According to Hund's rule, the lowest triplet state (T 1 ) is lower in energy than the lowest excited singlet state (S 1 ) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet−triplet energy gap (Δ ST ). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calculations that heptazine (1,3,4,6,7,9,9bheptaazaphenalene or tri-s-triazine) exhibits an inverted S 1 /T 1 energy gap (Δ ST ≈ −0.25 eV). This appears to be the first example of a stable closedshell organic molecule exhibiting S 1 /T 1 inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO− LUMO excitation character of the S 1 and T 1 states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S 1 /T 1 inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9bheptaazaphenalene (TAHz), a chemically stable heptazine derivative, in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S 1 lifetimes (of the order of hundreds of nanoseconds in nonaqueous solvents). These features of the heptazine chromophore have profound implications for organic optoelectronics as well as for water-splitting photocatalysis with heptazinebased polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.
a b s t r a c tSmall conjugated molecules are of great interest as promising alternatives to semiconducting polymers in organic photovoltaics (OPV). In this work, we introduce a more accurate assignment of the excited state of a promising squaraine (SQ) targeted for OPV application. From this assignment, we conclude that a mixed population of monomers and aggregates exists in spin-cast SQ:PC 61 BM bulk heterojunction (BHJ) films, where monomers indicate the presence of amorphous regions that could act as traps. Since crystallinity is critically important for efficient charge transport and exciton diffusion in the BHJ, we thermally anneal the as-cast films to reduce the amorphous regions. Our analysis of annealed films demonstrates a delicate trade-off between increased crystallinity and larger domain sizes. Crystallinity improves but often at the expense of larger crystal size, as supported by XRD and TEM study. Therefore, to achieve optimal OPV efficiency, we controlled the tradeoff to improve the crystallinity while maintaining a small, highly mixed BHJ morphology. We thus highlight the importance of chemical compatibility when designing small molecules for use in high efficiency BHJ devices. Significantly, we have connected theoretically validated spectroscopic assignment with the first full study of morphology and domain size control as they affect small molecule OPV active layers.
We demonstrate a completely heavy-atom-free red-to-yellow triplet–triplet annihilation (TTA) photon upconversion system using a thionated squaraine sensitizer, both in fluid solution and in a solid-state composite architecture. Previous works have shown that thionation introduces sulfur nonbonding (n) orbitals that invert the native energy ordering of the ππ*(S1) and nπ*(S2) singlet transitions on the squaraine core, opening a channel for efficient intersystem crossing in the thiosquaraine without relying on the heavy-atom effect, as expected based on El-Sayed’s rule. Our thiosquaraine [2-(4-(dibutylamino)phenyl)-4-(4-(dibutyliminio)cyclohexa-2,5-dien-1-ylidene)-3-thioxocyclobut-1-enethiolate] exhibits an intense red absorption band, no measurable room-temperature fluorescence, and a native triplet lifetime on the order of 20 μs. This triplet is readily quenched (k Q = 1.4 × 109 M–1 s–1) upon sensitizing the triplet excited state of rubrene as a model upconversion emitter. We observe a 0.27 eV anti-Stokes shift, with selective 685 nm excitation of the thiosquaraine resulting in upconverted rubrene fluorescence centered at 570 nm. The system shows an upconversion quantum efficiency of ∼1.5% in deaerated toluene solution. This quantum efficiency is defined based on a maximum 50% quantum efficiency for TTA upconversion. This system exhibits upconversion under filtered (650 nm long-pass) simulated solar illumination and an intensity transition from quadratic to linear optical power dependence at ∼150 W/cm2 under 685 nm laser diode illumination. We also apply this thiosquaraine system to demonstrate red-to-yellow photon upconversion in a solid-state polymer composite, a prerequisite for light-harvesting device integration. In contrast with traditional TTA upconversion photosensitizers incorporating cost-prohibitive precious metals or photolabile arylhalide groups, we present an easily tunable squaraine dye that serves as a promising red-absorbing heavy-atom-free upconversion sensitizer for increased scalability and photostability.
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