Increase In Two-photon Absorption Cross-section Research Articles

Responsive mechanism of 2-fluoro-5-nitrobenzoate based two-photon fluorescent probes for H2Sn detection: A theoretical perspective

Two-photon fluorescent probes with large two-photon absorption (TPA) cross sections have shown wide applications in biomedical domain. However, both the species and amounts of high efficient probes are far from meeting the requirements, one main reason is that the relationship between the molecular structures and the responsive mechanisms are not clear and theoretical framework in this field is not perfect. In this work, the photophysical properties including one- and two-photon absorption and emission of three newly synthesized fluorescent probes for hydrogen polysulfide (H2Sn) detection are investigated by density functional theory and time-dependent density functional theory with the polarizable continuum model in different solvents. Results indicate that the enhanced fluorescent intensity and enlarged TPA cross section can be found when the probes reacted with H2Sn. Moreover, the OPA intensity is largest and its fluorescent intensity is largely enhanced when detecting H2Sn for Pro2, this verifies its superior performance in the detection of H2Sn than Pro1 and Pro 3. Furthermore, the inner mechanism for the increase of TPA cross section is revealed, the responsive mechanisms for photo induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) processes are revealed through analyzing the energies and distributions of frontier orbitals. Our calculations provide theoretical perspectives for experimental measurements and could sever as a useful reference for developing advanced probes in biomedical fields.

Influence of laser and material parameters on two photon absorption in Rhodamine B and Rhodamine 6G solutions in MeOH

TPA is known to be a highly sensitive process. Open aperture Z-scan technique has been employed to measure two photon absorption cross section (TPCS) of Rhodamine B and Rhodamine 6G solutions in MeOH. An in depth analysis is carried out to assess the influence of medium as well as laser parameters on TPCS and relative TPA trends. The explored laser parameters are laser energy and pulse width. The varying medium parameters are dye solution concentrations and sample length (cuvette size). True value of TPCS is found to be independent of change in laser energy and pulse width (laser intensity). However, relative TPA is enhanced by increasing pulse energy due to increased transition probability. Shorter pulse width resulted in reduced TPA due to saturation effects. Increase in solution concentration has decreased TPCS due to agglomeration effects. An increase in TPCS with increased sample length is observed owing to an overall increase in TPA absorbers.

π-Extended diketopyrrolopyrrole-porphyrin arrays: one- and two-photon photophysical investigations and theoretical studies.

A complete one- and two-photon spectroscopic and photophysical characterization of three diketopyrrolopyrrole (DPP)-porphyrin conjugates is reported. The increased conjugation introduced by the incremental addition of one, two and four DPP units on the meso porphyrin positions strongly affects the optical properties of the systems. Ground and triplet excited state absorption spectra show a gradual broadening and bathochromic shift and a trend to lower energies is also observed for both fluorescence and phosphorescence emission. Interestingly, the fluorescence quantum yield increases along the series, leading to remarkable NIR emission properties for the larger derivatives. Unlike the model porphyrin, all derivatives exhibit high two-photon absorption activity. An increase in two-photon absorption cross-section in the regions 800-840 nm and 910-930 nm is observed moving from one DPP to two DPP appended units, with a value of the order of 4000 GM at 910 nm for the latter system. The four compounds show high efficiency in generating singlet oxygen, with yields ranging from 0.7 to 0.5, envisaging favourable applications in both one- and two-photon photodynamic therapies. A detailed theoretical exploration of both linear (absorption and emission) and non-linear (two-photon absorption) properties proposes an analysis of the experimental spectra and a comprehensive interpretation of the two-photon activity within the series of compounds.

Oligothiophene Dendrimers as New Building Blocks for Optical Applications

Thiophene branched structures have been proposed as candidates for photon harvesting and electron-hole transporting materials in novel organic light emitting diodes and solar energy conversion devices. To understand the photoinduced processes in a novel thiophene dendrimer system, the excited state dynamics and nonlinear optical properties of 3D oligothiophene dendrimers have been investigated. The key point of this contribution is that we have found that with these thiophene dendrimer systems, the excitation is delocalized over a large number of thiophene units in the dendrimer and there is an ultrafast energy transfer (200-300 fs) to the longest branch of dendrimer, which can be utilized for future optical devices. In terms of nonlinear optics, it was found that a super-linear increase of two-photon absorption cross-section is observed with an increase in thiophene dendrimer generation that can be explained by the increased excitation delocalization. Generation dependent torsional energy redistribution has also been observed, which planarizes the final emissive state on a picosecond time scale.