An internal charge transfer-dependent solvent effect in V-shaped azacyanines

Impact of Internal Charge Transfer on the Photophysical Properties of Pyridine-Fused Phosphorus-Nitrogen Heterocycles

The phosphaquinolinone scaffold has been previously studied as a modular core for a variety of fluorescent species where use of substituent effects has focused on increasing or decreasing electron density in the core rings. We now report the synthesis and analysis of several pyridine-containing phosphaquinolinone species exhibiting notable linear conjugation from the aryl-substituent to electron-withdrawing pyridyl nitrogen. Varying the nature of the aryl substituent from electron-withdrawing to electron-donating leads to the generation of an internal charge-transfer (ICT) band in the absorbance spectrum, which becomes the dominant absorbance in terms of intensity in the most electron-rich -NMe₂ example. This heterocycle exhibits improved photophysical properties compared to others in the set including high quantum yield and considerably red-shifted emission. The enhanced ICT can be observed in the X-ray data where a rare example of molecule co-planarity is observed. Computational data show increased localization of negative charge on the pyridyl nitrogen as the electron-donating character of the aryl-substituent increases.

Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives

Blends of donors and acceptors have been widely used in bulk-heterojunction solar cells to have exciton formation and charge separation by photoinduced electron transfer (PET). In this work, we have synthesized perylene diimide (PDI)-based materials having different aryl substituents at the bay positions (4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI) to understand the excited-state dynamics of electron transfer. The detailed photophysics was studied using steady-state as well as ultrafast dynamics of the excited states in different solvents. CBZ-N-Ph-PDI showed tremendous effects of the solvent on the electronic properties compared with the other two derivatives. The emission quantum yield of CBZ-N-Ph-PDI decreases drastically in dichloromethane and other polar solvents, indicating strong electron transfer. DFT calculations showed that in CBZ-N-Ph-PDI the HOMO is centered mostly on the N-phenylcarbazole and the LUMO is on the electron-poor PDI moieties. In addition, the energy levels of the HOMO and HOMO-1 in CBZ-N-Ph-PDI are estimated to be identical. The free energy change for charge separation (ΔG_CS) was calculated using electrochemical and photophysical data and found to be negative for CBZ-N-Ph-PDI. The ground- and excited-state dipole moment ratios suggest that the excited state of 4-Pyridyl-PDI (1.90) is less polar than that of 4-Anisyl-PDI (3.67), which provides an idea of the lower possibility of charge separation in 4-Anisyl-PDI and 4-Pyridyl-PDI. Ultrafast photodynamics studies of 4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI showed fast electron transfer only in CBZ-N-Ph-PDI and not in the other PDI derivatives. It was also observed that electron transfer is faster in DCM and THF than in toluene. Ultrafast dynamics studies showed the presence of an equilibrium between electron transfer and decay from the singlet excited state. Ultrafast studies also showed the features of the N-phenylcarbazole cation and PDI anion, further confirming the intramolecular electron transfer in CBZ-N-Ph-PDI.

The Synthesis and Photophysical Properties of Weakly Coupled Diketopyrrolopyrroles

Three centrosymmetric diketopyrrolopyrroles possessing either two 2-(2′-methoxyphenyl)benzothiazole or two 2-(2′-methoxyphenyl)benzoxazolo-thiophene scaffolds were synthesized in a straightforward manner, and their photophysical properties were investigated. Their emission was significantly bathochromically shifted as compared with that of simple DPPs reaching 650 nm. Judging from theoretical calculations performed with time-dependent density functional theory, in all three cases the excited state was localized on the DPP core and there was no significant CT character. Consequently, emission was almost independent of solvents’ polarity. DPPs possessing 2,5-thiophene units vicinal to DPP core play a role in electronic transitions, resulting in bathochromically shifted absorption and emission. Interestingly, as judged from transient absorption dynamics, intersystem crossing was responsible for the deactivation of the excited states of DPPs possessing para linkers but not in the case of dye bearing meta linker.

Novel Fluorescent Azacyanine Compounds: Improved Synthesis and Optical Properties

Benzothiazoles are known to possess a number of biological activities and therefore are considered to be an important scaffold in the design and synthesis of pharmacophores. In this study, an improved synthesis method for novel fluorescent benzothiazole-based cyclic azacyanine (CAC) dyes bearing different electron-donating/withdrawing groups on their scaffold is presented. The improved method enabled us to increase the synthesis yield for the previously reported CACs. More importantly, it allowed us to synthesize new CAC dyes that were not synthesizable with the previously reported method. The synthesized dyes were characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and mass spectrometry and their optical (absorption and fluorescence) properties were investigated. All of the synthesized CACs were found to be displaying strong absorption within the range of 387-407 nm. The spectral shifts observed in the absorption and fluorescence measurements suggested that the spectroscopic and optical properties of CACs can be directly modulated by the nature of the electron-donating/withdrawing substituents. The fluorescence quantum yields (QYs) of the unsubstituted (parent CAC) and substituted CACs were also measured and compared. The fluorescence QY of CACs with electron-donating substituents (methoxy or ethoxy) was found to be at least four times higher than that of the parent CAC with no substitutions.

Synthesis and structural analysis of dimethylaminophenyl-end-capped diketopyrrolopyrrole for highly stable electronic devices with polymeric gate dielectric

The synthesis and structural analysis of DPP(PhNMe₂)₂, a stable diketopyrrolopyrrole derivative end-capped with a strongly electron-donating dimethylaminophenyl moiety is reported and the origin of ambient stability is analyzed in detail.

The impact of interplay between electronic and steric effects on the synthesis and the linear and non-linear optical properties of diketopyrrolopyrrole bearing benzofuran moieties

Benzofuran has been proven to be the versatile substituent for tuning the optics of diketopyrrolopyrroles.