Locally excited state, conformation

Electronic excitation of either the donor or acceptor of the dyad generates two sets of locally excited states. In principle, the population of the folded set of excited states may be increased but in practice, this likelihood becomes negligible for long chain lengths n > 6) since competing ET and decay processes occur more rapidly than the rate of production of a sandwich conformation from an extended one, which would require rotation about several C-C bonds to achieve. 

Nmr analysis of Ice - Indicated that the meso isomer in alkane solvents at room temperature has mainly (>95Z) a TG chain conformation while at lower temperatures only the TG conformation is present.Nevertheless the time profile of the fluorescence at -50 C can not be analysed as a single exponential but good fits are obtained upon analysis of the locally excited state as a sum of two exponentials. Spectral information clearly indicates that at this temperature no back dissociation of the complex occurs. A substantial shift of the excimer emission maximum, a broadening of the full width at medium height (FWMH) of the excimer emission and the analysis of the fluorescence decay across the excimer band... 

Stationary state measurements (Longworth and Bovey, 1966, Bokobza et al., 1977) as well as measurements of the transient behavior of the excited states (De Schryver et al., 1982a) were reported. Longworth and Bovey (1966) published the uncorrected fluorescence spectra of meso Al and racemic Al at 298 and 77 K. At 77 K the spectra are, within experimental error, identical and resemble the spectrum of toluene at 77 K. At 298 K excimer emission is detected for meso Al and racemic Al. The ratio of the intensity of the excimer emission (Id, at 313 nm) over the intensity of the locally excited state emission (Im, at 283 nm) equals 2.45 for meso Al and 0.74 for racemic Al. The difference in excimer forming capacity is explained based on the NMR data reported above (Bovey et al., 1965). The meso isomer starting from the TG/GT form reaches the TT conformation via an easy bond rotation, leading to a full overlap of the phenyl chromophores. The racemic isomer is predominantly TT (75%) with about 25% of GG present at room temperature. 

In this scheme a direct equilibrium between one locally excited state and one excimer is proposed. The values of 8.4 and 19.2 kJmol" respectively for the activation energy of excimer formation for meso Al and racemic Al support the conclusion that meso Al forms more easily an excimer conformation than racemic Al. No justification was given for the use of the kinetic scheme in view of the different conformations present in racemic Al. 

The time dependent profile of the locally excited state emission of meso B3 in isooctane cannot be described with a two exponential decay function. The analysis is probably complicated by the large spectral overlap of excimer emission and the low quantum yield of locally excited state emission. The decay curves in the excimer region (430 nm) are described as the sum of three exponents. One of the exponents is very short (2.1 ns at 178 K) and has a negative preexponential factor. This complex behaviour of excited meso B3 is understood when ground state conformations are taken into account. The TTP ans a conformation will rapidly upon excitation form a partial overlap excimer, no rotation around a C-naphthyl bond is possible ( to form a full overlap excimer). 

The fluorescence spectrum of racemic A6 in solution at room temperature shows alkyl carbazole emission and an excimer emission band with maximum at 370 nm. When the sample is cooled to 133 K in isopentane (still liquid) the latter emission is almost the sole contributor to the spectrum (De Schryver et al., 1982b). Cooling the sample below 133 K (me-thylcyclohexane/isopentane, 1 1, glass formation) the contribution of the alkyl carbazole emission increases again and becomes the sole contribution to the spectrum at 84 K (Evers et al., 1983). This behaviour is explained by Vandendriessche et al. (1981). The most stable conformation in racemic A6 is the TT conformation. This conformation can form, upon excitation, a partial onverlap excimer via minor rotations of the chain and the chromophores (<20 deg).No real energy barrier is present between the TT locally excited state and the TT excimer state. At room temperature the 88 % TT will form efficiently the TT partial overlap excimer. The 12 % GG do not form the partial overlap excimer nor the full overlap excimer (no indication in fluorescence spectra and decay curves) and emits alkyl carbazole fluorescence. Upon cooling, the GG population decreases and more and more (TT) excimer will be observed. 

The decay curves obtained for meso A6 can be analyzed within scheme I (Table I). This can be understood when one assigns the locally excited state to the TG/GT conformation and the excimer state to the TT conformation. These two conformations are separated through a one rotation process. A full kinetical and thermodynamical analysis is reported for meso-A6 in isooctane (Vandendriessche et al., 1984). 

This interpretation is confirmed by means of the time correlated single photon counting technique at low temperatures, the decay of the locally excited state can be described by a two exponential function. Since the excimer dissociation to the locally excited state is only competitive at temperatures above -20 C, these two decay parameters are to be ascribed to two ground state conformations. From both lifetimes the ratio of their preexponentials and the lifetime of the unquenched chro-mophore all the rate constants present in the kinetic scheme and the ratio of the populations C7/C5 could be calculated. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

Due to the subpicosecond lifetimes of the monomeric bases there is little time for significant geometric rearrangements that might be necessary to form an excimer from an excited state initially localized on a single base. This suggests that excimers in nucleic acid polymers are more properly described as static excimers [30] in which two or more interacting chromophores are already pre-associated at the time of absorption. This implies that excimer states depend sensitively on conformation at the time of excitation, as the results presented here demonstrate. 

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