Dynamics of the Higher Lying Excited States of Cyanine Dyes. An Ultrafast Fluorescence Study

Abstract

The electronic relaxation dynamics of the second singlet excited states of several cyanine dyes was studied through the femtosecond fluorescence up-conversion technique. Our interest in these molecules comes from the potential applications of systems with upper excited singlet states with a long lifetime, which can include electron and energy transfer from the higher lying singlets after one- or two-photon absorption. We studied three series of cyanines with 4-quinolyl, 2-quinolyl, or benzothiazolyl type end groups, each with varying sp(2) carbon conjugation lengths in the methinic bridge. The dynamics after electronic excitation to singlet states above the fluorescent state vary significantly as a function of cyanine structure and conjugation length. In particular, for the 4-quinolyl series the cyanine with an intermediate conjugation length (three methinic carbons) has the slowest S2 decays with lifetimes of 5.4 ps in ethanol and 6.6 ps in ethylene glycol. On the other hand, we observed that the 2-quinolyl family has S2 decay times in the subpicosecond range independent of the conjugation length between the end groups. The slowest internal conversion was observed for the benzothiazolyl type cyanine with five methinic carbons, with an S2 lifetime of 17.3 ps in ethanol. For the planar cyanines of this study we observed for the first time a clear systematic trend in the S2 decay times which closely follow the energy gap law. It was also demonstrated that a slow S2 decay is as well observed upon excitation through degenerate two-photon absorption with near-IR pulses. The present study isolates the most important variables for the design of cyanines with long S2 lifetimes.

Synthesis and Photodynamics of Stilbenyl‐Azopyrroles: Two‐Photon Controllable Photoswitching Systems

“…For excited state dynamics for one‐ (400 nm) and two‐photon (800 nm) excitation, we used both femtosecond fluorescence up‐conversion and transient absorption spectroscopy. Both instruments are described in detail elsewhere . Briefly, the setups are based on a regenerative amplified Ti : Sapphire laser centered at 800 nm, with a repetition frequency of 1 kHz, temporal width of 180 fs and 0.8 mJ per pulse.…”

Fluorophore Release from a Polymethinic Photoremovable Protecting Group Through a Nonlinear Optical Process

“…) or TC5 and 4QC3 (see Ref. ), have 2PA cross‐sections between 100 and 2500 GM. The 850 nm wavelength was chosen to eliminate any contribution from single‐photon excitation since it was verified that linear absorption processes can exist at wavelengths below 820 nm due to tail‐absorption of the first transition.…”

“…Also, we recently reported that the second singlet excited state of these systems are atypically long‐lived. This was explained through the energy gap law implying relatively small internal conversion (S 2 →S 1 ) rates of the order of 10 11 s −1 . As stated, the AntOIR system was developed considering the cyanine section as a two‐photon‐absorbing PPG.…”

“…It should be mentioned that even if the dissociation process may not be a direct one (ultrafast in a dissociative potential), the long lifetime of the upper excited states of cyanine dyes (as shown in Figure ) can provide the necessary time for the system to find a dissociative pathway. The yield for the formation of the fluorescent AntOH product was ascertained by considering the pulse intensity (10 11 W cm −2 at the focal volume), the 2PA cross‐section (460 GM at 850 nm), and the 424 nm AntOH fluorescence measured on the irradiated solutions over a period of time.…”

Coherent multidimensional optical spectra measured using incoherent light

“…Although the lifetime of the excited singlet state is on the order of picoseconds in DTP, photoisomers in carbocyanines can live for microseconds or even milliseconds at room temperature in solution 45 . Probing photoisomerization in femtosecond spectroscopy typically requires significant data analysis of weak but long-lived features 50,51 . In contrast, in I (4) 2D ES the powerful unrestricted terms integrate over the long lifetime, and thus lead to strong spectral features, indicating the presence of photochemical species.…”