Electron-withdrawing Strength Research Articles

Why Does the Optimal Tuning Method of the Range Separation Parameter of a Long-Range Corrected Density Functional Fail in Intramolecular Charge Transfer Excitation Calculations?

We performed intra- and intermolecular charge transfer (CT) excitation energy calculations of (a) conjugated carbon chain [H2N-(CH=CH)n-X] and (b) its equidistant H2NH∙∙∙HX (n = 2~8) with various electron acceptors (X = NH2, OH, Cl, CHO, CN, and NO2) using EOM-CCSD, time-dependent (TD) Hartree-Fock (HF) and various density functional theory (DFT) functionals, such as BLYP, B3LYP, long-range corrected (LC) DFT, and LC-DFT with an optimally tuned (OT) range separation parameter (µ) using Koopman's theorem to investigate the effect of the electron-withdrawing (or -donating) strength of end-capped functional group (X) and CT distance (R) on intra- and intermolecular CT excitation energies. As the electron-withdrawing strength of X increases, both intra- and intermolecular CT excitation energies tend to decrease, since energy gaps between orbitals corresponding to CT excitations (e.g., HOMO and LUMO) decrease. However, the effect of the electron-withdrawing group on intramolecular CT excitation energy is negligible (at most 0.5 eV). OT-LC-DFT shows accurate intermolecular CT excitation energy, but worse results in intramolecular CT excitation energy than LC-DFT with the default µ value (0.47). Therefore, we conclude that the optimal tuning method is not effective in predicting intramolecular CT excitation energy. While intermolecular CT excitation energy has excitonic binding energy with asymptotic behavior to CT distance that is not affected by the choice of range separation parameter, intramolecular CT excitation energy is affected by orbital relaxation energy, which strongly depends on the choice of range separation parameter, which makes the OT method of range separation parameter ineffective in predicting intramolecular CT excitation energy as well as local excitation with high accuracy.

Rational Monomer Design for the Synthesis of Conjugated Polymers by Direct Heteroarylation Polymerization.

This study focuses on the design concepts that contribute to the C-H activation in bithiophene-flanked monomers incorporating naphthalene diimide (NDI), perylene diimide (PDI), and fluorene (FLU) and their polymerization by direct heteroarylation. Density functional theory (DFT) calculations reveal distinct energy requirements for C-H bond abstraction, which is dictated by the electron-withdrawing strength of the central aromatic core flanked by bithiophene. These provide insights into the reactivity of each monomer for C-H bond activation. Proton NMR spectroscopic experimental results confirm the favorable energetic profiles predicted by DFT, with NDI- and PDI-flanked monomers exhibiting lower energy requirements than fluorene-flanked monomers. Successful polymer synthesis is demonstrated for NDI and PDI, while the fluorene-flanked monomer shows challenges due to its higher energy demands.

Ring-chain tautomerism of vicinal hydroxyimines bearing sulfolane moiety

Cyclic sulfones and their derivatives have vast applications in biological, pharmaceutical, medicinal and in many other fields. Herein, we report synthesis of ten new hydroxyimines bearing sulfolane moiety by reaction of cis- and trans-amino alcohols with aromatic aldehydes. An extensive spectroscopic characterization of the products, using 1D and 2D NMR techniques (1H, 13C, NOE, NOESY, COSY, HSQC, HMBC) showed ring-chain tautomerism in deuterochloroform solution for cis-hydroxyimines. The content of the cyclic (1,3-oxazolidine) form increases with increasing electron-withdrawing strength of the substituent in the para position of the benzene ring of the imine. The structure of cis-(3S,4R)-3-hydroxy-4-((E)-(2-hydroxybenzylidene)amino)tetrahydrothiophene 1,1-dioxide was confirmed using XRD analysis. In addition, ADMET properties of products were evaluated in silico and showed high drug similarity.

Effects of Electron-Withdrawing Strengths of the Substituents on the Properties of 4-(Carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazole Derivatives as Blue Emitters for Doping-Free Electroluminescence Devices.

