Platt notation

In substituted benzenes, the symmetry is lowered and the transitions into the states that correlate to the B2u and B u states of benzene become allowed by IPA, 2PA, or both. However, when the substituents induce only a weak perturbation on the benzene yr-electron system, the IPA or 2PA spectra of the substituted compounds often closely resemble the spectrum of the unsubstituted parent molecule. Various theoretical models have been developed in an attempt to predict the type of change in the band intensity and characteristics in the 2PA spectra of substituted benzenes and, more generally, of alternant hydrocarbons [34-36]. It was found that the effect of a perturbation is quite different for IP and 2P allowed transitions. In particular, 2P transitions to the state correlated to the benzene B2u state (Lb) are affected more by vibronic coupling than transitions to the state correlated to the benzene Biu state (La, in Platt notation [31,32]). In contrast, inductive perturbations enhance the La band more than the Lb band. The effects of vibronic coupling and inductive substituents are reversed for IP transitions into these states. Experimental... 

Modified Notation.—The Platt notation is applied mainly to aromatic molecules and based on the conceptually simple perimeter model description of electronic excitations (7). Ground states are labeled A, the excited states involved in certain very high intensity transitions are labeled B and the excited states produced in partially forbidden transitions (i.e., those in which selection rules are violated) are labeled L and C. The notation is derived from selection rules appropriate for imaginary monocyclic aromatic systems. States to which transitions are forbidden because of a large change in angular momentum are L states. Transitions to C states are parity forbidden that is, they violate the g g, u u selection rule. In common aromatics other than benzene these selection rules break down and transitions to L and C states occur but at lower intensities relative to B states. 

Kearns points out (16) the Platt notation has the desirable features... 

Naphthalene, Anthracene, and Linear Polyacenes Linear polyacenes possess two low-lying excited states, which in the Platt notation used in spectroscopy, are labeled as La and Lb the former is Biu and the latter is 1 B2u (7,19). The Biu state is a monoionic state, while B2U is a covalent excited state, which as in benzene, is dipole forbidden, but can be accessed by two photon spectroscopy. In naphthalene and anthracene, is the second excited state, but in higher acenes, becomes the first excited state (16,19). We will now show how to construct and conceptualize this excited state. Already at the outset, we encounter here a problem of choice, namely, that the set of covalent Rumer VB structures involves many structures (e.g., 42 in naphthalene), and one has to restrict the VB structure set in order to understand the constitution of these... 

The absorption bands can be correlated to those of benzene (72T5873) the transition to the lowest energy level (band I in Table I) is the equivalent of the Bju - Atg transition of benzene at 256 nm ( Lj, band in Platt notation), and the much greater intensity in chalcogenopyrylium ions must depend on the lower symmetry of these compounds (point group CiJ-This transition in chalcogenopyrylium ions is A, it is allowed in... 

The nature of the electronic states of carbazole and N-alkyl substituted derivatives has been established on the basis of quantum-mechanical calculations as well as by spectroscopic methods involving polarized IR and Raman spectra and polarized absorption and fluorescence emission spectra Four n n electronic transitions to singlet excited states have been predicted and observed in the 350-230 nm region which, in the Platt notation, are designated as - A ( 345 nm), L, <- A ( 295 nm), <- A( 265 nm)and Bt, <- A ( 240 nm) All these bands diplay... 

We (fl) have reported the photophysical processes of a series of model esters of PET, and tentatively assigned the fluorescence and phosphorescence of the aromatic esters as (n, tt ) transitions, respectively. We (9) also performed an extensive study of the photophysical processes available to dimethyl terephthalate (DMT) in order to relate this monomeric species to the PET polymer. In 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (Table I), DMT has three major,absorptions which are according to Platt, s notation 191 nm, A- B, e = 40,620 1 mole" cm"1 244 nm, A-dLaT e = 23,880 1 mole-) cm" 289 nm, A U, e = 1780 1 mole")cm. ... 

Spectroscopists usually use notations based upon the symmetry properties of wave functions to describe excited states. Since many molecules have no special symmetry properties, such devices are not really strictly applicable in general. However, symmetry notation applicable to related molecules of high symmetry is often extended to unsymmetrical systems. The procedure has been treated formally by Platt (7) who introduced the concept of local symmetry. ... 

La and types in Platt s notation. The first state has the excited w-electron distributed on the atoms, i.e-, its nodal planes are across bonds whereas the latter state has its excited ir-electron distributed between atoms with its nodal planes containing the atoms. 

Platt s notation for molecular states is N M lt,pl,c lyStateSymmelry, where N labels the first, second, third, etc., states of the same type, the multiplicity is (25 + 1), A and B states are non-degenerate and degenerate, respectively, and g and u identify symmetric (gerade) and antisymmetric (ungerade) states, respectively. 

The three electronic absorption bands of benzene are listed in Table 3.1, identified by two common systems of notation. Platt developed the perimeter or free-electron model of the absorption bands of polycyclic aromatic hydrocarbons... 

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