Femto to millisecond observations of indole-based squaraine molecules photodynamics in solution

Abstract

We present femto-to-millisecond studies of the photodynamics of seven types of indole-based squaraine molecules (SQs) in solvents of different H-bonding ability and viscosity. These SQs can be classified into two families: SQs with two carboxylic groups in the side indole groups (symmetrical SQs) and with only one carboxylic group (asymmetrical SQs). Steady-state absorption and fluorescence techniques show narrow absorption and emission bands, with a small Stokes shift (about 300 cm(-1)). The femtosecond transient absorption spectra give a very short (∼100 fs) dynamics (assigned to IVR) and the associated spectra show two excited species assigned to two stereoisomers. A trans-cis photoisomerization occurs in a very fast time through a conical intersection. Pico-to-nanosecond emission experiments also reveal the presence of two fluorescing trans stereoisomers whose lifetimes show similar sensitivities to the nature of solvent. For example, lifetimes of 1.72, 0.46 and 0.29 ns were determined for the trans photoisomer of the SQ 41 in triacetin, dichloromethane and acetonitrile, respectively, reflecting the short decay of the S(1) state in highly polar and low viscous solvents. Flash photolysis experiments gave the transient absorption signals of the cis photoisomer that is formed after the twisting process at S(1). The cis-to-trans photoisomerization at the ground state happens in the μs time scale (1-4 μs), and it depends on the H-bonding ability and viscosity of the solvent. Thus, combining fs-ns and ns-μs experiments suggests that in the conical intersection region, only a small fraction of the twisted trans isomers are converted to the cis ones in the excited states. These results bring detailed and global insight into the large time window photodynamics of this family of SQs in solution.

Excited state photophysics of squaraine dyes for photovoltaic applications: an alternative deactivation scenario

“…The short time-constant τ 1 observed for VG1-C8 cannot be compatible neither with the rotational relaxation time (≈ 125 ps) nor with a fast intramolecular vibrational energy redistribution mechanism (IVR ≈ 70 fs) found by de Miguel et al. 34 Here we note that the rate of such component is weakly decreased upon increase of viscosity (from 9.8 ps, weight = 50% to 11 ps, weight = 40%), implying that this deactivation route does not require the motion of sterically demanding groups, i.e.…”

“…24 In fact, in this class of devices the effectiveness of charge separation, diffusion and collection processes depends sensibly on the degree of order [25][26][27][28][29][30] and conformational properties of the active materials. [31][32][33] More specifically, it has been proved that SQ dyes can undergo cis-trans photoisomerization via a twisted intramolecular charge transfer state (TICT) 34,35,27,28 , a process that provides a non-radiative deactivation mechanism of the SQs excited state, hence hindering a fast and quantitative electron injection into the semiconductor oxide in DSCs. 35,36,28,29 Therefore, the inhibition of this energy-loss mechanism would enhance the efficiency of such class of devices, as it has been observed for stilbene sensitizers exhibiting a trans-cis photoisomerization pattern.…”

Near-infrared emitting single squaraine dye aggregates with large Stokes shifts

“…3 The small Stokes shift of VG1-C8 (9 nm) is typical for symmetric squaraine derivatives, and it is indicative of comparable molecular configuration between the ground and excited electronic states of the molecule. 27,28 On the other hand, we noticed the spontaneous formation of shiny aggregates upon increase of the concentration to 10 mg/mL (14.03 mM) in ethanol (fig. 1c), with sizes ranging from 20 µm to 80 µm as measured by optical microscopy (fig.…”

Far‐Red‐ to NIR‐Emitting Adamantyl‐Functionalized Squaraine Dye: J‐Aggregation, Dissociation, and Cell Imaging

“…Typical assignments of aggregation state are based on aggregation‐induced spectral shifts in absorption spectra, where a red‐shift is characteristic of J‐aggregate and a blue‐shift is characteristic of H‐aggregate, as originally described by Kasha . Aggregation of squaraine compounds is significantly less efficient in neat organic solvents as compared to in aqueous/organic solvent mixtures or aqueous media . No observation of aggregates for AM‐1′ was found even in polar solvents such as MeOH and dimethyl sulfoxide (DMSO).…”