Photophysical Properties of Coumarin-152 and Coumarin-481 Dyes:  Unusual Behavior in Nonpolar and in Higher Polarity Solvents

Abstract

Solvent polarity and temperature effect on the photophysical properties of two 1,2-benzopyrone dyes, namely, coumarin-152 (C152) and coumarin-481 (C481), have been investigated using steady-state and time-resolved fluorescence measurements. In nonpolar (NP) solvents (e.g., hexane, cyclohexane, methylcyclohexane, 2-methylpentane, and 3-methylpentane), the Stokes' shifts (Δν̄) and fluorescence lifetimes (τf) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Δν̄ correlates linearly with the solvent polarity function, Δf = {(ε − 1)/(2ε + 1) − (n 2 − 1)/(2n 2 + 1)}, and the slopes of these plots indicate that the fluorescent states for both the dyes in these solvents are of intramolecular charge transfer (ICT) character. The unusually lower Δν̄ and τf values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields (Φf) and τf values of the two dyes also show an unusual reduction in higher polarity solvents (Δf > ∼0.2). Furthermore, unlike in all other solvents, in solvents with Δf > ∼0.2, the Φf and τf values are strongly temperature-dependent. These results indicated the involvement of a new activation-controlled nonradiative deexcitation channel in higher polarity solvents, assigned to the involvement of the nonfluorescent twisted intramolecular charge transfer (TICT) state. Though the activation barrier (ΔE a) for such processes usually decreases with solvent polarity, for the present systems the ΔE a is seen to increase with Δf. This unusual behavior has been rationalized assuming that the ΔE a arises because of the potential energy (PE) crossing of the TICT and ground states rather than that of the ICT and TICT states as is usually the case for most TICT molecules. On increasing Δf, since the highly polar TICT state not only gets better stabilization but also its PE surface becomes more steeper than the ICT state, the ΔE a effectively increases with solvent polarity.

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

“…Thus, the observed breaks in AOT/ACN RMs are definitely due to something other than involvement of the TICT state. This is in a sense also true for AOT/water RMs, because the nonemissive TICT state [49,53] is not expected to influence the spectral results (cf. Figure 2 A, inset), though it can have a direct effect on the f f and t f values.…”

Calculating singlet excited states: Comparison with fast time-resolved infrared spectroscopy of coumarins

“…56 Lower fluorescence quantum yields in highly polar solvents have been attributed to the formation of a separate, low-lying, twisted intramolecular charge transfer (TICT) S 1 state in 7-aminocoumarins containing flexible alkyl-substituted 7-NH 2 groups. 48,[54][55][56][57] This has been supported by TDDFT calculations, 58 and appears to require range-separated functionals for a correct description within TDDFT. 58 Furthermore, the conformational flexibility of these systems may also influence the IR spectra.…”

“…[41][42][43][44][45] This is important for comparing with TRIR data, as the B3LYP functional is also known to reproduce ground state experimental frequencies well using both scaled harmonic 49,50 and anharmonic 2, 51 techniques, while other commonly used functionals, such as PBE0 52 or CAM-B3LYP 25 and other range corrected functionals often predict frequencies of significantly lower quality. 50,53 Coumarins containing an unsubstituted 7-NH 2 group also show higher fluorescent quantum yields in polar solvents than in non-polar solvents, 54,55 attributed to an increase in positive charge on the N-atom and simultaneous flattening of the 7-NH 2 group in polar solvents, leading to suppression of non-radiative decay from the excited state. 56 Lower fluorescence quantum yields in highly polar solvents have been attributed to the formation of a separate, low-lying, twisted intramolecular charge transfer (TICT) S 1 state in 7-aminocoumarins containing flexible alkyl-substituted 7-NH 2 groups.…”

Studies of solution-processed organic light-emitting diodes and their materials

“…Quantum yield measurements were taken of the polymers in dilute solutions of chloroform relative to Coumarin-152 in acetonitrile. [38] PLEDs were fabricated on nominally 20 ohm/sq, 140 nm thick ITO-coated glass substrates (Colorado Concept Coatings). The substrates were first cleaned with a detergent and organic solvents and then treated in a UV/ozone oven to increase the work function of the ITO and hence facilitate hole injection, as described elsewhere.…”