Photophysical Properties of Benzoylgermane and para‐Substituted Derivatives: Substituent Effects on Electronic Transitions

Abstract

In the present study, a selection of basic substitution patterns on benzoyl(trimethyl)germane was investigated using time-dependent density-functional theory (TDDFT) to explore the influence on the stability and on the relative order of the lowest excited electronic states. The theoretical results are in agreement with absorption and fluorescence measurements. We show that electron-withdrawing groups decrease the energetic level of the lowest singlet and triplet state relative to the electron-pushing systems resulting in red-shifted radiative transitions (fluorescence). In the first triplet state electron-withdrawing groups lead to an increased dissociation barrier and a close approach with the singlet ground state before the transition state in the triplet state is reached, favoring radiationless ground-state recovery. The results are also in good agreement with empirical concepts of organic chemistry, therefore providing simple rules for synthetic strategies towards tuning the excited-state properties of benzoylgermanes.

Recent Advances in Germanium‐Based Photoinitiator Chemistry

“…The opposite behavior is observed when electron donating groups are attached. [18,19,25,36] Figure 2s hows the effect of Me and OMe substitution on the absorption behavior of selected tetraacylgermanes. Additionally,t he extinction coefficients increase almostl inearly according to the number of chromophores attached to the germaniumc enter ( Figure 3).…”

“…[46] Thep hotochemical Table 1. l max valuesa nd extinctionc oefficients for the longest wavelength absorption bandso fm onoacylgermanes 2a-d, [19,21] bisacylgermanes 5a-5f, [36] trisacylgermane 7, [25] and tetraacylgermanes 6a-k. [18,24] l max / e[L mol À1 cm À1 ] reactionp athways of acylgermanes in polar and non-polar media have been found to be analogous to those reported for acylsilanes. [43,46,47] Triplet-state a-cleavage of aG e À(CO) bond has been confirmed as the major photochemical reaction pathway of bisacylgermane 5a by Gescheidt and co-workersi n2 013, as evident from time-resolved EPR (TR-EPR, CIDEP) spectroscopy as well as femto-and nanosecond transienta bsorption spectroscopy (see Scheme 1).…”

“…Promoted by their outstanding absorption properties and their ability to efficiently produce reactive radicals (see Scheme 1), acylgermanes have become ap romisingc lass of photoinitiators. [17][18][19] Mono-a nd bisacylgermanes have first been reported as photoinitiators in 2008. [20,21] Since then,s ubstantiale fforts have been madeb ys everalw orkingg roups to elucidatet he photoreactivity and preparation of acylgermanes.…”

“…[20,21] Since then,s ubstantiale fforts have been madeb ys everalw orkingg roups to elucidatet he photoreactivity and preparation of acylgermanes. [18,19,[22][23][24][25][26] This Minireview focuses on the synthetic aspects and the photoreactivity of mono-to tetraacylgermanes as ap romising class of visible-light type IPIs.…”

Tetraacylstannanes as Long‐Wavelength Visible‐Light Photoinitiators with Intriguing Low Toxicity

“…[9] To date, phosphorousbased initiators (mono-a nd bisacylphosphane oxides) are well established, although UV light is required for sufficient photocuring and the parentc ompounds, as well as their photoproducts are in most cases highly toxic. [10][11][12] Thus, germanium-centered PI systems have emerged over the past few years as promising alternatives due to their low toxicity andt he bathochromic shift of their longest-wavelength absorption bands. [6,[13][14][15] However,asignificant drawback of the latter is the low abundance of germanium in the earth crust, resulting in high prices of germanium-based PIs.…”