Lowest excited states

A good example is the spectnun of naphthalene. The two lowest excited states have 62 and synnnetries and are allowed for one-photon transitions. A weak transition to one of these is observable in die two-photon spectnun [33], presumably made allowed by vibronic effects. Much stronger two-photon transitions are observable at somewhat higher energies to a and an A state lying quite close to the energies predicted by theory many years earlier [34]. 

Static properties of some molecules ([193,277-280]). More recently, pairs of ci s have been studied [281,282] in greater detail. These studies arose originally in connection with a ci between the l A and 2 A states found earlier in computed potential energy surfaces for C2H in symmetry [278]. Similar ci s appear between the potential surfaces of the two lowest excited states A2 and B2 iit H2S or of 82 and A in Al—H2 within C2v symmetry [283]. A further, closely spaced pair of ci s has also been found between the 3 A and 4 A states of the molecule C2H. Here the separation between the twins varies with the assumed C—C separation, and they can be brought into coincidence at some separation [282]. 

The UV spectra of thiirane 1-oxide and (15,25)-(+)-2-methylthiirane 1-oxide show a broad maximum at about 205 nm (e —23 000). The latter shows a positive Cotton effect at low energy followed by a negative effect at high energy. The lowest excited states of thiirane 1-oxide involve excitations from the two lone pairs of the oxygen atom (79G19). 2,3-Diphenylthiirene 1-oxide and 1,1-dioxide show absorption due to the 1,2-diphenyl-ethylene chromophore. 

CIS=(Rool=n) Specifies which excited state is to be studied (used for geometry optimizations, population analysis, and other single-state procedures). The default is the first (lowest) excited state (n=l). 

The first job step computes the energies of the three lowest excited states. The second job step uses its results to begin the optimization by including the Read option to the CIS keyword, Geom=Check, and Guess=Read (and of course the commands to name and save the checkpoint file). The Freq keyword computes the frequencies at the optimized structure. 

They also noted a strong solvent dependence in the lowest energy band, which corresponds to the lowest excited state of this molecule. This suggests that the dipole moment will change sign as the molecule moves from its ground state to the first excited state. 

For large interchain separations (8 A < R < 30 A), the LCAO coefficients of a given molecular orbital are localized on a single chain, as intuitively expected. The lowest excited state of these dimers results from a destructive interaction of the two intrachain transition dipole moments, whereas a constructive interaction prevails for the second excited stale. This result is fully consistent with the molcc-... 

Regarding the emission properties, AM I/Cl calculations, performed on a cluster containing three stilbene molecules separated by 4 A, show that the main lattice deformations take place on the central unit in the lowest excited state. It is therefore reasonable to assume that the wavefunction of the relaxed electron-hole pair extends at most over three interacting chains. The results further demonstrate that the weak coupling calculated between the ground state and the lowest excited state evolves in a way veiy similar to that reported for cofacial dimers. 

In order to address the possible influence of positional disorder, we have chosen to analyze the way basic operations such as translations and rotations affect the properties calculated for highly symmetric configurations. This approach could provide guidelines to prevent the loss of significant optical coupling between the ground state and the lowest excited state, and hence the quenching of luminescence in the solid state. 

In a regime of strong interaction between the chains no optical coupling between the ground slate and the lowest excited state occurs. The absence of coupling, however, has a different origin. Indeed, below 7 A, the LCAO coefficients start to delocalize over the two chains and the wavefunclions become entirely symmetric below 5 A due to an efficient exchange of electrons between the chains. This delocalization of the wavcfunclion is not taken into account in the molecular exciton model, which therefore becomes unreliable at short chain separations. Analysis of the one-electron structure of the complexes indicates that the... 

The above results indicate that the selcelion rules are relaxed when the geometry modifications taking place upon pholoexcitalion are considered. Although the transition dipole moment between the ground state and the lowest excited state remains small, the luminescence is no longer entirely quenched by the interchain in-... 

Figure 4-13. Evolution with the size of the sexithienyl cluster of the excitation energies from the ground stale to the lowest excited state (open circles), to the high-lying excited slate strongly coupled to the ground state (open squares), and to the lowest charge transfer-excited stale (open triangles). In all cases, only inlralayer interactions have been considered.

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

The lowest excited states in molecular crystals are singlet and triplet excitons [3]. Since it costs coulombic energy to transfer an electron that has been excited optically from the HOMO (highest occupied molecular orbital) to the LUMC)... 

Recently, a symmetry rule for predicting stable molecular shapes has been developed by Pearson Salem and Bartell" . This rule is based on the second-order, or pseudo, Jahn-Teller effect and follows from the earlier work by Bader . According to the symmetry rule, the symmetries of the ground state and the lowest excited state determine which kind of nuclear motion occurs most easily in the ground state of a molecule. Pearson has shown that this approximation is justified in a large variety of inorganic and small organic molecules. 

The symmetries of the lowest excited states listed in Table 1 are nothing but the symmetries to which the most soft second-order bond distortions belong. It is seen that the types of symmetry reduction predicted using the symmetry rule are in complete agreement with those obtained on the basis of the dynamic theory. 

Molecule (symmetry) Hiickel SCF Lowest excited state Symmetry reduction ... 

Azulene (XI) possesses a transannular bond which has the same effect as those of bowtiene (Fig. 3). The splittings of the top filled and bottom empty degenerate orbitals of cyclododecapentaene in this case are half the corresponding splitting in the case of bowtiene, and are not large enough to produce an effective vibronic interaction between the ground and lowest excited states of the resultant azulene molecule. 

If the full molecular symmetry is assumed, the ground states of the cation radical of fulvalene and the anion radical of heptafulvalene are both predicted to be of symmetry by using the semiempirical open-shell SCF MO method The lowest excited states of both radicals are of symmetry and are predicted to be very close to the ground states in the framework of the Hiickel approximation these states are degenerate in both cases (Fig. 4). Therefore, it is expected that in both these radicals the ground state interacts strongly with the lowest excited state through the nuclear deformation of symmetry ( — with the result that the initially assumed molecular... 

On the other hand, in the anion radical of fulvalene and the cation radical of heptafulvalene, the energy gaps between the ground and lowest excited state (which is in both cases doubly degenerate in the Hiickel approximation (Fig. 4)) are predicted to be reasonably large (1.4 and 1.7 eV, respectively), so that these radicals would not suffer a symmetry reduction. 

The orbital arrangement for pentalene shown in Fig. 2 serves to indicate how close the second excited state is to the first excited state when two more electrons are placed in the nonbonding orbital to form the dianion. The very small (E2 — E1) values for fulvalene and hepta-fulvalene are realized from the orbital arrangements shown in Fig. 4 in both molecules the two lowest excited states ( 3 and 211) have the same energy in the Huckel picture. 

The symmetry of the most soft distortion in the lowest excited state is given by the direct product of the symmetry of the first excited state (shown in Table 1) and that of the second excited state (shown in Table 2). These symmetries are b3g(R ) for 1 and VII 2(1 ) for XVII and IV- hi (z) for XXI and XXIII, and fli(z) for XXII. The symmetries of the lowest excited states are then predicted to be Cj, Q, and C2 , respectively. It should be noted that despite the strong vibronic coupling with the second excited state, the first excited state of sesqui-fulvalene (XXII) does not undergo a symmetry reduction. 

Using the same method as described in II.B, Binsch and Heil-bronner have examined the second-order bond distortion in the lowest excited states of nonalternant hydrocarbons (I, IV—VII, X, XI, XIII — XV and XVII), and have shown that, of the molecules examined, only VI and XVII suffer a molecular-symmetry reduction in the lowest... 

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