The synthesis of four 4-(carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazoles with extra groups ((3-methyl)-phenyl-, 4-fluorophenyl-, quinolinyl-, or (3-trifluoromethyl)-phenyl-) in the acceptor fragment has been reported. The effects of substituents with different electron-withdrawing strengths on the thermal, electrochemical, photophysical, and electroluminescence properties of the synthesized compounds are discussed. The results of X-ray analyses and density functional theory (DFT) calculations support unusual molecular packing and electronic properties. The compounds are capable of glass formation with glass transition temperatures ranging from 54-84 °C. Ionization potentials of the compounds are in the range of 5.98-6.22 eV and electron affinities range from 3.09 to 3.35 eV. Under ultraviolet excitation, the neat films of the compounds exhibit blue emission with photoluminescence quantum yields ranging from 18 to 27%. The films of selected compounds are used for the preparation of host-free light-emitting layers of organic light-emitting diodes with very simple device structures and an external quantum efficiency of 4.6%.

The full spectrum tuning of fluorescent molecules via a one-pot multicomponent reaction

Fluorogenic probes for imaging enable visualization and analysis of difficult-to-reach cells and organelles. However, there are limited efficient examples of tuning these fluorescent molecules to higher wavelengths. This is vital since different tissues are sensitive to varying wavelength emissions. To address this need, we report the discovery, tuning, structure-photophysical property relationships (SPPR), and time-dependent DFT (TD-DFT) computations of 400–700+ nm fluorescent pyrido[2′,1′:2,3]imidazo[4,5-c]isoquinolines and substituted imidazo[1,2-a]pyridin-3-amines. The syntheses involve the trimethylsilylcyanide (TMSCN) modified Groebke-Blackburn-Bienaymé (GBB) multicomponent reaction as well as the TMSCN modified GBB combined with subsequent condensation of an aldehyde, and Aza-Friedel-Crafts-Intramolecular Cyclization-Oxidation all in one pot. The SPPR reveals that electron-withdrawing strength in the para-position of the aminopyridine starting material has direct control over the absorption and fluorescence emission wavelengths of these molecules. The TD-DFT computations show the changes in the natural transition orbitals (NTOs) with differing substitutions to the parent molecule that dictate the observed excitations, emissions, and fluorescence intensities. These findings give insights and directions for tuning the fluorescent properties of these motifs for various uses as probes and imaging agents.

Extremely Electron-Withdrawing Lewis-Paired CN Groups for Organic p-Dopants.

P-type chemical doping (p-doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p-dopants have insufficient electron-withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron-withdrawing Lewis-paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p-dopants with excellent doping stability. Various Lewis acids are paired with CN-functionalized conjugated molecules in the solution state, which strengthens the electron-withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long-term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

Precise Electron-Withdrawing Strength Modulation of ESIPT Probes for Ultrasensitive and Specific Fluorescence Sensing.

The precise regulation of the electron-withdrawing/electron-donating strength in a probe is of great significance for the design of reaction-based fluorescent probes with specific functionalities. Here, a family of excited-state intramolecular proton transfer (ESIPT)-based probes with fluorescence turn-on sensing properties toward KMnO4 was designed by precisely modulating the electron-withdrawing strength of the substituents located at the para-position of the recognition group. It is found that -F, -CHO, and -H as the electron-withdrawing groups bound at the probe can specifically recognize KMnO4, which ensures a blue emission displayed by the reaction products. Especially with -CHO as the electron-withdrawing group, the reaction product shows the most stable fluorescence. The probe 2-(benzo[d]oxazol-2-yl)-4-formylphenyl acrylate (BOPA-CHO) demonstrated a more superior sensing performance toward KMnO4, including a low limit of detection (LOD, 0.96 nM), a rapid response (<3 s), and a rather good selectivity even in the presence of 21 interferents. Moreover, the practicality of the probe was further verified by a test pen comprising a BOPA-CHO-embedded sponge, which is capable of detecting KMnO4 solid with a naked-eye LOD of 11.62 ng. The present probe design and modulation strategy would open up a new path for the design of high-performance fluorescent probes.

Molecular engineering of fluorescence probe for real-time non-destructive visual screening of meat freshness.

