NIR-Sensitized Cationic and Hybrid Radical/Cationic Polymerization and Crosslinking.

NIR-sensitized cationic polymerization proceeded with good efficiency, as was demonstrated with epoxides, vinyl ether, and oxetane. A heptacyanine functioned as sensitizer while iodonium salt served as coinitiator. The anion adopts a special function in a series selected from fluorinated phosphates (a: [PF6 ]- , b: [PF3 (C2 F5 )3 ]- , c: [PF3 (n-C4 F9 )3 ]- ), aluminates (d: [Al(O-t-C4 F9 )4 ]- , e: [Al(O(C3 F6 )CH3 )4 ]- ), and methide [C(O-SO2 CF3 )3 ]- (f). Vinyl ether showed the best cationic polymerization efficiency followed by oxetanes and oxiranes. DFT calculations provided a rough pattern regarding the electrostatic potential of each anion where d showed a better reactivity than e and b. Formation of interpenetrating polymer networks (IPNs) using trimethylpropane triacrylate and epoxides proceeded in the case of NIR-sensitized polymerization where anion d served as counter ion in the initiator system. No IPN was formed by UV-LED initiation using the same monomers but thioxanthone/iodonium salt as photoinitiator. Exposure was carried out with new NIR-LED devices emitting at either 805 or 870 nm.

NIR-Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free-Radical Photopolymerization.

Cyanines comprising either a benzo[e]- or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR-LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso-position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.

The effects of substituents and solvents on the ground-state π-electronic structure and electronic absorption spectra of a series of model merocyanine dyes and their theoretical interpretation.

Two series of new merocyanine dyes have been synthesised and the dependence of their electronic structure on substituents and solvents has been studied by NMR spectroscopy, by using both the NMR (13)C chemical shifts between adjacent C atoms in the polymethine chain and the (3)J(H,H) coupling constants for trans-vicinal protons. The widely used valence bond (VB) model based on two contributing structures cannot account theoretically for the observed alternating π-electron density in the polymethine chain. In addition, the prediction of zero-π-bond order alternation (or zero-bond length alternation) by this model is also incorrect. However, the results are consistent with the predictions of a qualitative VB model which considers the resonance of a positive charge throughout the whole polymethine chain. Based on this model and the Franck-Condon principle the effect of substituents and solvents on the fine structure of the electronic spectra of these dyes can be explained as vibronic transitions from the vibrational state v = 0 to v', where v is the vibrational quantum number of the totally symmetric C=C valence vibration of the polymethine chain in the electronic ground state and v' is that in the electronic excited state. In contrast, neither the effects of substituents or solvents on the electronic structure of merocyanines and their electronic spectra can be accounted for by the simple two state VB model.

Relationship between the molecular structure of cyanine dyes and the vibrational fine structure of their electronic absorption spectra.

Electronic absorption spectra of symmetrical cyanine dyes show vibronic sub-bands, attributed to the symmetric C-C valence vibration of the polymethine chain in the electronic excited state. Displacements in the equilibrium configuration between electronic ground and excited states of cyanine dyes lead to longer C-C bonds in the excited state. Additionally, in the electronic ground state, a small degree of bond localisation always remains in the chain depending on the different heterocyclic terminal groups. Our investigations suggest that we can use (3)J(H,H) coupling constants in the polymethine chain to characterise the bond localisation within the chain. Based on these values and the Franck-Condon principle, the intensity distribution among the vibrational sub-bands can be explained.