Nature of Excited States

Bases stacked rather than hydrogen bonded have also been studied with quantum chemical methods [182, 244-247]. The nature of excited states in these systems has been debated and theoretical calculations are called to decide on the degree of excited state localization or delocalization, as well as the presence and energy of charge transfer states. The experimentally observed hypochromism of DNA compared to its individual bases has been known for decades [248], Accurate quantum chemical calculations are limited in these systems because of their increased size. Many of the reported studies have used TDDFT to calculate excited states of bases stacked with other bases [182, 244, 246, 247], However, one has to be cautious when us-... 

Negres RA, Przhonska OV, Hagan DJ, Van Stryland EW, Bondar MV, Slominsky YL, Kachkovski AD (2001) The nature of excited-state absorption in polymethine and squarylium molecules. IEEE J Sel Top Quantum Electron 7 849-863... 

The second reason for delocalization of energy losses is the collective nature of excited states. This collectivity may exist even for excited electronic states of a single molecule. The simplest example is the excitation of zr-electron states, which are delocalized along the molecule. When a fast electron excites such a molecule, it transfers its energy to the whole ensemble of tt electrons. As a result, the energy absorption is delocalized along the molecule, and the latter can be a long (e.g., a polymer molecule). 

It is meaningless trying to find a relation between the nature of excited states and the ability of complexes to undergo photodimerization, since in the corresponding addition reactions ground-state molecules participate. The formation of coordinatively unsaturated species should occur from the excited states involving the central atom as one of the central atom - axial ligand bonds must be cleaved. 

Compound Nature of excited state Singlet energy/ k mol-1 Triplet ener kj moH... 

Although the general characteristics of organic LEDs can be derived from theoretical description of energy supply modes, excitation mechanisms, and nature of excited states, quantitative properties of specific materials and EL structures can rarely be obtained from one theory alone. For example, the concept of molecular excitons for excited states is well... 

Fig. 4. Nature of excited state of diaza [2.2] spirene from an extended Huckel-calculation...

Whereas the distinction between collective and cooperative effects can appear artificial, it is obvious that, since optical responses are gs properties, their nonadditivity cannot be ascribed to the delocalized nature of excited states. On the other hand, static responses can be calculated from sum-over-state (SOS) expressions involving excited state energies and transition dipole moments [35]. And in fact tlie exciton model has been recently used by several authors to calculate and/or discuss linear and non-linear optical responses of mm [36, 37, 38, 39, 40, 41, 42]. But tlie excitonic model hardly accounts for cooperativity and one may ask if there is any link between collective effects related to the delocalized nature of exciton states and cooperative effects in the gs, related to the self-consistent dependence of tlie local molecular gs on the surrounding molecules. 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

Time-resolved emission spectroscopy has provided valuable information on the nature of excited states in polymers. Two distinct types of excimers have been observed in poly(Y-vinylcarbazole) using picosecond time-resolved fluorescence. The sandwich-type excimer emitting at 420 nm was formed in several nanoseconds, whereas a second excimer emitting at 375 nm was formed immediately after a lOps electron pulse. (Scheme 13). Similar observations were also... 

Excitation into electronic absorption results in intensity enhancement of normal modes of vibration which are coupled to electronic transition by either Franck-Condon or Herzberg-Teller coupling allows for study of chromophoric active sites in biological molecules at low concentrations allows assignment of CT (and in some cases LF) transitions based on nature of excited state distortion can provide information on metal-ligand bonding as described above for vibrational spectroscopy... 

As described above, the molecular mechanics method is based on an empirical (classical) force field. As such, it is very intuitive and relatively easy for a student to comprehend, compared to solving and interpreting Schrddinger s equation. Hence, for routine insight into the (predicted) molecular structure of ground-state molecules, with no unusual valence, molecular mechanics is preferred over quantum mechanics. This is aside from the issue of computational requirements. Of course, molecular mechanics does not provide insight into electron distribution or the nature of excited states of molecules. 

It is interesting to note that the one-dimensional polymer 98, also exhibited the same maximum at that found for 97 (also very broad emission centered at about 420nm) [66]. Steric must certainly play a major role in the choice of nature of excited state in these unsaturated gold species. 

Experimental evidence on excitation transfer derives from different sources. In strongly and weakly coupled systems most of the information has been obtained from absorption spectra and is, thus, rather on the delocalization of excitation. In weakly coupled systems the direct observation of actual excitation transfer is the only source. Depending on the nature of excited states involved, different effects are to be used here, such as fluorescence, phosphorescence, electron-spin resonance and even chemical reactivity. 

It is worth noting that not just the occupied KS orbitals but also the virtual KS orbitals and orbital energies can be used for qualitative interpretation. They surely are not devoid of physical meaning, but they are directly related to excitation energies and to the electronic nature of excited states. This has become evident very recently from the development of time-dependent DFT methods for response properties, in particular excitation energies [30-32]. (see also the results in Ref. [33]). 

Excitons The Nature of Excited States in Conjugated Polymers... 

Gave promise of control of photochemistry [as a result of a more fundamental understanding of the nature of excited states]. 

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