Spoiled meat poses a great challenge to food security and human health, which should be addressed by the early monitoring and warning of the meat freshness. We herein exploited a molecular engineering strategy to construct a set of fluorescence probes (PTPY, PTAC, and PTCN) with phenothiazine as fluorophore and cyanovinyl as recognition site for the facile and efficient monitoring of meat freshness. These probes produce an obvious fluorescence color transition from dark red to bright cyan in response to cadaverine (Cad) through the nucleophilic addition/elimination reaction. The sensing performances were elaborately improved to achieve quick response (16s), low detection limit (LOD=3.9nM), and high contrast fluorescence color change by enhancing the electron-withdrawing strength of cyanovinyl moiety. Furthermore, PTCN test strips were fabricated for portable and naked-eye detection of Cad vapor with fluorescence color change from crimson to cyan, and accurate determination of Cad vapor level with RGB color (red, green, blue) mode analysis. The test strips were employed to detect the freshness of real beef samples, and demonstrated a good capability of non-destructive, non-contact and visual screening meat freshness on site.

N-Type Doping of Naphthalenediimide-Based Random Donor–Acceptor Copolymers to Enhance Transistor Performance and Structural Crystallinity

An integrated strategy of molecular design and conjugated polymer doping is proposed to improve the electronic characteristics for organic field effect transistor (OFET) applications. Here, a series of soluble naphthalene diimide (NDI)-based random donor-acceptor copolymers with selenophene π-conjugated linkers and four acceptors with different electron-withdrawing strengths (named as rNDI-N/S/NN/SS) are synthesized, characterized, and used for OFETs. N-type doping of NDI-based random copolymers using (12a,18a)-5,6,12,12a,13,18,18a,19-octahydro-5,6-dimethyl-13,18[1',2']-benzenobisbenzimidazo[1,2-b:2',1'-d]benzo[i][2.5]benzodiazocine potassium triflate adduct (DMBI-BDZC) is successfully demonstrated. The undoped rNDI-N, rNDI-NN, and rNDI-SS samples exhibit ambipolar charge transport, while rNDI-S presents only a unipolar n-type characteristic. Doping with DMBI-BDZC significantly modulates the performance of rNDI-N/S OFETs, with a 3- to 6-fold increase in electron mobility (μe) for 1 wt % doped device due to simultaneous trap mitigation, lower contact resistance (RC), and activation energy (EA), and enhanced crystallinity and edge-on orientation for charge transport. However, the doping of intrinsic pro-quinoidal rNDI-NN/SS films exhibits unchanged or even reduced device performance. These findings allow us to manipulate the energy levels by developing conjugated copolymers based on various acceptors and quinoids and to optimize the dopant-polymer semiconductor interactions and their impacts on the film morphology and molecular orientation for enhanced charge transport.

Terminal Groups of Nonfullerene Acceptors: Design and Application

Materials design plays a critical role in improving the power conversion efficiency (PCEs) of organic solar cells (OSCs). Recent advances in nonfullerene acceptors (NFAs) have achieved great success and made a large contribution to the rapid increase in PCEs. The current state-of-the-art OSCs have PCEs of >19%, demonstrating their great potential for use in practical applications. All high-efficiency NFAs adopt an A–D–A or A–DA′D–A (A = acceptor and D = donor) structure. Modulating the electron push–pull effect using an alternating D–A structure has proven to be an effective molecular design method. Specially, for NFAs, the design and application of terminal groups are of great significance. In this Perspective, a brief introduction is given to the development of terminal groups and representative materials. We highlight the critical role of the molecular electrostatic potential in evaluating the electron-withdrawing strength. The applications of terminal groups in designing wide-, middle-, and low-bandgap NFAs are discussed. Then, an outline of the other functions of terminal groups in affecting the intermolecular packing, blend morphology, and energy loss is presented. Furthermore, insights into the key issues that should be considered when developing new terminal groups, including efficiency, cost, and stability, are discussed.

