3.3. Absorption spectra of Dyes
The allowed transitions of singlet-singlet nature were calculated using TD-DFT for D1, D1A, D2, D2A, D3 and D3A via UV-Vis spectra at GGA-PBE, SCC-DFTB and Hybrid-B3LYP levels of theory, as shown in figure 5. In case of GGA-PBE, the main absorption peak of D1 is found at 539 nm (2.3 eV) with oscillator strength of 1.01 and small absorption peak is found at 6391 nm with very small oscillator strength of 0.1. For D1A the main absorption peak is at 639 nm (1.94 eV) with oscillator strength 1.08 and a peak is found at 539 nm with oscillator strength 0.7. For SCC-DFTB calculations, the main absorption peaks for D1 and D1A are found at 652 nm (1.9 eV),516 nm (2.4 eV), 688 nm (1.8 eV) and 590 nm (2.1 eV) having oscillator strengths of 0.68, 0.44, 0.68 and 0.46 respectively. In case of Hybrid-B3LYP, the absorption peaks are found at 495 nm (2.5 eV) and 539 nm (2.3 eV), 619 nm (2.0 eV) having oscillator strengths 0.95, 0.98 and 0.51 respectively for D1 and D1A. The spectra shows a red shift and broadening of peaks with increase in oscillator strength when π-spacer length has increased for D1A in agreement with literature [59, 61-65]. The main absorption peak for D2 is found at 688 nm (1.8 eV) with oscillator strength 0.16 using GGA-PBE whereas two absorption peaks are found at 619 nm (2.0 eV) and 495 nm (2.59 eV) with oscillator strength 0.38 and 0.66 respectively using SCC-DFTB. For D2A, there are two main absorption peaks at 688 nm and 563 nm with oscillator strength 0.27 and 0.24 in case of GGA-PBE where as there are three absorption peaks at 688 nm (1.8 eV), 619 nm (2.0 eV) and 539 nm (2.3 eV) having oscillator strengths 0.67, 0.45 and 0.67 respectively for SCC-DFTB. In case of Hybrid-B3LYP functional, the absorption peaks are found at 413 nm (3.0 eV), 479 nm (2.6 eV) and 476 nm (2.6 eV) with oscillator strengths 0.90, 0.59 and 0.96 respectively for D2 and D2A.The oscillator strength has increased with increase in π-bridge length with small red shifts [63, 65-69].
There exist two absorption peaks for D3 at 774 nm (1.6 eV) and 688 nm (1.8 eV) with oscillator strength of 0.29 and 0.20 respectively. In case of D3A there is only one main absorption peak at 885 nm (1.4 eV) having oscillator strength of 0.45 with GGA-PBE. However, in case of SSC-DFTB, the absorption peaks are found at 688 nm (1.8 eV), 563nm (2.2 eV), 744 nm (1.6 eV), 619 nm (2.0 eV) with excitation respective values of oscillator strength as 0.53, 0.81 and 0.77,0.87 for D3 and D3A. Similarly, for Hybrid-B3LYP absorption peaks are found at 563 nm (2.2 eV), 413 nm (3.0 eV) and 476 nm (2.0 eV) with respective values of oscillator strength as 0.86, 0.90 and 0.85 for D3 and D3A.This is evident from the result that a red shift occurs and oscillator strength also enhanced in absorption spectrum when an length of π-bridge was increased for D3A [65-69].