Structural Disorder as the Origin of Optical Properties and Spectral Dynamics in Squaraine Nano-Aggregates

Abstract

In contrast to regular J-and H-aggregates, thin film squaraine aggregates usually have broad absorption spectra containing both J-and H-like features, which are favorable for organic photovoltaics. Despite being successfully applied in organic photovoltaics for years, a clear interpretation of these optical properties by relating them to specific excited states and an underlying aggregate structure has not been made. In this work, by static and transient absorption spectroscopy on aggregated n-butyl anilino squaraines, we provide evidence that both the red-and blue-shifted peaks can be explained by assuming an ensemble of aggregates with intermolecular dipole−dipole resonance interactions and structural disorder deriving from the four different nearest neighbor alignments�in sharp contrast to previous association of the peaks with intermolecular charge-transfer interactions. In our model, the next-nearest neighbor dipole−dipole interactions may be negative or positive, which leads to the occurrence of J-and Hlike features in the absorption spectrum. Upon femtosecond pulse excitation of the aggregated sample, a transient absorption spectrum deviating from the absorbance spectrum emerges. The deviation finds its origin in the excitation of two-exciton states by the probe pulse. The lifetime of the exciton is confirmed by the band integral dynamics, featuring a single-exponential decay with a lifetime of 205 ps. Our results disclose the aggregated structure and the origin of red-and blue-shifted peaks and explain the absence of photoluminescence in squaraine thin films. Our findings underline the important role of structural disorder of molecular aggregates for photovoltaic applications.

Ultrafast Photoinduced Dynamics of Quaterthiophene-Squaraine Hybrid Aggregates

“…This suggested that these two molecules exhibit an excited-state interaction only in their aggregated state. The SQR molecules exhibited emission at 658 nm in the THF solution (Figure S1, olive), but this emission completely disappeared in the case of SQR aggregates (Figure C, olive) and QTH-SQR aggregates (Figure C, blue), indicating the purely dark lower-energy states in SQR aggregates …”

“…Squaraine (SQR) dyes exhibit several advantageous properties including strong absorption in the visible to NIR region, high molar extinction coefficients, and good photostability compared to traditional cyanine dyes due to their unique rigid and planar structure . In addition, they easily form aggregates via strong intermolecular interactions due to their quadrupolar structure . Therefore, they find potential in photovoltaic and biomedical applications. , They contain a four-membered electron-deficient ring, which acts as an electron acceptor. , …”

“…43 In addition, they easily form aggregates via strong intermolecular interactions due to their quadrupolar structure. 44 Therefore, they find potential in photovoltaic and biomedical applications. 43,45 They contain a four-membered electron-deficient ring, which acts as an electron acceptor.…”

Tuning the HOMO Energy of the Triarylamine Molecules with Orthogonal HOMO and LUMO Using Functional Groups

“…73,74,[78][79][80][81][82][83][84][85] Finally, we mention that the structures are sensitive to the environments, such as presence of other ligands, 86 and aggregation. [87][88][89][90][91][92][93][94][95][96][97] The suitability of these molecules in the application of solar cells and compatibility with the acceptors 84,[98][99][100][101][102][103][104][105][106] for electron transport will need detailed electronic properties, which are discussed below.…”

DFT and TDDFT Studies of Silicon Analogs of Fluorescein Derivatives

“…The influence of the replacement of different elements such as carbon, oxygen and germanium with silicon were also investigated to understand effect of different Furthermore, we also investigated the aggregation effect of the silicon analog of fluorescein since dye aggregation is very commonly observed in many fluorescein and rhodamine derivatives [81][82][83][84][85] and important in organic small molecule materials. [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105] Most dyes can be shown to produce the spectrum of the unassociated dye for sufficiently diluted solutions. The spectral changes observed upon increasing concentrations have been attributed to dye association in the form of either parallel or head-to-tail manner.…”