PREFACE
2.2. Results and Discussion
2.2.3. Photophysical properties
Figure 2.17. TGA plots of compound 3a-d (heating rate of 10 oC/min, Nitrogen atmosphere).
of 19-22 nm was observed for these molecules. Compounds 3a-d showed broad emission band centered around 476 nm with a Stoke’s shift of 27-36 nm (Figure 2.18b). Emission spectra of all bay-annulated perylene tetraesters are also following same trend as in absorption spectra i.e. shows blue shift in comparison to parent tetraester, while emission spectra of parent tetraester 5b and N-annulated perylene tetraester 1b showed slight red shift, while S-annulated perylene tetraester 2b showed slight blue shift in comparison to Se-annulated tetraester 3b. Absorption and emission spectra of all these molecules did not vary with the chain length.17a,15b,28 Solutions of compounds 1a-d exhibit bright green fluorescence under UV light of long wavelength, solutions of compounds 2a-d exhibit sky blue fluorescence under UV light of long wavelength while solutions of compounds 3a-d exhibited weak fluorescence under UV light of long wavelength (Figure 2.19). UV-Vis absorption and emission spectra of bay-annulated perylene tetraesters were taken on diluting the micromolar solution. On dilution of the micromolar solutions the pattern of the absorption and emission spectra did not change much except a reduction in the intensity.
As a representative case we have measured the quantum yield of compound 1b, 2b, 3b and 5b with respect to flourescein. The quantum yields for N- and S-annulated perylene tetraesters 1b and 2b was found to almost unity whereas the quantum yield of compound 3b was very low and found to be 0.07 (Table 2.9).
Figure 2.18. Absorption spectra (a) and emission spectra (b) of compounds 5b, 1b, 2b and 3b in micromolar THF solution.
Fluorescence lifetime and steady state anisotropy measurements were carried out in dilute solutions (3μM solution in dichloromethane). The measured fluorescence lifetime for bay-annulated perylene tetraesters were tabulated in Table 2.5 and found to be lower than perylene (5.2 ns in ethanol solution)29. The decreased fluorescence lifetime may be
(a) (b)
Table 2.5. Photophysical properties of DLCsa in solution.
Table 2.6. Photophysical properties of DLCsa in thin film.
Entry Absorption (nm) Emissionb (nm) Stokes shift nm (nm-1)
1b 416,479 556 140 (6053)
2b 408,471 534 126 (5783)
3b 419,476 552 133 (5750)
5b 497,458 534, 562 37 (1394)
aprepared by the spin-coating of millimolar solution in toluene; bexcited at the absorption maxima.
Figure 2.19. Micromolar solutions and spin coated thin-films of perylene tetraester 5b, N-annulated perylene tetraester 1b, S-annulated perylene tetraester 2b and Se-annulated perylene tetraester 3b.
Entry Absorption (nm)
Emissionb (nm)
Stokes shift nm (cm-1)
λonset
(nm) ΔEg,optc,d τe,f (ns)
Steady state anisotropy
1a 459,434,410 493 34 (1502) 501 2.48 3.61 0.182
1b 459, 434, 410 492 33 (1461) 501 2.48 3.60 0.185
1c 459,434,410 495 36 (1584) 501 2.48 3.62 0.179
1d 459,434,410 493 34 (1502) 504 2.47 3.59 0.178
2a 442, 417, 398 461, 481 19 (933) 465 2.67 3.01 0.024
2b 442, 417, 398 464, 482 22 (1073) 463 2.68 3.05 0.029
2c 442, 417, 398 464, 482 22 (1073) 463 2.68 3.02 0.037
2d 442, 417, 398 462, 481 20 (979) 463 2.68 3.01 0.032
3a 449,422 476 36 (1653) 471 2.64 0.27 0.043
3b 449,422 476 33 (1525) 471 2.64 0.11 0.075
3c 449,422 476 27 (1263) 471 2.64 0.15 0.015
3d 449,422 476 27 (1263) 471 2.64 0.13 0.069
5b 470, 441 489, 517 19 (827) 504 2.46 4.04 0.031
ain micromolar solutions in THF; bthe excitation wavelength λex = absorption maxima; ccalculated from the red edge of the absorption band; din eV; eFluorescence life time; f3 M solution in dichloromethane.
due to higher non-radiative decay. This may be due to the molecular structure and intermolecular interactions. Heteroatoms incorporation into the molecular structure may be another factor, which led to this observation. The steady state anisotropy values for all bay-annulated perylene tetraesters were also found to be comparable to the values reported for perylene tetraester.17g The lower anisotropy values observed could be due to higher rotational diffusion which may occur during the life time of the excited state and displaces the emission dipole of the fluorophore.
Millimolar solutions of compound 1b, 2b and 3b in toluene were used to prepare the thin film on glass slide by spin coating (500-2000 rpm). The absorption spectrum of N-annulated perylene tetraester 1b obtained showed two maxima at 416 nm and 479 nm respectively, while the emission spectrum showed a single maximum at 556 nm and Thin film showed orange luminescence under the UV light of 365 nm wavelength (Figure 2.20, inset). The absorption spectrum of of S-annulated perylene tetraester 2b showed two maxima at 408 nm with a shoulder at 471 nm respectively, while the emission spectrum exhibits an emission maximum at 534 nm and Thin film showed yellowish green luminescence under the UV light of 365 nm wavelength (Figure 2.21, inset). The absorption spectrum of Se-annulated perylene tetraester 3b showed a maximum at 419 nm and a shoulder 476 nm, while the emission spectrum exhibited a maximum at 552 nm and thin film showed very weak luminescence under the UV light of 365 nm wavelength (Figure 2.22, inset). The absorption maxima is hypsochromically shifted with respect to that of monomer (solution state) and the emission spectrum in the solid state was bathochromically shifted for all bay-annulated perylene tetraesters. The observed red shifted emission in the case of thin film is due to the lowered energy level which is resulted from the close overlap of cores in the thin film state.30 Kasha et al., described that molecular aggregation in ground state lead to exciton coupling, which in turns splits the excited state aggregates into two energy levels (Davydov splitting).31 When the molecules are stacked one above the other, which are known as H-aggregates, the transition to the upper level is allowed. A blue shifted absorption band compared to that of the monomer is the characteristic of this phenomenon. Transition to the lower level is allowed when the molecules in aggregates are arranged in a slip stack fashion (J-aggregates). The blue shifted
absorption maxima in the case of thin film of compounds points to the formation of H-aggregates, with the molecules being stacked in a co-facial manner within the
aggregates.31,32 Thus the fluorescence gets quenched due to the aggregation.
Figure 2.20. Absorption (black trace) and emission (red trace) spectra obtained for the spin coated thin film for compound 1b (Inset shows the film under UV light of wavelength 365 nm).
Figure 2.21. Absorption (black trace) and emission (red trace) spectra obtained for the spin coated thin film for compound 2b (Inset shows the film under UV light of wavelength 365 nm).
Figure 2.22. Absorption (black trace) and emission (red trace) spectra obtained for the spin coated thin film for compound 3b (Inset shows the film under UV light of wavelength 365 nm).