Real-Time Fluorescence Screening Platform for Meat Freshness

Meat's freshness is closely related to food safety and human health and has received increasing attention nowadays. To on-site visually screen meat freshness in a fast and non-destructive manner, we rationally constructed a series of fluorescent probes (JDCN, JDNS, and JDPY) with distinct electron-withdrawing substitution groups based on julolidine-fused coumarin. These probes underwent an aza-Michael addition followed by an elimination reaction with cadaverine to generate a colorimetric and ratiometric fluorescence response, and their sensing performance was rationally enhanced by improving the electron-withdrawing strength of substitution groups. Particularly, JDCN with a dicyanovinyl group as the reaction site exhibited outstanding sensing performance including rapid response (∼60 s), high selectivity, and low detection limit (14 nM). Furthermore, JDCN was fabricated into test kits to detect cadaverine vapor with a high-contrast fluorescence change from red to green. Based on two-color visualization of cadaverine vapor, on-site non-contact and non-destructive monitoring of meat freshness was successfully achieved. The good sensing performance rendered JDCN test kits a promising real-time fluorescence screening platform for rapid, non-destructive, and accurate evaluation of meat freshness.

Acceptor modulation for improving thermally activated delayed fluorescence emitter in through-space charge transfer on spiroskeletons

Through-space charge transfer (TSCT) is regarded as an effective way to develop thermally activated delayed fluorescence (TADF) emitters. Based on this strategy, many molecular frameworks have been proposed, among which spirobased scaffolds have been extensively studied due to their unique advantages. In this work, we developed three emitters SPS, SPO, and SPON, which were constructed with the same donor and various acceptors to explore the influence of acceptor modulation at the C9 position of fluorene for spirostructure TSCT emitters. The results show that the acceptor with too weak electron-withdrawing ability will cause the emitter to not have TADF properties, while the acceptor with too much steric hindrance will weaken the face-to-face π-π stacking interaction between donor/acceptor (D/A). Since SPO balances the electron-withdrawing strength and steric hindrance of the acceptor, it achieves the highest external quantum efficiency (EQE) of 17.75%. This work shows that appropriate acceptor selection is essential for the TADF properties and high efficiency of the spirobased scaffold TSCT emitter

Bithienopyrroledione-based polymeric donors for efficient fullerene- and non-fullerene-based organic photovoltaic cells

Thienopyrroledione (TPD) is a promising electron-accepting core building block of donor-acceptor-type semiconducting polymers for use in high-performance organic photovoltaic (OPV) cells. In this study, bithienopyrroledione (biTPD) derivatives with different lengths of side alkyl chains were designed to utilize specific ability including stronger electron-withdrawing strength and extended π-conjugation compared with the mono TPD unit. The synthesized copolymers, poly(3-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-6-octylthieno[3,2-b]thiophen-2-yl)-5,5′-bis(2-butyloctyl)-3'-(6-octylthieno[3,2-b]thiophen-2-yl)-4H,4′H-[1,1′-bithieno[3,4-c]pyrrole]-4,4′,6,6′(5H,5′H)-tetraone) (PBDT-biTPD(BO)) and poly(3-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-6-octylthieno[3,2-b]thiophen-2-yl)-5,5′-bis(2-hexyldecyl)-3'-(6-octylthieno[3,2-b]thiophen-2-yl)-4H,4′H-[1,1′-bithieno[3,4-c]pyrrole]-4,4′,6,6′(5H,5′H)-tetraone) (PBDT-biTPD(HD)), composed of biTPD and typical benzo[1,2-b:4,5-b’]dithiophene (BDT) units had broad absorption with deep frontier molecular orbital energy levels. In addition, PBDT-biTPD(BO) and PBDT-biTPD(HD) showed predominant face-on orientation confirmed by the grazing-incidence wide-angle X-ray scattering (GIWAXS). The OPV device exhibited the maximum power conversion efficiency (PCE) of 8.48% and 8.99% for PBDT-biTPD(BO):PC71BM and PBDT-biTPD(HD):PC71BM, respectively, with a high open-circuit voltage (VOC). In particular, PBDT-biTPD(HD):IT-4F showed the best PCE of 9.49% with improved short-circuit current, despite the slightly decreased VOC compared to that of PBDT-biTPD(HD):PC71BM.

