FROM JABLONSKI TO FEMTOSECONDS: THE EVOLUTION OF CONTEMPORARY MOLECULAR EXCITATION CONCEPTS*

Michael Kasha

Institute of Molecular Biophysics and Department of Chemistry
Florida State University, Tallahassee, Florida 32601-4380

    Alexander Jablonski in his landmark papers on the luminescence spectroscopy of molecular dyes set the stage for a dramatic evolution of the field of molecular photophysics. He stated as a general principle that in the case an ‘energetically isolated’ dye molecule is phosphorescence capable, then in this molecule there must exist at least one metastable level.
    As a first classification, simple molecular excitation defined as a one-photon, one-molecule interaction will be outlined.  Extending the concept of a low-lying metastable state to non-dye organic molecules, with the recognition of the metastable state as the lowest triplet state of a molecule, was the first revolutionary step distinguishing complex molecule spectroscopy from atomic and diatomic spectroscopy.
    Unlike atomic and diatomic electronic systems, in polyatomic molecules relativistic spin-orbital coupling effects are observed in light-atom (low Z-atom) polyatomics as the dominant phenomenon determining excitation pathways, occasioned by the prominence of the radiationless process of internal conversion and intersystem crossing as ultrafast and ubiquitous processes in such molecules.
    The acceptance of the idea of facile excitation of the lowest triplet state led to a great exploration of new molecular state properties: photomagnetism, triplet state lifetimes, transient Tn ¬ T1 absorption, electron spin resonance, triplet state photosensitization, and in addition, the extra spin-orbital perturbation of internal and external high-Z (heavy) atom effects on radiationless transitions, lifetimes of triplet states, induced singlet-triplet absorptions, etc.
    A second major extension came from the full range of orbital characterization in molecules [s , p ; n-, and l- lone pairs (non-bonding, and conjugatable, resp.)], with corresponding orbital classification of state electronic configurations. In the Cartesian framework of molecules, the orbital classification yielded not only symmetry classification of electronic states with consequent radiative selection rules and polarization of transitions, but also the special features of a Cartesian framework which distinguish polyatomic molecules from atomic and diatomic systems: antisymmetric vibrations, which may permit dipole-forbidden transitions to appear (with the exception of polyatomics which may be ‘vibrationally deficient,’ i.e., lacking the antisymmetric vibrational mode required to overcome an electronic restriction on a dipole-allowed transition). Of greater importance is the consequence of the skeletal framework on the effect of the angular momentum operator in spin-orbital coupling. The appropriate Cartesian component of the angular momentum operator acting as a rotation operator has the effect of rotating an in-plane n-orbital to coincide with an out-of-plane p -orbital. This has the consequence of generating non-vanishing one-center spin-orbital matrix elements, with a gain of a factor of 1000 or more on the probability of singlet-triplet mixing for n ® p * excitation (radiative and radiationless), compared with p ® p * excitations. The forgoing is yet another contribution to the dominance of spin-orbital effects on complex molecules consisting of C, N, O, and H atoms. All of the foregoing was securely established by 1960.
    Since 1960, non-simple molecular excitations have dominated the interest of many molecular spectroscopists: one-photon-multi-molecule and many-photon-one-molecule excitations. The principle ones among many new extensions include inter-molecular-unit interactions (coupled molecular units, as well as two-molecule complexes) including charge-transfer states, molecular exciton systems (dimers, trimers, …, helical polymers, molecular sheets, spherical aggregates, etc.), simultaneous transitions (in which two excited molecules emit one photon, or one photon excites two molecules in collision – even if unlike, as evidenced in H2, CO mixtures, singlet-molecular-oxygen spectroscopy, molecular-oxygen-aromatic-molecule collisional-pair interactions, etc.), biphotonic absorption (two-photon, one-molecule absorption) with new selection rules, etc.
    Another class of non-simple excitations involves internal structural and electronic changes, such as excitation-induced internal torsion (EIT), sometimes including twisting intramolecular charge transfer (TICT); and the complex cases of excited state intramolecular proton-transfer (single proton ESIPT or biprotonic ESDPT) tautomerization.
    Jablonski also stated a second general principle: the surrounding embedding-medium molecules and the dissolved dyestuff molecule together form a ‘dyestuff center’ – which then in condensed matter spectroscopy governs the observable phenomena. A remarkable example of this effect is demonstrated by the study of the proton-transfer spectroscopy of the flavonol quercetin.
    Quercetin (3,3? ,4? ,5,7-pentahydroxyflavone) exhibits no luminescence in room temperature solvents and little luminescence in 77K protic glasses (ethanol, glycerol). However on a time-scale of 10 minutes, a time-dependent induction of proton-transfer tautomer fluorescence is observed, together with normal tautomer fluorescence; this is understood as a photophysical induction, in which the solute interacts with the H-bonded solvation envelope in the rigid glass state, the time-dependence arising from the thermal annealing (arising from the vibronic relaxation of the Franck-Condon excitation) of the H-bonded solvent cage structures, permitting a decoupling of the 5-OH carbonyl H-bond, and a proton-transfer at the H-bond of the 3-OH to carbonyl. Thus the
Jablonski model for a solute-solvent-cage molecular excitation center is effected.
    Femtosecond and picosecond spectroscopy have now taken a major rôle in contemporary molecular excitation studies, in which the subtleties of molecular excitation dynamics of the intricate pathways of non-simple excitation modes have taken center stage. The multifarious aspects of radiationless transitions in these cases lead to the sub-picosecond dynamics regime, in numerous variations.
    Jablonski’s key papers of 1935 and 1936 have had a powerful catalytic effect in the intervening six decades on the unfolding of the great panorama of molecular electronic behavior, with its many and diverse branches. Novel experimental developments (such as the evolution of laser technology) and conceptual developments (in the quantum theory of excitation) have accelerated the evolution of contemporary molecular spectroscopy, but we can clearly acknowledge the clean first path cut through the jungle of facts by Jablonski’s concise statements defining elementary molecular photophysics.

(*)This paper is a natural sequel to the previous paper: M. Kasha, Fifty Years of the Jablonski Diagram, Acta Physica Polonica A71, 661-670 (1987).