarbons derive, the first true chromophores in cosmic history. We contrast coherent, parametric optical processes with dissipative, non-parametric photochemistry, identifying thermodynamic irreversibility, vibrational relaxation and the Stokes shift as the distinguishing features of molecular fluorescence. The thread that holds the sequence together is symmetry, in the role of the rule that establishes which decay channels are accessible to an excited state: in the first molecules an inversion center cancels the permanent dipole moment and closes an entire radiative channel; in aromatic chromophores parity and vibronic coupling grade its intensity, while rules of orbital nature govern the non-radiative channels; and in flexible chromophores the breaking of planarity opens access to conical intersections and hence to non-radiative decay. Based on this, we outline experimental and theoretical strategies for characterizing non-radiative channels and excited-state dynamics. We also discuss how these elements bear on the reconstruction of the chemical pathways that lead from interstellar polycyclic aromatic hydrocarbons to the chromophores of terrestrial biochemistry. Here, the symmetry of the molecular framework is joined by the broken symmetry of their chiral environment—a connection that we propose, together with the experiments that would test it.