Linear and Nonlinear Optical Processes Controlling S₂ and S₁ Dual Fluorescence in Cyanine Dyes

Abstract

We report on the changes in the dual fluorescence of two cyanine dyes IR144 and IR140 as a function of viscosity and probe their internal conversion dynamics from S2 to S1 via their dependence on a femtosecond laser pulse chirp. Steady-state and time-resolved measurements performed in methanol, ethanol, propanol, ethylene glycol, and glycerol solutions are presented. Quantum calculations reveal the presence of three excited states responsible for the experimental observations. Above the first excited state, we find an excited state, which we designate as S1′, that relaxes to the S1 minimum, and we find that the S2 state has two stable configurations. Chirp-dependence measurements, aided by numerical simulations, reveal how internal conversion from S2 to S1 depends on solvent viscosity and pulse duration. By combining solvent viscosity, transform-limited pulses, and chirped pulses, we obtain an overall change in the S2/S1 population ratio of a factor of 86 and 55 for IR144 and IR140, respectively. The increase in the S2/S1 ratio is explained by a two-photon transition to a higher excited state. The ability to maximize the population of higher excited states by delaying or bypassing nonradiative relaxation may lead to the increased efficiency of photochemical processes.

“Awakening” of the S2 Emission of Tricarbocyanine Dyes Stimulated by Interaction with Carbon Quantum Dots

“…All spectra were measured immediately after the preparation of solutions except for the cyan curve in (a), which shows the aging effect of the violet curve after ∼1 h. The dotted violet curve in (a) is a superposition of blue and black curves. S 2 absorption band near 440 nm is consistent with a similar effect observed in high-viscosity solvents due to suppression of low-frequency molecular vibrations, as was reported by Laboe et al 23 Suppression of the low-frequency vibrations both in the ground and excited states can be concluded, on one hand, from the reduced QY of the dye anti-Stokes photoluminescence (ASPL) assisted by the hot band absorption (HBA) (Figure 1d), where the low-frequency vibrations of the ground state play a major role in promoting the HBA-assisted ASPL 24 and, on the other hand, from the narrowing S 0 → S 1 absorption band near 840 nm via reduced contribution of the high-energy vibronic tail (Figure 2a). Third, dilution of the composite solution leads to recovery of the initial spectral shape of the absorption of the free dye (Figure S4), which indicates there is a weak physisorption of the dye molecules on the surface of CQD.…”

“…Third, time-resolved PL spectra confirm that the decay kinetics of the emission at ∼510 nm is different from that for the neat CQD and is relevant for this type of dye. 23 Specifically, the emission kinetics of S 2 possesses two decay components (i.e., the fast component of ∼280 ps and a slower decay with a time constant of 1.8 ns; Figure 4 and Table 1). Such biexponential kinetics can be associated with two relaxation channels corresponding to two different configurations of the excited state of carbocyanine dyes as was shown recently by Dantus et al 23 The two configurations give rise to two minima in the S 2 curve, where one of the configurations has a much slower S 2 to S 1 IC and primarily exhibits the S 2 → S 0 emission.…”

“…23 Specifically, the emission kinetics of S 2 possesses two decay components (i.e., the fast component of ∼280 ps and a slower decay with a time constant of 1.8 ns; Figure 4 and Table 1). Such biexponential kinetics can be associated with two relaxation channels corresponding to two different configurations of the excited state of carbocyanine dyes as was shown recently by Dantus et al 23 The two configurations give rise to two minima in the S 2 curve, where one of the configurations has a much slower S 2 to S 1 IC and primarily exhibits the S 2 → S 0 emission. Compared to the S 1 , the relatively high excitation energy leads to significant distortion of both S 2 configurations, which may correspond to twisting of the bulky diphenylamino meso-group, and distortion of polymethine chain itself.…”

Anti-Kasha Fluorescence in Molecular Entities: Central Role of Electron–Vibrational Coupling

“…These are generally very fast, ultimately leading to the population of the lowest vibrational level of the S 1 state with almost the unity of quantum yield. 14 Because IC processes between the manifold of S n are generally very fast, other possible decay processes such as e.g., anti-Kasha S n → S 0 fluorescence and/or S n → T m intersystem crossing (ISC) are not competitive to IC 15 (we note that luminescence or anti-Kasha reactivity from a higher-lying excited state than n,m = 2 is possible but often less likely possible as n,m increases). 14 This is the ultimate reason that Kasha's rule applies for most of the molecular entities.…”

“…14 Because IC processes between the manifold of S n are generally very fast, other possible decay processes such as e.g., anti-Kasha S n → S 0 fluorescence and/or S n → T m intersystem crossing (ISC) are not competitive to IC 15 (we note that luminescence or anti-Kasha reactivity from a higher-lying excited state than n,m = 2 is possible but often less likely possible as n,m increases). 14 This is the ultimate reason that Kasha's rule applies for most of the molecular entities. For instance, in the specific case of azulene, its anti-Kasha fluorescence originates from the very large energy gap between S 2 and S 1 (i.e., > 1.5 eV).…”