Order of Magnitude Effects of Thiazole Regioisomerism on the Near‐IR Two‐Photon Cross‐Sections of Dipolar Chromophores

Abstract

We have investigated computationally the two‐photon absorption (2PA) properties of donor‐acceptor dipolar chromophores, the conjugated backbones of which contain two five‐membered heterocyclic groups which may be electron‐rich (thiophene‐2,5‐diyl) and/or electron‐deficient (thiazole‐2,5‐diyl). Quantum‐chemical calculations (INDO/MRDCI/S‐tensor and Sum‐Over‐States calculations based on DFT‐optimized geometries) indicate that the two‐photon cross‐sections into the lowest two excited states S1 and S2 can be tuned by more than an order of magnitude by varying the nature, order, and, in the case of thiazole, orientation of the heterocycles. Going from one thiazole regioisomer to the other has the strongest impact on the 2PA spectra and can even invert the ratio between the 2PA cross‐sections of S1 and S2. An essential‐state analysis reveals that different channels dominate 2PA into S1 and S2. The sensitivity of 2PA into S1 towards the orientation of the thiazole ring stems from a local modulation on the thiazole ring of the change in state dipole moment upon excitation to S1, Δµ01, whereas the dominant essential parameter through which the thiazole orientation affects 2PA into S2 is the transition dipole moment between S1 and S2, µ12.

Benzothiazoles with Tunable Electron-Withdrawing Strength and Reverse Polarity: A Route to Triphenylamine-Based Chromophores with Enhanced Two-Photon Absorption

“…The same “ umpolung ” strategy can be also extended to TPA phenomena, as proved by our calculations on a series of dipolar benzothiazole-containing molecules using the quadratic response time-dependent density functional theory (DFT) method − (cf., Tables S1 and S2 in Supporting Information for results obtained by using the Coulomb-attenuated CAM-B3LYP functional in vacuo and in solution, respectively). Similar effects of the positional isomerism on TPA properties have been also theoretically predicted very recently in a family of triazole- and thiazole-based chromophores, respectively . Although the DFT method often provides an improper description of the excited states with pronounced charge-transfer character, this problem is overcome to some extent by the use of long-range corrected functionals such as CAM-B3LYP .…”

Regioisomeric Effects on the Electronic Features of Indenothiophene‐Bridged D–π‐A′–A DSSC Sensitizers

“…The regioisomeric effects upon introducing respectively D and A moieties onto the unsymmetrical IDT core would possibly perform subtle influence on the ICT character and hence the light-harvesting behavior. These two new molecules were designed based on the following merits: 1) the aliphatic n-hexyloxy groups on D impart the final molecules to suppress the charge recombination by blocking the electrolyte diffusion, [26] 2) the integration of two electron-deficient acceptors (i.e., pyrimidine and cyanoacrylic acid) appears to give appropriate energy levels and essential ICT character, 3) the peripheral p-tolyl substitutions on the 4-position of IDT offer the sufficient steric hindrance, and hence inhibit the propensity of forming undesired aggregations on the TiO 2 surface, and 4) the electronic difference on the 2-and 6-position of the IDT ring provides the possibility to implement orientation-matched and -mismatched arrangements [27][28][29] of the IDT linker as connecting to primary electrondonor (D) and electron-acceptor (A'-A) groups, thereby providing the insights of the final ICT character. As depicted in Scheme 2, the matched (A-IDT-D) and mismatched (D-IDT-A) orientations follow the D/A arrangements based on the directions (i.e., along the molecular long axis) of the molecular dipole of the IDT ring and the charge transfer dominated by the electron donor (D) and the electron acceptor (A'-A).…”

Synthesis, Characterization, and Optical Properties of 4H-Pyran-4-ylidene Donor-Based Chromophores: The Relevance of the Location of a Thiophene Ring in the Spacer

“…On the other hand, the nature of the heterocyclic system and its position in the spacer can determine the second-order NLO properties of the molecule because the heterocycle can act as an auxiliary donor or acceptor group . For instance, significantly enhanced NLO responses have been obtained with a pyrrole attached to the donor − and a thiazole ring in the acceptor side. − The thiophene ring is certainly one of the most widely employed heteroaromatic spacers in push–pull systems, and as a π-excessive heterocycle, it should behave as an auxiliary donor when it is closer to the donor. ,, However, several authors have found greater nonlinearities when thiophene is next to the acceptor group, and therefore, the general rule is not always followed. ,− …”