Substituent engineering of covalent organic frameworks modulates the crystallinity and electrochemical reactivity

Covalent organic frameworks (COFs) are emerging as powerful electrochemical energy storage/conversion materials benefiting from the controlled pore and chemical structures, which are usually determined by the regulation of the molecular building blocks. In contrast, the substituents are not considered significant for the electrochemical reactivity as they are usually removed during carbonization, which is necessary for improving the electrical conductivity of an electrode material. Here we show that the substituents play key roles not only in synthesizing COFs but also in controlling the COF structures during carbonization and thus the related electrochemical reactivity. Five characteristic substituents were used when synthesizing a new COF structure and it was found that electron-withdrawing strength of the substituents significantly influences the crystallinity of the COFs by tuning the reactivity of building blocks, or even determines whether the crystalline COF can be constructed. Moreover, the differences in chemical groups, sizes, and thermal stabilities of the substituents result in varied pore-collapse behaviors and the structures of the carbonized COFs, which show diverse effects on the electrochemical performances. An optimal material shows the highest surface area of 2131 m2/g, rich pores around 1 nm, and the highest ratio of sp2 carbon among the samples, corresponding to the largest double-layer specific capacity over 125 F/g in an ionic liquid electrolyte, while another material with the lowest surface area and N-doping level exhibits a high H2O2 production selectivity over 80% through selective oxygen reduction. This study shows guiding significance for the design of building blocks and substituents for COFs and further the carbonized carbons, and also exhibits the great potential of substituent engineering in modulating the electrochemical reactivity.

Franck-Condon Tuning of Optical Cycling Centers by Organic Functionalization.

Laser induced electronic excitations that spontaneously emit photons and decay directly to the initial ground state ("optical cycling transitions") are used in quantum information and precision measurement for state initialization and readout. To extend this primarily atomic technique to large, organic compounds, we theoretically investigate optical cycling of alkaline earth phenoxides and their functionalized derivatives. We find that optical cycle leakage due to wave function mismatch is low in these species, and can be further suppressed by using chemical substitution to boost the electron-withdrawing strength of the aromatic molecular ligand through resonance and induction effects. This provides a straightforward way to use chemical functional groups to construct optical cycling moieties for laser cooling, state preparation, and quantum measurement.

Flavylium Fluorophores as Near-Infrared Emitters.

This theoretical work rationalizes the absorption and fluorescence emission properties of conjugated dyes composed of dimethylamino flavylium heterocycles linked by a polymethine chain, which were recently reported to act as efficient shortwave infrared emitters. Density functional theory is used to characterize the electronic structure of the low-lying excited states as a function of the polymethine chain length. Decomposition of the computed excitations in terms of diabatic states is also performed to deconvolute the excited states wavefunction into charge-transfer intramolecular excitations. Based on these results, chemical substitution patterns consisting in enhancing the electron-withdrawing strength of the polymethine bridge and the electron-donating ability of the lateral flavylium fragments, are proposed to further redshift the photoluminescence of the fluorophores.

Low-bandgap D-A1-D-A2 type copolymers based on TPTI unit for efficient fullerene and nonfullerene polymer solar cells

To obtain low-bandgap polymers paired well with fullerene and nonfullerene acceptors, here we adopted a D-A1-D-A2 motif to develop two new low-bandgap copolymers PTPTI-T-BDD and PTPTI-T-FBT, where thiophene was used as the D unit, thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline-5,11(4H,10H)-dione (TPTI) was used as the A1 unit, and benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (BDD) and 5,6-difluoro-2,1,3-benzothiadiazole (FBT) were employed as the A2 unit, respectively. Effects of the electron-withdrawing strength of A2 unit on optoelectronic and photovoltaic properties of the PTPTI-T-BDD- and PTPTI-T-FBT-based fullerene and nonfullerene polymer solar cells (PSCs) were systematically investigated. When blended with PC71BM and ITIC, PTPTI-T-FBT-based PSCs showed a power conversion efficiency (PCE) of 6.20% and 6.03%, respectively, both of which are higher than that of PTPTI-T-BDD-based PSCs. Furthermore, ternary PSCs based on PBDB-T:PTPTI-T-FBT:PC71BM exhibited an improved PCE of 7.92%. This work suggests that constructing D-A1-D-A2 copolymers is a promising strategy to develop low-bandgap copolymers for efficient fullerene and nonfullerene PSCs with a reduced energy loss.

Photobleaching Efficiency Parallels the Enhancement of Membrane Damage for Porphyrazine Photosensitizers.

