Abstract
Designing photoactive molecules with tailored photophysical properties, particularly charge-transfer (CT) behaviour and excited-state dynamics, is essential for the advancement of a wide range of applications. This thesis investigates the excited-state dynamics of organic donor–acceptor systems and metal compounds exhibiting CT character, featuring diverse electron-donating and electron-withdrawing groups. A combination of spectroscopic methods, supported by density functional theory (DFT) calculations, is employed to characterize and interpret these electronic states.
The first series of organic donor–acceptor systems, featuring a fused dithiophene (dctp) core, was systematically designed to explore excited-state properties. Variations in donor groups (triphenylamine and carbazole) and acceptor units, 2-(3-oxo-2,3-dihydro 1H-inden-1-ylidene)malononitrile (InOCN) and 1H-indene-1,3(2H)-dione (IndO) enabled modulation of CT behavior. Spectroscopic analyses revealed CT absorption bands in the 17,300–15,000 cm−1 range and singlet-state CT emission between 14,700–13,100 cm−1. Additionallly, Resonance Raman spectroscopy (RRS) validated time dependent density functional theory (TD-DFT) predictions, confirming CT band transitions, occurring from the donor-π system to the acceptor unit in 2−((6−(9−ethylcarbazol−3−yl)−4,4−dihexylcyclopenta[2,1 −b : 3,4−b′]dithiophen−2−yl)methylene)indene−1,3−dione (PX134) and 2−(2−((6−(9−ethylcarbazol−3−yl)−4,4−dihexylcyclopenta[2,1− b : 3,4 − b′]dithiophen − 2 − yl)methylene) − 3 − oxo − 2,3 − dihydroinden − 1 − ylidene)malononitrile (PX135) while it proceeds from the donor to the π-acceptor units in ((6−(4−(diphenylamino)phenyl)−4,4−dihexylcyclopenta[2,1−b : 3,4−b′]dithiophen− 2−yl)methylene)indene−1,3−dione (PX137) and 2−(2−((6−(4−(diphenylamino)phenyl)− 4, 4 − dihexylcyclopenta[2,1 − b : 3,4 − b′]dithiophen − 2 − yl)methylene) − 3 − oxo − 2, 3 − dihydroinden − 1 − ylidene)malononitrile (PX138). Solvatochromism was quantified using Lippert–Mataga, McRae, and Weller plots. This indicates dipole moment shifts with minimal solvent-specific interactions. DFT modeling showed high molecular planarity and good agreement with experimental Raman spectra, validating the structures. Malononitrile-based acceptors enhanced the CT character relative to the IndO acceptor. Variable-temperature (VT) emission studies revealed blue-shifted emission with no evidence of aggregation.
Bipyridine ligands substituted at the 4,4′-positions with phenyleneethynylene (PE) units, 4,4′-bis(phenylethynyl)-2,2′-bipyridine (LH), 4,4′-bis((4-nitrophenyl)ethynyl)-2,2′ bipyridine (LNO2 ), 4,4′-bis((4-methoxyphenyl)ethynyl)-2,2′-bipyridine (LOMe), and 4,4′ bis((perfluorophenyl)ethynyl)-2,2′-bipyridine (LPFP) were synthesized via Sonogashira coupling, and their photophysical properties were investigated. The influence of PE substituents on photophysical properties was systematically evaluated. DFT calculations confirmed planar geometries of the ligands. Absorption spectra exhibited distinct π–π∗ transitions. LH, LOMe and LPFP exhibit blue-shifted emission relative to LNO2 . Monoexponential decay in CH2Cl2 confirmed the presence of a single emissive species. VT studies revealed blue-shifted emission and reduced quantum yields with increase in temperature.
The photophysical properties of three new heteroleptic Cu(I) complexes that incorporate bis[2-(diphenylphosphino)phenyl] (POP) and 4,4′-bis(phenylethynyl)-2,2′-bipyridine derivatives bearing electron-donating or electron-withdrawing phenyleneethynylene (PE) substituents were investigated. The derivatives include LH, LOMe, and LPFP. These complexes exhibit distinct absorption maxima at 423–445 nm, corresponding to metal to ligand charge transfer (MLCT) transitions. Spectroscopic techniques and TD-DFT calculations were employed to elucidate their excited-state behavior. TD-DFT predictions reveal that variations in ligand electronics play a key role in tuning the relative MLCT and ILCT energies. In particular, electron-donating LOMe derivatives favour ILCT character, whereas the electron-withdrawing LPFP substituent lead to a predominantly MLCT transition. These trends are further validated by RRS measurements, which show distinct vibrational enhancement patterns consistent with the computed electronic transitions. Transient absorption spectroscopy identifies a singlet MLCT excited state, while emission studies show broad MLCT (S1 → S0) bands. These results demonstrate that altering bipyridine ligand electronics effectively controls the excited-state behaviour of Cu(I) complexes, offering pathways for improved optoelectronic performance.
A related series of Cu(I) complexes incorporating the bulky 4,4′-dimesityl-2,2′ bipyridine (diMESbipy) ligand and the same PE-substituted bipyridines (LH, LOMe, LNO2 , and LPFP) was also examined. Redox behavior indicaties a geometry shifts from tetrahedral to square planar upon oxidation. Absorption spectra showed bands in the range 498–522 nm, assigned to MLCT transitions. TD-DFT predicts both MLCT and ILCT transitions in these complexes, and the energies of the charge-transfer states can be effectively tuned through electron-withdrawing or electron-donating sub stituents. These trends are further corroborated by RRS measurements, which rein force the substituent-dependent modulation of CT behaviour.Transient absorption spec troscopy shows that electron-donating substituents tend to prolong the excited-state life time, whereas electron-withdrawing groups shorten it. This demonstrates that ligand electronic effects play a key role in tuning excited-state dynamics and photophysical behavior
Finally, the photophysical properties of heteroleptic Re(I) fac-tricarbonyl complexes featuring ancillary ligands bromide (Br–) and 4-(dimethylamino)pyridine (DMAP), combined with 4,4′-bis(phenylethynyl)-2,2′-bipyridine derivatives (LH, LOMe, LPFP) bearing electronically tuned PE substituents, were systematically investigated. Based on spectroscopic and TD-DFT results, the ligand electronics modulate both ground and excited-state properties. Absorption spectra showed broad MLCT bands (422–436 nm), attributed to MLCT transitions. Electron-withdrawing substituents drive MLCT transitions to ward longer wavelengths, while electron-donating substituents shift them to shorter wave lengths. TD-DFT analysis confirms the presence of MLCT transitions, with substitution of Br− by the stronger σ-donor DMAP producing a noticeable blue shift in the transition energy. Furthermore, the complex [Re(CO)3(LOMe)(DMAP)]+, which incorporates both DMAP and the electron-donating LOMe group, displays ILCT character in contrast to the other Re complexes. This behaviour is further corroborated by RRS measurements, which support the TD-DFT predictions and underscore the key role of ligand substitution in modulating MLCT and ILCT energetics. Transient absorption spectroscopy indicates ligand-dependent shifts in GSB and ESA bands. VT results indicate that Br− complexes show greater loss in emission intensity and quantum yield, suggesting greater structural distortion, attributable to the halide effect and enhanced XLCT character. DMAP substituted complexes remain spectrally stable, show a smaller decrease in emission intensity, reflecting excited-state rigidity and suppressed nonradiative decay. σ-donor DMAP thus stabilizes the MLCT state and increases lifetime.