Shortwave infrared polymethine dyes for bioimaging: ultrafast relaxation dynamics and excited-state decay pathways

Excited-state relaxation in two prototypical shortwave infrared (SWIR) polymethine dyes developed for bioimaging, heptamethine chromenylium Chrom7 and flavylium Flav7, is studied by means of femtosecond transient absorption with broadband ultraviolet-to-SWIR probing complemented by steady-state and time-resolved fluorescence and phosphorescence measurements. The relaxation processes of the dyes in dichloromethane are resolved with sub-100 fs temporal resolution using SWIR, near-IR, and visible photoexcitation. Different population members of the ground-state inhomogeneous ensemble are found to equilibrate via skeletal deformation changes with time constants of 90 fs and either 230 fs (Chrom7) and 350 fs (Flav7) followed by slower evolution matching the 1-ps timescale of diffusive solvation dynamics. Molecules excited into high-lying singlet electronic states (Sn) by visible excitation repopulate with time constants of 400 fs (Chrom7) and 450 fs (Flav7) the corresponding first excited singlet S1 states, which decay within several hundreds of picoseconds in dichloromethane and chloroform solvents. Vibrational relaxation in S1 for both Chrom7 and Flav7 in dichloromethane occurs with time constants of 350 and 800 fs for excess of vibrational energy of ∼1000 and 10 000 cm-1 deposited by near-IR and visible excitation, respectively. Two competing non-radiative processes are present in S1: temperature-independent internal conversion, and thermally-activated twisting about a carbon-carbon bond of the conjugated chain, which is substantial at room temperature but essentially nonreactive, producing traces of isomer product. Intersystem crossing in S1, and thus the triplet quantum yield, is minor. The importance of absorption bands from the excited S1 state in applications requiring high-intensity excitation conditions is discussed.

Sensitization of Nd3+ Luminescence by Simultaneous Two-Photon Excitation through a Coordinating Polymethinic Antenna

We have designed and synthesized two new cyaninic Nd³⁺ complexes where the lanthanide emission can be induced from simultaneous two-photon absorption followed by energy migration. These complexes correspond to a molecular design that uses an antenna ligand formed by the functionalization of a heptamethine dye with 5-ol-phenanthroline or 4-phenyl-terpyridine derivatives. These complexes employ the important nonlinear optical properties of symmetric polymethines to sensitize the lanthanide ion. We verified that simultaneous biphotonic excitation indirectly induces the ⁴F_3/2 → ⁴I_11/2 Nd³⁺ emission using femtosecond laser pulses tuned below the first electronic transition of the antenna. The simultaneous two-photon excitation events initially form the nonlinear-active second excited singlet of the polymethine antenna, which rapidly evolves into its first excited singlet. This state in turn induces the formation of the emissive Nd³⁺ states through energy transfer. The role of the first excited singlet of the antenna as the donor state in this process was verified through time resolution of the antenna's fluorescence. These measurements also provided the rates for antenna-lanthanide energy transfer, which indicate that the phenanthroline-type ligand is approximately five times more efficient for energy transfer than the phenyl-terpyridine derivative due to their relative donor-acceptor distances. The simultaneous two-photon excitation of this polymethine antenna allows for high spatial localization of the Nd³⁺excitation events.

Revealing the Photophysical and Photochemical Reaction Processes of Carprofen in Different Solutions via Ultrafast Femtosecond to Nanosecond Transient Absorption

Carprofen (CP), one kind of a nonsteroidal anti-inflammatory drug, exhibits phototoxic side effects in physiology, while its phototoxic mechanism is ambiguous. To uncover CP's photophysical and photochemical reaction processes, femtosecond to nanosecond transient absorption spectroscopies were employed to directly detect excited states and transient intermediates of CP upon UV irradiation in pure acetonitrile (MeCN), MeCN/water 1:1, and acid/alkaline buffer solutions. The transient absorption data together with DFT calculations were integrated to elucidate mechanisms for photochemical reactions of CP in different solutions. The associated photophysical and photochemical reaction pathways are dependent on various solution environments. In a pure MeCN solvent, CP is excited to a singlet state (S₁) and rapidly interacts with the solvent to proceed solvent rearrangement (SR). It then undergoes vibrational cooling (VC) and proceeds intersystem crossing (ISC) to produce the lowest triplet state (³CP). ³CP finally decays to the ground state. While in a MeCN/water 1:1 solution, deprotonated S₁ of CP experiences SR and VC processes, and then it is promoted to a deprotonated triplet state (³CP^-). ³CP^- undergoes the parallel reactions: dechlorination to a phenyl radical (²CP^-) and decarboxylation to a T₁ anion (³CP^-(de-CO₂)). Finally, both intermediates produce the radical anion species ²CP^-(de-CO₂). In a pH = 7.4 (MeCN/PBS 1:1) solution, ³CP^- can be converted into ²CP^-(de-CO₂) more quickly. Interestingly, we found that the dechlorination step can be promoted in an alkaline solution. Phenyl and chlorine radicals produced in an aqueous solution may be the root cause of the drug's harmful side effects on the human body. This may be useful to guide the design of related CP drugs with minimal phototoxicity in the pharmaceutical process.

All-optical modulation and detection using a gain medium in a pulse shaper

We demonstrate all-optical modulation and ultrafast detection using an on-resonance optical gain medium, combined with spectral splitting in a Fourier transform pulse shaper. Multiple spectral channels of one optical beam can be independently modulated in time by another beam, allowing high-rate modulation and multiplexing without requiring ultrafast response from the gain medium. For detection of sub-picosecond signals we demonstrate a method of ultrafast signal detection (temporal imaging with no spatial resolution) that utilizes the spatio-temporal tilt of an optical pulse in a pulse shaper. The proposed methods can find applications in optical information technology and ultrafast imaging.