Large Inverted Singlet-Triplet Energy Gaps Are Not Always Favorable for Triplet Harvesting: Vibronic Coupling Drives the (Reverse) Intersystem Crossing in Heptazine Derivatives.

Heptazine derivatives are promising dopants for electroluminescent devices. Recent studies raised the question whether heptazines exhibit a small regular or an inverted singlet-triplet (IST) gap. It was argued that the S1 ← T1 reverse intersystem crossing (RISC) is a downhill process in IST emitters and therefore does not require thermal activation, thus enabling efficient harvesting of triplet excitons. Rate constants were not determined in these studies. Modeling the excited-state properties of heptazine proves challenging because fluorescence and intersystem crossing (ISC) are symmetry-forbidden in first order. In this work, we present a comprehensive theoretical study of the photophysics of heptazine and its derivative HAP-3MF. The calculations of electronic excitation energies and vibronic coupling matrix elements have been conducted at the density functional theory/multireference configuration interaction (DFT/MRCI) level of theory. We have employed a finite difference approach to determine nonadiabatic couplings and derivatives of spin-orbit coupling and electric dipole transition matrix elements with respect to normal coordinate displacements. Kinetic constants for fluorescence, phosphorescence, internal conversion (IC), ISC, and RISC have been computed in the framework of a static approach. Radiative S1 ↔ S0 transitions borrow intensity mainly from optically bright E' π → π* states, while S1 ↔ T1 (R)ISC is mediated by E″ states of n → π* character. Test calculations show that IST gaps as large as those reported in the literature are counterproductive and slow down the S1 ← T1 RISC process. Using the adiabatic DFT/MRCI singlet-triplet splitting of -0.02 eV, we find vibronically enhanced ISC and RISC to be fast in the heptazine core compound. Nevertheless, its photo- and electroluminescence quantum yields are predicted to be very low because S1 → S0 IC efficiently quenches the luminescence. In contrast, fluorescence, IC, ISC, and RISC proceed at similar time scales in HAP-3MF.

Internal conversion of singlet and triplet states employing numerical DFT/MRCI derivative couplings: Implementation, tests, and application to xanthone.

We present an efficient implementation of nonadiabatic coupling matrix elements (NACMEs) for density functional theory/multireference configuration interaction (DFT/MRCI) wave functions of singlet and triplet multiplicity and an extension of the Vibes program that allows us to determine rate constants for internal conversion (IC) in addition to intersystem crossing (ISC) nonradiative transitions. Following the suggestion of Plasser et al. [J. Chem. Theory Comput. 12, 1207 (2016)], the derivative couplings are computed as finite differences of wave function overlaps. Several measures have been taken to speed up the calculation of the NACMEs. Schur's determinant complement is employed to build up the determinant of the full matrix of spin-blocked orbital overlaps from precomputed spin factors with fixed orbital occupation. Test calculations on formaldehyde, pyrazine, and xanthone show that the mutual excitation level of the configurations at the reference and displaced geometries can be restricted to 1. In combination with a cutoff parameter of tnorm = 10-8 for the DFT/MRCI wave function expansion, this approximation leads to substantial savings of cpu time without essential loss of precision. With regard to applications, the photoexcitation decay kinetics of xanthone in apolar media and in aqueous solution is in the focus of the present work. The results of our computational study substantiate the conjecture that S1 T2 reverse ISC outcompetes the T2 ↝ T1 IC in aqueous solution, thus explaining the occurrence of delayed fluorescence in addition to prompt fluorescence.

Acridones: Strongly Emissive HIGHrISC Fluorophores.

An acridone derivative (N-methyl-difluoro-acridone, NMA-dF) is characterized with respect to its utility as an emitter in organic light emitting diodes (OLEDs). Using steady-state and time-resolved spectroscopy as well as quantum chemistry, its ability to convert singlet and triplet excitons into light was scrutinized. NMA-dF emits in the deep blue range of the visible spectrum. Its fluorescence emission occurs with quantum yields close to 1 and a radiative rate constant of ≈5 × 108 s-1. So, it processes singlet excitons very efficiently. Using 1,4-dichlorobenzene as a sensitizer, it is shown that NMA-dF also converts triplet excitons into light. With the aid of quantum chemistry, this is related to a reverse intersystem crossing starting from a higher triplet state (HIGHrISC).

Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines.

In an effort to explain the experimentally observed variation of the photocatalytic activity of t Bu 3 P, n Bu 3 P and (MeO) 3 P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine- and phosphite-IC 4 F 9 adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of t Bu 3 P-IC 4 F 9 and (MeO) 3 P-IC 4 F 9 into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in n Bu 3 P-IC 4 F 9 , in particular with regard to the P-I-C bending angle, reduce the Franck-Condon factors and thus the absorption probability compared to t Bu 3 P-IC 4 F 9 . Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO) 3 P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C-I cleavage of perfluorobutane iodide involves S1 ← S0 absorption of the adduct followed by intersystem crossing to the photochemically active T 1 state.

Impact of fluorination on the photophysics of the flavin chromophore: a quantum chemical perspective.

10-Methylisoalloxazine (MIA) and its mono-fluorinated derivatives (6-9F-MIA) were investigated by means of quantum chemistry, looking into the influence of fluorination on fluorescence, absorption and inter-system crossing (ISC). A maximized fluorescence quantum yield (ΦFl) of this chromophore is desirable for application as a potential fluorescence marker in biodiagnostics/photobiological studies. An enhanced triplet quantum yield ΦT on the other hand may open a perspective for photodynamic therapies (PDT) in cancer treatment. Determination of equilibrium geometries was carried out employing (time-dependent) Kohn-Sham density functional theory and electronic properties were obtained using a combined density functional theory and multi-reference configuration interaction (DFT/MRCI) method. In the gas phase, El-Sayed-favored 1(ππ*) [radiolysis arrow - arrow with voltage kink] 3(nπ*)-ISC enables population transfer to the triplet domain on a timescale of 109 s-1, i.e. significantly faster than fluorescence (kFl ≈ 107 s-1). Two different models were applied to investigate the influence of aqueous medium on absorption and relaxation: the implicit solvation model A is the well-established conductor-like screening model (COSMO) and hybrid model B combines quantum mechanical micro-hydration and conductor-like screening. A polar, protic environment leads to a significant blue-shift of the nπ* potentials, slowing down the ISC process to 107-108 s-1, now enabled by vibronic spin-orbit coupling. Simple principles are derived that demonstrate the effect of fluorination at different positions on the spectroscopic properties. These principles can be utilized with respect to multiply fluorinated derivatives and even further substitution to enlarge effects on the population decay and quantum yields.