Photostability is considered a key asset for photosensitizers (PS) used in medical applications as well as for those used in energy conversion devices. In light-mediated medical treatments, which are based on PS-induced harm to diseased tissues, the photoinduced cycle of singlet oxygen generation has always been considered to correlate with PS efficiency. However, recent evidence points to the fundamental role of contact-dependent reactions, which usually cause PS photobleaching. Therefore, it seems reasonable to challenge the paradigm of photostability versus PS efficiency in medical applications. We have prepared a series of Mg(II) porphyrazines (MgPzs) having similar singlet oxygen quantum yields and side groups with different electron-withdrawing strengths that fine-tune their redox properties. A detailed investigation of the photobleaching mechanism of these porphyrazines revealed that it is independent of singlet oxygen, occurring mainly via photoinduced electron abstraction of surrounding electron rich molecules (solvents or lipids), as revealed by the formation of an air-stable radical anion intermediate. When incorporated into phospholipid membranes, photobleaching of MgPzs correlates with the degree of lipid unsaturation, indicating that it is caused by an electron abstraction from the lipid double bond. Interestingly, upon comparing the efficiency of membrane photodamage between two of these MgPzs (with the highest and the lowest photobleaching efficiencies), we found that the higher the rate of PS photobleaching the faster the leakage induced in the membranes. Our results therefore indicate that photobleaching is a necessary step toward inflicting irreversible biological damage. We propose that the design of more efficient PS for medical applications should contemplate contact-dependent reactions as well as strategies for PS regeneration.

Conjugacy of organic cathode materials for high-potential lithium-ion batteries: Carbonitriles versus quinones

Identification of redox-active moieties other than well-known carbonyl is essential to design the best candidates for organic cathodes in lithium-ion batteries among a broad-array of organic materials. In this study, an advanced computational modeling approach is employed to comprehensively investigate the redox properties for two representative organic classes, namely carbonitriles and quinones. It is highlighted that organic molecules with conjugating (non-conjugating) backbones would show higher redox potential for longer (shorter) backbone, owing to the strengthened inductive effect. Further examination verifies the importance of the relative electron-withdrawing strength of functionality over hydrogen on the redox potential and battery performance, suggesting a molecular design direction toward a carbonitrile with a longer conjugating backbone functionalized by an acyl chloride for highly-durable, high-performance cathodes. All these findings are comprehensively understood by the established universal correlations on redox property-electron affinity-solvation energy, highlighting two-stage transition behaviors of carbonitriles during the discharging process.

Donor-acceptor preassociation, excited state solvation threshold, and optical energy cost as challenges in chemical applications of photobases.

Photobases are molecules with increased pKa in the excited state that can serve to transduce light energy into proton removal capability. They can be used to control chemical reactions using light, such as removing protons from a catalytic site in reactions that are rate-limited by proton transfer. We identify and explore several major challenges toward their practical applications. Two important challenges are the need for pre-association (or ground state hydrogen bonding) between the proton donor and the photobase, and the need for excited state solvation of the photogenerated products. We investigate these two challenges with the photobase 5-methoxyquinoline as the proton acceptor and a low-pKa alcohol, 2,2,2-trifluoroethanol, as the proton donor. We vary the concentration of the donor in a background non-hydrogen-bonding solvent. Using absorption spectroscopy, we have identified that the donor-acceptor concentration ratio must exceed 100 : 1 to achieve appreciable ground state hydrogen bonding. Interestingly, emission spectroscopy reveals that the onset of ground state hydrogen bonding does not guarantee successful excited state proton transfer. It takes an additional order of magnitude increase in donor-acceptor ratio to achieve that goal, revealing that it is necessary to have excess donor molecules to reach the solvation threshold for the photogenerated products. The next challenge is reducing the large ground-excited state energy gap, which often requires UV photons to drive proton transfer. We show experimental and computational data comparing the photobasicity and optical energy gap for a few N-aromatic heterocyclic photobases. In general, we find that reducing the energy gap by increasing the conjugation size necessarily reduces photobasicity, while adding substituents of varying electron-withdrawing strength allows some fine-tuning of this effect. The combination of these two factors provide a preliminary design space for creating new photobasic molecules.