Photophysics of H- and J-Aggregates of Indole-Based Squaraines in Solid State

Abstract

The photodynamics of six types of indole-based squaraines (SQs) molecules deposited onto quartz substrates by using the spin-coating method have been studied by means of steady-state and time-resolved techniques. Enhanced scattering signal at the resonance wavelengths together with the broadening and shift of the absorption bands to the red side with respect to those in solution prove the formation of H-and J-aggregates in the thin film samples. Excited state deactivation dynamics were studied by the femtosecond (fs) transient absorption spectroscopy and nanosecond (ns) flash photolysis. Faster kinetics were obtained for the solid state samples as compared to those in solution. Singular value decomposition (SVD) analysis of the fs transient signal proves the presence of monomers and aggregates, enabling the separation of the kinetics for each one. For the SQ monomers, the dynamics of the singlet excited state does not depend on the pump fluence, and we thus propose that the quenching of the excited monomers is due to an energy transfer process to the aggregates. For the SQ aggregates, nonexponential fit of the experimental time profiles along with the power dependence of the transient absorption signal indicates a singlet− singlet annihilation process (γ′ ≈ 3 × 10 −15 cm 3 s −1/2 ). The combination of the long-range Forster-type mechanism and quantum delocalization is found to be the physical process that accounts for the singlet−singlet annihilation. In the flash photolysis experiments, we observed transient signals with a maximum intensity at 710 nm and lifetimes of 30 and 35 ns for SQ 41 and SQ 26, respectively. We have attributed these signals to the cis photoisomers of the aggregates deactivating to the trans photoisomers through the back photoisomerization reaction. These results provide information for a better understanding of the photodynamics of squaraines.

Solar cells sensitized with near-infrared absorbing dye: Problems with sunlight conversion efficiency revealed in ultrafast laser spectroscopy studies

“…When the dye is attached to TiO2, its absorption shows a strong contribution related to aggregates. The absorption of monomers is probably red-shifted with respect to that in solution and has a maximum around 850 nm, while another, shorter wavelength band with absorption at around 750 nm is due to Haggregates [24,25,27]. The amplitude ratio of the longer-wavelength band to the shorterwavelength one depends on the concentration of CDCA: when it changes from 2 mM to 20 mM, the H-aggregate contribution decreases ( Figure 1C).…”

“…the ability of a particular dye to attach to the titania surface), the wavelength dependence is worth noticing. Self-quenching processes, either due to energy transfer between monomers and H-aggregates [25] or due to singlet-singlet annihilation process within the same species [22,24,27], are usually based on the resonance Förster energy transfer mechanism. In this mechanism, the energy transfer rate depends on the spectral overlap integral (between donor emission and acceptor absorption bands), which, in turn, scales with the wavelength as λ 4 .…”

Strategies for optimizing the performance of carbazole thiophene appended unsymmetrical squaraine dyes for dye-sensitized solar cells

“…18 H-aggregation involves a plane to plane 15 alignment that is dominated by molecules having less steric effects. 31,32 More detailed insight into the effect of aggregation on photovoltaic parameters is outside the scope of the current paper and will be reported in detail elsewhere. State of the art squaraine sensitizers reported by Grätzel et al 33 Table 2).…”

Molecular aspects of squaraine dyes aggregation and its influence on spectroscopic properties

“…Squaraine dyes have attracted attention for their unique properties [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A one-dimensional molecule stacking observed in squaraine solutions is of similar nature as that recognized for cyanine or merocyanine dyes.…”